Migrate main navigation to Navigation 3.

app/src/androidTest/java/org/thoughtcrime/securesms/main/MainNavigationLaunchTest.kt

@@ -62,10 +62,12 @@ class MainNavigationLaunchTest {
   private val recipient: RecipientId get() = harness.others.first()
 
   /**
-   * Share-target cold-launch regression test. Pre-fix, wrapNavigator() re-routed the
-   * early-staged Conversation through goTo(), whose async wallpaper-prefetch path emitted
-   * a SECOND internalDetailLocation with a fresh ConversationArgs — recreating the
-   * fragment and dropping share data.
+   * Share-target cold-launch regression test. Originally, replaying a deferred navigation request
+   * once the navigator went live re-routed the early-staged Conversation through goTo(), whose async
+   * wallpaper-prefetch path pushed a SECOND entry with a fresh ConversationArgs — recreating the
+   * fragment and dropping share data. The deferral is gone now that the back stacks are owned by the
+   * view-model and can be pushed to whether or not a composition is alive, but the double-create this
+   * guards against is worth keeping a test on.
    */
   @Test
   fun coldLaunch_shareIntent_createsFragmentExactlyOnceWithShareData() {
@@ -93,9 +95,9 @@ class MainNavigationLaunchTest {
             appendLine("--- diagnostic dump ---")
             appendLine("fragments observed: ${recorder.allCreated}")
             appendLine("activity fragments: ${launched.activity.supportFragmentManager.fragments.map { it::class.simpleName }}")
-            appendLine("vm.currentListLocation: ${vm.mainNavigationState.value.currentListLocation}")
+            appendLine("vm.currentListLocation: ${vm.mainNavigationBarState.value.currentListLocation}")
             appendLine("vm.detailLocation: ${vm.detailLocation.value}")
-            appendLine("vm.chatsBackStackEntries: ${vm.chatsBackStackEntries.toList()}")
+            appendLine("vm.navigator[MainListRoute.Chats]: ${vm.navigator[MainListRoute.Chats].toList()}")
           }
         }
         throw IllegalStateException("${e.message}\n$state", e)
@@ -225,22 +227,22 @@ class MainNavigationLaunchTest {
         "Expected shareDataTimestamp=-1 for notification path, got ${args.shareDataTimestamp}"
       }
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
-      check(vm.mainNavigationState.value.currentListLocation == MainNavigationListLocation.CHATS) {
-        "Expected currentListLocation=CHATS, got ${vm.mainNavigationState.value.currentListLocation}"
+      check(vm.mainNavigationBarState.value.currentListLocation == MainListRoute.Chats) {
+        "Expected currentListLocation=CHATS, got ${vm.mainNavigationBarState.value.currentListLocation}"
       }
     }
   }
 
   @Test
   fun coldLaunch_tabIntent_setsListLocation() {
-    val intent = tabIntent(MainNavigationListLocation.CALLS)
+    val intent = tabIntent(MainListRoute.Calls)
     launchSync(intent).use { launched ->
       val recorder = launched.recorder
-      awaitListFragment(launched, MainNavigationListLocation.CALLS)
+      awaitListFragment(launched, MainListRoute.Calls)
 
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
-      check(vm.mainNavigationState.value.currentListLocation == MainNavigationListLocation.CALLS) {
-        "Expected VM CALLS, got ${vm.mainNavigationState.value.currentListLocation}"
+      check(vm.mainNavigationBarState.value.currentListLocation == MainListRoute.Calls) {
+        "Expected VM CALLS, got ${vm.mainNavigationBarState.value.currentListLocation}"
       }
       Thread.sleep(750)
       check(recorder.createdArgs.isEmpty()) {
@@ -256,7 +258,7 @@ class MainNavigationLaunchTest {
    */
   @Test
   fun coldLaunch_detailLocationIntent_isNoOpToday() {
-    val intent = detailLocationIntent(MainNavigationDetailLocation.Chats.ConversationSettings(recipient))
+    val intent = detailLocationIntent(MainDetailRoute.Chats.ConversationSettings(recipient))
     launchSync(intent).use { launched ->
       val recorder = launched.recorder
       Thread.sleep(1500)
@@ -265,7 +267,7 @@ class MainNavigationLaunchTest {
           "starts handling it on cold launch, update or delete this test. Got: ${recorder.allCreated}"
       }
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
-      val staged = runOnMainSync { vm.chatsBackStackEntries.filterNot { it is MainNavigationDetailLocation.Empty } }
+      val staged = runOnMainSync { vm.navigator[MainListRoute.Chats].filterIsInstance<MainDetailRoute>() }
       check(staged.isEmpty()) {
         "Expected no detail to be staged on the chats back stack, got $staged"
       }
@@ -277,11 +279,11 @@ class MainNavigationLaunchTest {
     val intent = deepLinkIntent(Uri.parse("https://signal.org/test-not-a-real-deeplink"))
     launchSync(intent).use { launched ->
       val recorder = launched.recorder
-      awaitListFragment(launched, MainNavigationListLocation.CHATS)
+      awaitListFragment(launched, MainListRoute.Chats)
 
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
-      check(vm.mainNavigationState.value.currentListLocation == MainNavigationListLocation.CHATS) {
-        "Expected CHATS for deep-link launch, got ${vm.mainNavigationState.value.currentListLocation}"
+      check(vm.mainNavigationBarState.value.currentListLocation == MainListRoute.Chats) {
+        "Expected CHATS for deep-link launch, got ${vm.mainNavigationBarState.value.currentListLocation}"
       }
       check(recorder.createdArgs.isEmpty()) {
         "Expected no ConversationFragment for deep-link launch, got ${recorder.createdArgs.size}"
@@ -294,16 +296,16 @@ class MainNavigationLaunchTest {
     val intent = Intent(context, MainActivity::class.java)
     launchSync(intent).use { launched ->
       val recorder = launched.recorder
-      awaitListFragment(launched, MainNavigationListLocation.CHATS)
+      awaitListFragment(launched, MainListRoute.Chats)
 
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
-      check(vm.mainNavigationState.value.currentListLocation == MainNavigationListLocation.CHATS) {
-        "Expected default CHATS, got ${vm.mainNavigationState.value.currentListLocation}"
+      check(vm.mainNavigationBarState.value.currentListLocation == MainListRoute.Chats) {
+        "Expected default CHATS, got ${vm.mainNavigationBarState.value.currentListLocation}"
       }
       Thread.sleep(750)
       val detailLocation = runOnMainSync { vm.detailLocation.value }
-      check(detailLocation == MainNavigationDetailLocation.Empty) {
-        "Expected Empty detail location, got $detailLocation"
+      check(detailLocation == null) {
+        "Expected no detail location, got $detailLocation"
       }
       check(recorder.createdArgs.isEmpty()) {
         "Expected no ConversationFragment for bare launch, got ${recorder.createdArgs.size}"
@@ -353,23 +355,23 @@ class MainNavigationLaunchTest {
       }
       val baseline = recorder.createdArgs.size
 
-      val warmIntent = detailLocationIntent(MainNavigationDetailLocation.Empty)
+      val warmIntent = MainActivity.clearTopAndExitDetail(context)
       runOnMainSync {
         InstrumentationRegistry.getInstrumentation().callActivityOnNewIntent(launched.activity, warmIntent)
       }
 
-      await(description = "no new ConversationFragment after Empty detail intent") {
+      await(description = "no new ConversationFragment after exit-detail intent") {
         recorder.createdArgs.size == baseline
       }
 
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
 
-      await(description = "conversation cleared from chats back stack after Empty detail intent") {
-        vm.chatsBackStackEntries.none { it is MainNavigationDetailLocation.Conversation }
+      await(description = "conversation cleared from chats back stack after exit-detail intent") {
+        vm.navigator[MainListRoute.Chats].none { it is MainDetailRoute.Conversation }
       }
 
-      check(vm.mainNavigationState.value.currentListLocation == MainNavigationListLocation.CHATS) {
-        "Expected CHATS, got ${vm.mainNavigationState.value.currentListLocation}"
+      check(vm.mainNavigationBarState.value.currentListLocation == MainListRoute.Chats) {
+        "Expected CHATS, got ${vm.mainNavigationBarState.value.currentListLocation}"
       }
     }
   }
@@ -377,18 +379,18 @@ class MainNavigationLaunchTest {
   @Test
   fun warmStart_onNewIntent_tabIntent_switchesList() {
     launchSync(Intent(context, MainActivity::class.java)).use { launched ->
-      awaitListFragment(launched, MainNavigationListLocation.CHATS)
+      awaitListFragment(launched, MainListRoute.Chats)
 
-      val warmIntent = tabIntent(MainNavigationListLocation.CALLS)
+      val warmIntent = tabIntent(MainListRoute.Calls)
       runOnMainSync {
         InstrumentationRegistry.getInstrumentation().callActivityOnNewIntent(launched.activity, warmIntent)
       }
 
-      awaitListFragment(launched, MainNavigationListLocation.CALLS)
+      awaitListFragment(launched, MainListRoute.Calls)
 
       val vm = runOnMainSync { launched.activity.mainNavigationViewModel() }
-      check(vm.mainNavigationState.value.currentListLocation == MainNavigationListLocation.CALLS) {
-        "Expected VM CALLS, got ${vm.mainNavigationState.value.currentListLocation}"
+      check(vm.mainNavigationBarState.value.currentListLocation == MainListRoute.Calls) {
+        "Expected VM CALLS, got ${vm.mainNavigationBarState.value.currentListLocation}"
       }
       check(launched.recorder.createdArgs.isEmpty()) {
         "Expected no ConversationFragment for tab switch, got ${launched.recorder.createdArgs.size}"
@@ -427,22 +429,22 @@ class MainNavigationLaunchTest {
 
   @Test
   fun recreate_midTab_restoresTab() {
-    launchSync(tabIntent(MainNavigationListLocation.CALLS)).use { launched ->
-      awaitListFragment(launched, MainNavigationListLocation.CALLS)
+    launchSync(tabIntent(MainListRoute.Calls)).use { launched ->
+      awaitListFragment(launched, MainListRoute.Calls)
 
       runOnMainSync { launched.activity.recreate() }
 
       // Verify the user-visible tab content rebinds after recreate, not just the VM. The
       // recorder removes destroyed fragments, so this only passes once the post-recreate
       // CallLogFragment instance is attached.
-      awaitListFragment(launched, MainNavigationListLocation.CALLS)
+      awaitListFragment(launched, MainListRoute.Calls)
 
       // launched.activity returns the *latest* MainActivity (the holder updates in
       // onActivityCreated), so this reads the post-recreate VM instance.
       val location = runOnMainSync {
-        launched.activity.mainNavigationViewModel().mainNavigationState.value.currentListLocation
+        launched.activity.mainNavigationViewModel().mainNavigationBarState.value.currentListLocation
       }
-      check(location == MainNavigationListLocation.CALLS) {
+      check(location == MainListRoute.Calls) {
         "Expected VM CALLS post-recreate, got $location"
       }
       check(launched.recorder.createdArgs.isEmpty()) {
@@ -573,12 +575,12 @@ class MainNavigationLaunchTest {
     }
   }
 
-  private fun tabIntent(tab: MainNavigationListLocation): Intent {
+  private fun tabIntent(tab: MainListRoute): Intent {
     return Intent(context, MainActivity::class.java)
       .putExtra("STARTING_TAB", tab)
   }
 
-  private fun detailLocationIntent(location: MainNavigationDetailLocation): Intent {
+  private fun detailLocationIntent(location: MainDetailRoute): Intent {
     return Intent(context, MainActivity::class.java)
       .putExtra("DETAIL_LOCATION", location)
   }
@@ -742,11 +744,11 @@ class MainNavigationLaunchTest {
    * attached, so a tab assertion that reads the FragmentManager is a real user-visible
    * signal — strictly stronger than reading the VM's `currentListLocation`.
    */
-  private fun listFragmentClass(location: MainNavigationListLocation): Class<out Fragment> = when (location) {
-    MainNavigationListLocation.CHATS -> ConversationListFragment::class.java
-    MainNavigationListLocation.ARCHIVE -> ConversationListArchiveFragment::class.java
-    MainNavigationListLocation.CALLS -> CallLogFragment::class.java
-    MainNavigationListLocation.STORIES -> StoriesLandingFragment::class.java
+  private fun listFragmentClass(location: MainListRoute): Class<out Fragment> = when (location) {
+    MainListRoute.Chats -> ConversationListFragment::class.java
+    MainListRoute.Archive -> ConversationListArchiveFragment::class.java
+    MainListRoute.Calls -> CallLogFragment::class.java
+    MainListRoute.Stories -> StoriesLandingFragment::class.java
   }
 
   /**
@@ -775,7 +777,7 @@ class MainNavigationLaunchTest {
     return roots
   }
 
-  private fun awaitListFragment(launched: LaunchedActivity, location: MainNavigationListLocation) {
+  private fun awaitListFragment(launched: LaunchedActivity, location: MainListRoute) {
     val expected = listFragmentClass(location)
     try {
       await(timeoutMs = 10_000, description = "${expected.simpleName} attached for $location") {

app/src/main/java/org/thoughtcrime/securesms/MainActivity.kt

@@ -9,7 +9,6 @@ import android.annotation.SuppressLint
 import android.app.Activity
 import android.content.Context
 import android.content.Intent
-import android.graphics.Color
 import android.os.Build
 import android.os.Bundle
 import android.view.MotionEvent
@@ -24,12 +23,12 @@ import androidx.activity.result.ActivityResultLauncher
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.activity.viewModels
 import androidx.compose.foundation.background
-import androidx.compose.foundation.interaction.MutableInteractionSource
 import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.BoxWithConstraints
 import androidx.compose.foundation.layout.BoxWithConstraintsScope
 import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.WindowInsets
 import androidx.compose.foundation.layout.WindowInsetsSides
@@ -44,22 +43,18 @@ import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.systemBarsPadding
 import androidx.compose.foundation.layout.windowInsetsPadding
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
 import androidx.compose.material3.adaptive.ExperimentalMaterial3AdaptiveApi
-import androidx.compose.material3.adaptive.layout.PaneAdaptedValue
-import androidx.compose.material3.adaptive.layout.PaneExpansionAnchor
-import androidx.compose.material3.adaptive.layout.ThreePaneScaffoldRole
-import androidx.compose.material3.adaptive.layout.rememberPaneExpansionState
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.key
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.rememberCoroutineScope
-import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.toArgb
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.platform.LocalFocusManager
@@ -68,19 +63,12 @@ import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import androidx.core.content.ContextCompat
 import androidx.fragment.app.DialogFragment
-import androidx.fragment.compose.AndroidFragment
-import androidx.fragment.compose.rememberFragmentState
 import androidx.lifecycle.Lifecycle
 import androidx.lifecycle.LifecycleOwner
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.lifecycle.createSavedStateHandle
 import androidx.lifecycle.lifecycleScope
 import androidx.lifecycle.repeatOnLifecycle
-import androidx.lifecycle.viewmodel.navigation3.rememberViewModelStoreNavEntryDecorator
-import androidx.navigation3.runtime.NavKey
-import androidx.navigation3.runtime.entryProvider
-import androidx.navigation3.runtime.rememberSaveableStateHolderNavEntryDecorator
-import androidx.navigation3.ui.NavDisplay
 import androidx.recyclerview.widget.RecyclerView
 import com.google.android.material.dialog.MaterialAlertDialogBuilder
 import io.reactivex.rxjava3.subjects.PublishSubject
@@ -92,9 +80,16 @@ import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import org.signal.core.ui.BottomSheetUtil
+import org.signal.core.ui.NavigationType
 import org.signal.core.ui.compose.Snackbars
+import org.signal.core.ui.compose.split.ListDetailNavDisplay
+import org.signal.core.ui.compose.split.ListDetailPaneLayout
+import org.signal.core.ui.compose.split.ListDetailPaneMetrics
+import org.signal.core.ui.compose.split.ListPaneChrome
+import org.signal.core.ui.compose.split.PaneAnchor
+import org.signal.core.ui.compose.split.rememberListDetailPaneLayout
+import org.signal.core.ui.compose.split.rememberListDetailPaneMetrics
 import org.signal.core.ui.compose.theme.SignalTheme
-import org.signal.core.ui.navigation.TransitionSpecs
 import org.signal.core.ui.permissions.Permissions
 import org.signal.core.ui.rememberIsSplitPane
 import org.signal.core.util.AppForegroundObserver
@@ -109,14 +104,12 @@ import org.thoughtcrime.securesms.backup.v2.ArchiveRestoreProgressState
 import org.thoughtcrime.securesms.backup.v2.ui.CouldNotCompleteBackupRestoreSheet
 import org.thoughtcrime.securesms.backup.v2.ui.verify.VerifyBackupKeyActivity
 import org.thoughtcrime.securesms.calls.YouAreAlreadyInACallSnackbar.show
-import org.thoughtcrime.securesms.calls.callsNavEntries
 import org.thoughtcrime.securesms.calls.log.CallLogFilter
 import org.thoughtcrime.securesms.calls.log.CallLogFragment
 import org.thoughtcrime.securesms.calls.new.NewCallActivity
 import org.thoughtcrime.securesms.calls.quality.CallQuality
 import org.thoughtcrime.securesms.calls.quality.CallQualityBottomSheetFragment
 import org.thoughtcrime.securesms.chats.ConversationTransitionState
-import org.thoughtcrime.securesms.chats.chatsNavEntries
 import org.thoughtcrime.securesms.components.DebugLogsPromptDialogFragment
 import org.thoughtcrime.securesms.components.PromptBatterySaverDialogFragment
 import org.thoughtcrime.securesms.components.compose.ConnectivityWarningBottomSheet
@@ -135,7 +128,6 @@ import org.thoughtcrime.securesms.components.voice.VoiceNoteMediaControllerOwner
 import org.thoughtcrime.securesms.conversation.ConversationIntents
 import org.thoughtcrime.securesms.conversation.NewConversationActivity
 import org.thoughtcrime.securesms.conversation.v2.MotionEventRelay
-import org.thoughtcrime.securesms.conversationlist.ConversationListArchiveFragment
 import org.thoughtcrime.securesms.conversationlist.ConversationListFragment
 import org.thoughtcrime.securesms.conversationlist.RelinkDevicesReminderBottomSheetFragment
 import org.thoughtcrime.securesms.conversationlist.RestoreCompleteBottomSheetDialog
@@ -145,14 +137,14 @@ import org.thoughtcrime.securesms.devicetransfer.olddevice.OldDeviceExitActivity
 import org.thoughtcrime.securesms.groups.ui.creategroup.CreateGroupActivity
 import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.lock.v2.CreateSvrPinActivity
+import org.thoughtcrime.securesms.main.EmptyDetailScreen
 import org.thoughtcrime.securesms.main.MainBottomChrome
 import org.thoughtcrime.securesms.main.MainBottomChromeCallback
 import org.thoughtcrime.securesms.main.MainBottomChromeState
-import org.thoughtcrime.securesms.main.MainContentLayoutData
+import org.thoughtcrime.securesms.main.MainDetailRoute
+import org.thoughtcrime.securesms.main.MainListRoute
 import org.thoughtcrime.securesms.main.MainMegaphoneState
 import org.thoughtcrime.securesms.main.MainNavigationBar
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-import org.thoughtcrime.securesms.main.MainNavigationListLocation
 import org.thoughtcrime.securesms.main.MainNavigationRail
 import org.thoughtcrime.securesms.main.MainNavigationRouter
 import org.thoughtcrime.securesms.main.MainNavigationViewModel
@@ -164,6 +156,7 @@ import org.thoughtcrime.securesms.main.MainToolbarMode
 import org.thoughtcrime.securesms.main.MainToolbarState
 import org.thoughtcrime.securesms.main.MainToolbarViewModel
 import org.thoughtcrime.securesms.main.Material3OnScrollHelperBinder
+import org.thoughtcrime.securesms.main.rememberDecoratedDetailEntries
 import org.thoughtcrime.securesms.mediasend.MediaSendLauncher
 import org.thoughtcrime.securesms.megaphone.Megaphone
 import org.thoughtcrime.securesms.megaphone.MegaphoneActionController
@@ -177,8 +170,6 @@ import org.thoughtcrime.securesms.service.BackupMediaRestoreService
 import org.thoughtcrime.securesms.service.KeyCachingService
 import org.thoughtcrime.securesms.starred.StarredMessagesActivity
 import org.thoughtcrime.securesms.stories.Stories
-import org.thoughtcrime.securesms.stories.landing.StoriesLandingFragment
-import org.thoughtcrime.securesms.stories.storiesNavEntries
 import org.thoughtcrime.securesms.util.AppStartup
 import org.thoughtcrime.securesms.util.CachedInflater
 import org.thoughtcrime.securesms.util.CommunicationActions
@@ -187,12 +178,6 @@ import org.thoughtcrime.securesms.util.Material3OnScrollHelper
 import org.thoughtcrime.securesms.util.SplashScreenUtil
 import org.thoughtcrime.securesms.util.TopToastPopup
 import org.thoughtcrime.securesms.util.viewModel
-import org.thoughtcrime.securesms.window.AppPaneDragHandle
-import org.thoughtcrime.securesms.window.AppScaffold
-import org.thoughtcrime.securesms.window.AppScaffoldAnimationStateFactory
-import org.thoughtcrime.securesms.window.AppScaffoldNavigator
-import org.thoughtcrime.securesms.window.NavigationType
-import org.thoughtcrime.securesms.window.rememberThreePaneScaffoldNavigatorDelegate
 import org.whispersystems.signalservice.api.websocket.WebSocketConnectionState
 import kotlin.time.Duration.Companion.minutes
 import org.signal.core.ui.R as CoreUiR
@@ -212,7 +197,11 @@ class MainActivity :
 
     private const val KEY_STARTING_TAB = "STARTING_TAB"
     private const val KEY_DETAIL_LOCATION = "DETAIL_LOCATION"
-    const val RESULT_CONFIG_CHANGED = Activity.RESULT_FIRST_USER + 901
+    private const val KEY_EXIT_DETAIL = "EXIT_DETAIL"
+    const val RESULT_CONFIG_CHANGED = RESULT_FIRST_USER + 901
+
+    /** Width the navigation rail occupies inside the list pane. */
+    private val RAIL_WIDTH = 80.dp
 
     @JvmStatic
     fun clearTop(context: Context): Intent {
@@ -221,14 +210,23 @@ class MainActivity :
     }
 
     @JvmStatic
-    fun clearTopAndOpenTab(context: Context, startingTab: MainNavigationListLocation): Intent {
+    fun clearTopAndOpenTab(context: Context, startingTab: MainListRoute): Intent {
       return clearTop(context).putExtra(KEY_STARTING_TAB, startingTab)
     }
 
     @JvmStatic
-    fun clearTopAndOpenDetail(context: Context, location: MainNavigationDetailLocation): Intent {
+    fun clearTopAndOpenDetail(context: Context, location: MainDetailRoute): Intent {
       return clearTop(context).putExtra(KEY_DETAIL_LOCATION, location)
     }
+
+    /**
+     * Opens the main screen with the current tab's detail content dropped, leaving its list displayed.
+     * Used by screens that finish having invalidated whatever the detail pane was showing.
+     */
+    @JvmStatic
+    fun clearTopAndExitDetail(context: Context): Intent {
+      return clearTop(context).putExtra(KEY_EXIT_DETAIL, true)
+    }
   }
 
   private val dynamicTheme = DynamicNoActionBarTheme()
@@ -241,8 +239,8 @@ class MainActivity :
     get() = mediaController
 
   private val mainNavigationViewModel: MainNavigationViewModel by viewModel {
-    val startingTab = intent.extras?.getSerializableCompat(KEY_STARTING_TAB, MainNavigationListLocation::class.java)
-    MainNavigationViewModel(it.createSavedStateHandle(), startingTab ?: MainNavigationListLocation.CHATS)
+    val startingTab = intent.extras?.getSerializableCompat(KEY_STARTING_TAB, MainListRoute::class.java)
+    MainNavigationViewModel(it.createSavedStateHandle(), startingTab ?: MainListRoute.Chats)
   }
 
   private val vitalsViewModel: VitalsViewModel by viewModel {
@@ -303,7 +301,7 @@ class MainActivity :
           mainNavigationViewModel.navigationEvents.collectLatest {
             when (it) {
               MainNavigationViewModel.NavigationEvent.STORY_CAMERA_FIRST -> {
-                mainBottomChromeCallback.onCameraClick(MainNavigationListLocation.STORIES)
+                mainBottomChromeCallback.onCameraClick(MainListRoute.Stories)
               }
             }
           }
@@ -383,27 +381,23 @@ class MainActivity :
 
     setContent {
       val mainToolbarState by toolbarViewModel.state.collectAsStateWithLifecycle()
-      val megaphone by mainNavigationViewModel.megaphone.collectAsStateWithLifecycle()
-      val mainNavigationState by mainNavigationViewModel.mainNavigationState.collectAsStateWithLifecycle()
+      val mainNavigationState by mainNavigationViewModel.mainNavigationBarState.collectAsStateWithLifecycle()
 
       LaunchedEffect(mainNavigationState.currentListLocation) {
         when (mainNavigationState.currentListLocation) {
-          MainNavigationListLocation.CHATS -> toolbarViewModel.presentToolbarForConversationListFragment()
-          MainNavigationListLocation.ARCHIVE -> toolbarViewModel.presentToolbarForConversationListArchiveFragment()
-          MainNavigationListLocation.CALLS -> toolbarViewModel.presentToolbarForCallLogFragment()
-          MainNavigationListLocation.STORIES -> toolbarViewModel.presentToolbarForStoriesLandingFragment()
+          MainListRoute.Chats -> toolbarViewModel.presentToolbarForConversationListFragment()
+          MainListRoute.Archive -> toolbarViewModel.presentToolbarForConversationListArchiveFragment()
+          MainListRoute.Calls -> toolbarViewModel.presentToolbarForCallLogFragment()
+          MainListRoute.Stories -> toolbarViewModel.presentToolbarForStoriesLandingFragment()
         }
       }
 
       val isActionModeActive = mainToolbarState.mode == MainToolbarMode.ACTION_MODE
       val isSearchModeActive = mainToolbarState.mode == MainToolbarMode.SEARCH
-      val isNavigationRailVisible = mainToolbarState.mode != MainToolbarMode.SEARCH
-      val isNavigationBarVisible = mainToolbarState.mode == MainToolbarMode.FULL
-      val isBackHandlerEnabled = mainToolbarState.destination != MainNavigationListLocation.CHATS && !isActionModeActive && !isSearchModeActive
+      val isBackHandlerEnabled = mainToolbarState.destination != MainListRoute.Chats && !isActionModeActive && !isSearchModeActive
 
       BackHandler(enabled = isBackHandlerEnabled) {
-        mainNavigationViewModel.setFocusedPane(ThreePaneScaffoldRole.Secondary)
-        mainNavigationViewModel.goTo(MainNavigationListLocation.CHATS)
+        mainNavigationViewModel.goTo(MainListRoute.Chats)
       }
 
       BackHandler(enabled = isActionModeActive) {
@@ -421,35 +415,18 @@ class MainActivity :
         }
       }
 
-      val mainBottomChromeState = remember(mainToolbarState.destination, mainToolbarState.mode, megaphone) {
-        MainBottomChromeState(
-          destination = mainToolbarState.destination,
-          mainToolbarMode = mainToolbarState.mode,
-          megaphoneState = MainMegaphoneState(
-            megaphone = megaphone,
-            mainToolbarMode = mainToolbarState.mode
-          )
-        )
-      }
-
       val isSplitPane = LocalResources.current.rememberIsSplitPane()
-      val contentLayoutData = MainContentLayoutData.rememberContentLayoutData(mainToolbarState.mode)
+      val contentLayoutData = rememberListDetailPaneMetrics(listPaddingStart = mainToolbarState.mode.listPaddingStart)
 
       MainContainer {
-        val wrappedNavigator = rememberNavigator(isSplitPane, contentLayoutData, maxWidth)
-        val listPaneWidth = contentLayoutData.rememberDefaultPanePreferredWidth(maxWidth)
-        val navigationType = NavigationType.rememberNavigationType()
-
         val detailLocation by mainNavigationViewModel.detailLocation.collectAsStateWithLifecycle()
-        val isConversationFullscreen = !isSplitPane &&
-          wrappedNavigator.scaffoldValue.primary == PaneAdaptedValue.Expanded &&
-          detailLocation is MainNavigationDetailLocation.Conversation
+        val isConversationFullscreen = !isSplitPane && detailLocation is MainDetailRoute.Conversation
 
         val context = LocalContext.current
         val isDarkTheme = isSystemInDarkTheme()
         val navBarColor = when {
           isSplitPane -> SignalTheme.colors.colorSurface1.toArgb()
-          isConversationFullscreen -> Color.TRANSPARENT
+          isConversationFullscreen -> Color.Transparent.toArgb()
           else -> ContextCompat.getColor(context, CoreUiR.color.signal_colorSurface2)
         }
 
@@ -467,125 +444,40 @@ class MainActivity :
           }
         }
 
-        val anchors = remember(contentLayoutData, mainToolbarState, listPaneWidth, navigationType) {
-          val halfPartitionWidth = contentLayoutData.partitionWidth / 2
+        val convoTransitionState = ConversationTransitionState.remember(isSplitPane)
 
-          val detailOffset = when {
-            mainToolbarState.mode == MainToolbarMode.SEARCH -> 0.dp
-            navigationType == NavigationType.BAR -> 0.dp
-            else -> 80.dp
-          }
-
-          val detailOnlyAnchor = PaneExpansionAnchor.Offset.fromStart(detailOffset + contentLayoutData.listPaddingStart + halfPartitionWidth)
-          val detailAndListAnchor = PaneExpansionAnchor.Offset.fromStart(listPaneWidth + halfPartitionWidth)
-          val listOnlyAnchor = PaneExpansionAnchor.Offset.fromEnd(contentLayoutData.detailPaddingEnd - halfPartitionWidth)
-
-          listOf(detailOnlyAnchor, detailAndListAnchor, listOnlyAnchor)
+        DisposableEffect(convoTransitionState) {
+          mainNavigationViewModel.setChatListSnapshotCaptureProvider { convoTransitionState.writeGraphicsLayerToBitmap() }
+          onDispose { mainNavigationViewModel.setChatListSnapshotCaptureProvider(null) }
         }
 
-        val (detailOnlyAnchor, detailAndListAnchor, listOnlyAnchor) = anchors
+        val paneAnchor by mainNavigationViewModel.paneAnchor.collectAsStateWithLifecycle()
+        val hasDetailContent by mainNavigationViewModel.hasDetailContent.collectAsStateWithLifecycle()
 
-        val paneExpansionState = rememberPaneExpansionState(
-          key = wrappedNavigator.scaffoldValue.paneExpansionStateKey,
-          anchors = anchors,
-          initialAnchoredIndex = 1
+        val tabEntries = rememberDecoratedDetailEntries(mainNavigationViewModel, convoTransitionState, isSplitPane)
+
+        val paneLayout = rememberMainPaneLayout(
+          contentLayoutData = contentLayoutData,
+          maxWidth = maxWidth,
+          toolbarMode = mainToolbarState.mode,
+          paneAnchor = paneAnchor
         )
 
-        val paneAnchorIndex = rememberSaveable(paneExpansionState.currentAnchor) {
-          anchors.indexOf(paneExpansionState.currentAnchor)
+        val listPaneChrome: ListPaneChrome = remember {
+          { content -> MainListPaneChrome(content = content) }
         }
 
-        LaunchedEffect(anchors) {
-          val index = when {
-            paneAnchorIndex < 0 -> 1
-            paneAnchorIndex > anchors.lastIndex -> anchors.lastIndex
-            else -> paneAnchorIndex
-          }
-
-          if (index in anchors.indices) {
-            val anchor = anchors[index]
-            paneExpansionState.animateTo(anchor)
-          }
+        val emptyDetailContent: @Composable () -> Unit = remember {
+          { EmptyDetailScreen() }
         }
 
-        val convoTransitionState = ConversationTransitionState.remember(isSplitPane)
-        val mutableInteractionSource = remember { MutableInteractionSource() }
-
-        LaunchedEffect(convoTransitionState) {
-          mainNavigationViewModel.setChatListSnapshotCaptureProvider { convoTransitionState.writeGraphicsLayerToBitmap() }
-        }
-
-        LaunchedEffect(isSplitPane) {
-          mainNavigationViewModel.onSplitPaneChanged(isSplitPane)
-        }
-
-        val scope = rememberCoroutineScope()
-
-        BackHandler(paneExpansionState.currentAnchor == detailOnlyAnchor) {
-          mainNavigationViewModel.goTo(MainNavigationDetailLocation.Empty)
-          scope.launch {
-            paneExpansionState.animateTo(listOnlyAnchor)
-          }
-        }
-
-        LaunchedEffect(paneExpansionState.currentAnchor, detailOnlyAnchor, listOnlyAnchor, detailAndListAnchor) {
-          val isFullScreenPane = when (paneExpansionState.currentAnchor) {
-            listOnlyAnchor, detailOnlyAnchor -> {
-              true
-            }
-
-            else -> {
-              false
-            }
-          }
-
-          mainNavigationViewModel.onPaneAnchorChanged(isFullScreenPane)
-        }
-
-        LaunchedEffect(paneExpansionState.currentAnchor) {
-          when (paneExpansionState.currentAnchor) {
-            listOnlyAnchor -> {
-              mainNavigationViewModel.setFocusedPane(ThreePaneScaffoldRole.Secondary)
-            }
-
-            detailOnlyAnchor -> {
-              mainNavigationViewModel.setFocusedPane(ThreePaneScaffoldRole.Primary)
-            }
-
-            else -> Unit
-          }
-        }
-
-        val paneFocusRequest by mainNavigationViewModel.paneFocusRequests.collectAsStateWithLifecycle(null)
-        LaunchedEffect(paneFocusRequest) {
-          if (paneFocusRequest == null) {
-            return@LaunchedEffect
-          }
-
-          if (paneFocusRequest == ThreePaneScaffoldRole.Secondary && paneExpansionState.currentAnchor == detailOnlyAnchor) {
-            paneExpansionState.animateTo(listOnlyAnchor)
-          }
-
-          if (paneFocusRequest == ThreePaneScaffoldRole.Primary && paneExpansionState.currentAnchor == listOnlyAnchor) {
-            paneExpansionState.animateTo(detailOnlyAnchor)
-          }
-        }
-
-        val noEnterTransitionFactory = remember {
-          AppScaffoldAnimationStateFactory(
-            enabledStates = AppScaffoldNavigator.NavigationState.entries.filterNot {
-              it == AppScaffoldNavigator.NavigationState.ENTER
-            }.toSet()
-          )
-        }
-
-        AppScaffold(
-          navigator = wrappedNavigator,
-          modifier = convoTransitionState.writeContentToGraphicsLayer(),
-          paneExpansionState = paneExpansionState,
+        Scaffold(
+          containerColor = Color.Transparent,
           contentWindowInsets = WindowInsets(),
           snackbarHost = {
-            if (wrappedNavigator.scaffoldValue.primary == PaneAdaptedValue.Expanded) {
+            // MainBottomChrome renders its own host over the list, but only in single pane, so this one
+            // has to cover both split pane and whatever fills the window in single pane.
+            if (isSplitPane || hasDetailContent) {
               MainSnackbar(
                 hostKey = SnackbarHostKey.Global,
                 onDismissed = mainBottomChromeCallback::onSnackbarDismissed,
@@ -593,170 +485,20 @@ class MainActivity :
               )
             }
           },
-          bottomNavContent = {
-            if (isNavigationBarVisible) {
-              Column(
-                modifier = Modifier
-                  .clip(contentLayoutData.navigationBarShape)
-                  .background(color = SignalTheme.colors.colorSurface2)
-              ) {
-                MainNavigationBar(
-                  state = mainNavigationState,
-                  onDestinationSelected = mainNavigationCallback
-                )
-
-                if (!LocalResources.current.rememberIsSplitPane()) {
-                  Spacer(Modifier.navigationBarsPadding())
-                }
-              }
-            }
-          },
-          navRailContent = {
-            if (isNavigationRailVisible) {
-              MainNavigationRail(
-                state = mainNavigationState,
-                mainFloatingActionButtonsCallback = mainBottomChromeCallback,
-                onDestinationSelected = mainNavigationCallback
-              )
-            }
-          },
-          secondaryContent = {
-            val listContainerColor = if (isSplitPane) {
-              SignalTheme.colors.colorSurface1
-            } else {
-              MaterialTheme.colorScheme.surface
-            }
-
-            Column(
-              modifier = Modifier
-                .padding(start = contentLayoutData.listPaddingStart)
-                .fillMaxSize()
-                .background(listContainerColor, contentLayoutData.shape)
-                .clip(contentLayoutData.shape)
-            ) {
-              MainToolbar(
-                state = mainToolbarState,
-                callback = toolbarCallback
-              )
-
-              Box(
-                modifier = Modifier.weight(1f)
-              ) {
-                when (val destination = mainNavigationState.currentListLocation) {
-                  MainNavigationListLocation.CHATS -> {
-                    val state = key(destination) { rememberFragmentState() }
-                    AndroidFragment(
-                      clazz = ConversationListFragment::class.java,
-                      fragmentState = state,
-                      modifier = Modifier.fillMaxSize()
-                    )
-                  }
-
-                  MainNavigationListLocation.ARCHIVE -> {
-                    val state = key(destination) { rememberFragmentState() }
-                    AndroidFragment(
-                      clazz = ConversationListArchiveFragment::class.java,
-                      fragmentState = state,
-                      modifier = Modifier.fillMaxSize()
-                    )
-                  }
-
-                  MainNavigationListLocation.CALLS -> {
-                    val state = key(destination) { rememberFragmentState() }
-                    AndroidFragment(
-                      clazz = CallLogFragment::class.java,
-                      fragmentState = state,
-                      modifier = Modifier.fillMaxSize()
-                    )
-                  }
-
-                  MainNavigationListLocation.STORIES -> {
-                    val state = key(destination) { rememberFragmentState() }
-                    AndroidFragment(
-                      clazz = StoriesLandingFragment::class.java,
-                      fragmentState = state,
-                      modifier = Modifier.fillMaxSize()
-                    )
-                  }
-                }
-
-                MainBottomChrome(
-                  state = mainBottomChromeState,
-                  callback = mainBottomChromeCallback,
-                  megaphoneActionController = megaphoneActionController,
-                  modifier = Modifier.align(Alignment.BottomCenter)
-                )
-              }
-            }
-          },
-          primaryContent = {
-            Box(
-              modifier = Modifier
-                .padding(end = contentLayoutData.detailPaddingEnd)
-                .clip(contentLayoutData.shape)
-                .background(color = MaterialTheme.colorScheme.surface)
-                .fillMaxSize()
-            ) {
-              when (mainNavigationState.currentListLocation) {
-                MainNavigationListLocation.CHATS, MainNavigationListLocation.ARCHIVE -> {
-                  NavDisplay<NavKey>(
-                    backStack = mainNavigationViewModel.chatsBackStackEntries,
-                    onBack = { mainNavigationViewModel.popChatsDetailLocation() },
-                    transitionSpec = { TransitionSpecs.HorizontalSlide.transitionSpec },
-                    popTransitionSpec = { TransitionSpecs.HorizontalSlide.popTransitionSpec },
-                    predictivePopTransitionSpec = { TransitionSpecs.HorizontalSlide.predictivePopTransitionSpec },
-                    entryProvider = entryProvider { chatsNavEntries(convoTransitionState) }
-                  )
-                }
-
-                MainNavigationListLocation.CALLS -> {
-                  NavDisplay<NavKey>(
-                    backStack = mainNavigationViewModel.callsBackStackEntries,
-                    onBack = { mainNavigationViewModel.popCallsDetailLocation() },
-                    transitionSpec = { TransitionSpecs.HorizontalSlide.transitionSpec },
-                    popTransitionSpec = { TransitionSpecs.HorizontalSlide.popTransitionSpec },
-                    predictivePopTransitionSpec = { TransitionSpecs.HorizontalSlide.predictivePopTransitionSpec },
-                    entryDecorators = listOf(
-                      rememberSaveableStateHolderNavEntryDecorator(),
-                      rememberViewModelStoreNavEntryDecorator()
-                    ),
-                    entryProvider = entryProvider { callsNavEntries(isSplitPane) }
-                  )
-                }
-
-                MainNavigationListLocation.STORIES -> {
-                  NavDisplay<NavKey>(
-                    backStack = mainNavigationViewModel.storiesBackStackEntries,
-                    onBack = { mainNavigationViewModel.popStoriesDetailLocation() },
-                    transitionSpec = { TransitionSpecs.HorizontalSlide.transitionSpec },
-                    popTransitionSpec = { TransitionSpecs.HorizontalSlide.popTransitionSpec },
-                    predictivePopTransitionSpec = { TransitionSpecs.HorizontalSlide.predictivePopTransitionSpec },
-                    entryDecorators = listOf(
-                      rememberSaveableStateHolderNavEntryDecorator(),
-                      rememberViewModelStoreNavEntryDecorator()
-                    ),
-                    entryProvider = entryProvider { storiesNavEntries() }
-                  )
-                }
-              }
-            }
-          },
-          paneExpansionDragHandle = if (contentLayoutData.hasDragHandle()) {
-            {
-              AppPaneDragHandle(
-                paneExpansionState = paneExpansionState,
-                mutableInteractionSource = mutableInteractionSource
-              )
-            }
-          } else {
-            null
-          },
-          animatorFactory = if (mainNavigationState.currentListLocation.isChatsTab) {
-            noEnterTransitionFactory
-          } else {
-            AppScaffoldAnimationStateFactory.Default
-          }
-        )
+          modifier = convoTransitionState.writeContentToGraphicsLayer()
+        ) { paddingValues ->
+          ListDetailNavDisplay(
+            entries = tabEntries,
+            isSplitPane = isSplitPane,
+            paneAnchor = paneAnchor,
+            onBack = { mainNavigationViewModel.popCurrentDetailLocation() },
+            onExitDetail = { mainNavigationViewModel.exitDetailLocation() },
+            layout = paneLayout,
+            listPaneChrome = listPaneChrome,
+            emptyDetailContent = emptyDetailContent,
+            modifier = Modifier.padding(paddingValues)
+          )
+        }
       }
     }
 
@@ -784,26 +526,118 @@ class MainActivity :
   }
 
   /**
-   * Creates and wraps a scaffold navigator such that we can use it to operate with both
-   * our split pane and legacy activities.
+   * Builds the geometry for the list/detail split and keeps it following the view-model's anchor.
    */
-  @OptIn(ExperimentalMaterial3AdaptiveApi::class)
   @Composable
-  private fun rememberNavigator(
-    isSplitPane: Boolean,
-    contentLayoutData: MainContentLayoutData,
-    maxWidth: Dp
-  ): AppScaffoldNavigator<Any> {
-    val scaffoldNavigator = rememberThreePaneScaffoldNavigatorDelegate(
-      isSplitPane = isSplitPane,
-      horizontalPartitionSpacerSize = contentLayoutData.partitionWidth,
-      defaultPanePreferredWidth = contentLayoutData.rememberDefaultPanePreferredWidth(maxWidth)
+  private fun rememberMainPaneLayout(
+    contentLayoutData: ListDetailPaneMetrics,
+    maxWidth: Dp,
+    toolbarMode: MainToolbarMode,
+    paneAnchor: PaneAnchor
+  ): ListDetailPaneLayout {
+    val navigationType = NavigationType.rememberNavigationType()
+
+    return rememberListDetailPaneLayout(
+      paneAnchor = paneAnchor,
+      maxWidth = maxWidth,
+      onAnchorSelected = { mainNavigationViewModel.onPaneAnchorSelected(it) },
+      metrics = contentLayoutData,
+      // Searching hides the rail, leaving nothing of the list pane behind once the detail fills the window.
+      collapsedListWidth = when {
+        toolbarMode == MainToolbarMode.SEARCH -> 0.dp
+        navigationType == NavigationType.BAR -> 0.dp
+        else -> RAIL_WIDTH
+      }
     )
+  }
 
-    val coroutine = rememberCoroutineScope()
+  /**
+   * The chrome belonging to the list pane — navigation rail or bar, toolbar, and the floating buttons and
+   * megaphones layered over the list — wrapped around [content].
+   *
+   * Handed to [ListDetailNavDisplay] as a [ListPaneChrome] rather than as a scene parameter: a scene excludes
+   * its content lambda from equality, so anything captured there would go stale when an equal instance is
+   * retained.
+   */
+  @Composable
+  private fun MainListPaneChrome(
+    modifier: Modifier = Modifier,
+    content: @Composable () -> Unit
+  ) {
+    val mainToolbarState by toolbarViewModel.state.collectAsStateWithLifecycle()
+    val mainNavigationState by mainNavigationViewModel.mainNavigationBarState.collectAsStateWithLifecycle()
+    val megaphone by mainNavigationViewModel.megaphone.collectAsStateWithLifecycle()
 
-    return remember(scaffoldNavigator, coroutine) {
-      mainNavigationViewModel.wrapNavigator(coroutine, scaffoldNavigator)
+    val isSplitPane = LocalResources.current.rememberIsSplitPane()
+    val contentLayoutData = rememberListDetailPaneMetrics(listPaddingStart = mainToolbarState.mode.listPaddingStart)
+    val navigationType = NavigationType.rememberNavigationType()
+
+    val bottomChromeState = remember(mainToolbarState.destination, mainToolbarState.mode, megaphone) {
+      MainBottomChromeState(
+        destination = mainToolbarState.destination,
+        mainToolbarMode = mainToolbarState.mode,
+        megaphoneState = MainMegaphoneState(
+          megaphone = megaphone,
+          mainToolbarMode = mainToolbarState.mode
+        )
+      )
+    }
+
+    val listContainerColor = if (isSplitPane) {
+      SignalTheme.colors.colorSurface1
+    } else {
+      MaterialTheme.colorScheme.surface
+    }
+
+    Row(modifier = modifier.fillMaxSize()) {
+      if (navigationType == NavigationType.RAIL && mainToolbarState.mode != MainToolbarMode.SEARCH) {
+        MainNavigationRail(
+          state = mainNavigationState,
+          mainFloatingActionButtonsCallback = mainBottomChromeCallback,
+          onDestinationSelected = mainNavigationCallback
+        )
+      }
+
+      Column(
+        modifier = Modifier
+          .weight(1f)
+          .fillMaxSize()
+          .background(listContainerColor, contentLayoutData.shape)
+          .clip(contentLayoutData.shape)
+      ) {
+        MainToolbar(
+          state = mainToolbarState,
+          callback = toolbarCallback
+        )
+
+        Box(modifier = Modifier.weight(1f)) {
+          content()
+
+          MainBottomChrome(
+            state = bottomChromeState,
+            callback = mainBottomChromeCallback,
+            megaphoneActionController = megaphoneActionController,
+            modifier = Modifier.align(Alignment.BottomCenter)
+          )
+        }
+
+        if (navigationType == NavigationType.BAR && mainToolbarState.mode == MainToolbarMode.FULL) {
+          Column(
+            modifier = Modifier
+              .clip(contentLayoutData.navigationBarShape)
+              .background(color = SignalTheme.colors.colorSurface2)
+          ) {
+            MainNavigationBar(
+              state = mainNavigationState,
+              onDestinationSelected = mainNavigationCallback
+            )
+
+            if (!isSplitPane) {
+              Spacer(Modifier.navigationBarsPadding())
+            }
+          }
+        }
+      }
     }
   }
 
@@ -855,19 +689,24 @@ class MainActivity :
 
     val extras = intent.extras ?: return
 
-    val detailLocation = extras.getParcelableCompat(KEY_DETAIL_LOCATION, MainNavigationDetailLocation::class.java)
+    if (extras.getBoolean(KEY_EXIT_DETAIL, false)) {
+      mainNavigationViewModel.exitDetailLocation()
+      return
+    }
+
+    val detailLocation = extras.getParcelableCompat(KEY_DETAIL_LOCATION, MainDetailRoute::class.java)
     if (detailLocation != null) {
       goTo(detailLocation)
       return
     }
 
-    val startingTab = extras.getSerializableCompat(KEY_STARTING_TAB, MainNavigationListLocation::class.java)
+    val startingTab = extras.getSerializableCompat(KEY_STARTING_TAB, MainListRoute::class.java)
 
     when (startingTab) {
-      MainNavigationListLocation.CHATS -> mainNavigationViewModel.onChatsSelected()
-      MainNavigationListLocation.ARCHIVE -> mainNavigationViewModel.onArchiveSelected()
-      MainNavigationListLocation.CALLS -> mainNavigationViewModel.onCallsSelected()
-      MainNavigationListLocation.STORIES -> {
+      MainListRoute.Chats -> mainNavigationViewModel.onChatsSelected()
+      MainListRoute.Archive -> mainNavigationViewModel.onArchiveSelected()
+      MainListRoute.Calls -> mainNavigationViewModel.onCallsSelected()
+      MainListRoute.Stories -> {
         if (Stories.isFeatureEnabled()) {
           mainNavigationViewModel.onStoriesSelected()
         }
@@ -1057,8 +896,8 @@ class MainActivity :
         return
       }
 
-      mainNavigationViewModel.goTo(MainNavigationListLocation.CHATS)
-      mainNavigationViewModel.goTo(MainNavigationDetailLocation.Conversation(ConversationIntents.readArgsFromBundle(extras)))
+      mainNavigationViewModel.goTo(MainListRoute.Chats)
+      mainNavigationViewModel.goTo(MainDetailRoute.Conversation(ConversationIntents.readArgsFromBundle(extras)))
       intent.action = null
       setIntent(intent)
     }
@@ -1109,7 +948,7 @@ class MainActivity :
   private fun handleQuickRestoreIntent(intent: Intent) {
     intent.data?.let { data ->
       CommunicationActions.handlePotentialQuickRestoreUrl(this, data.toString()) {
-        onCameraClick(MainNavigationListLocation.CHATS, isForQuickRestore = true)
+        onCameraClick(MainListRoute.Chats, isForQuickRestore = true)
       }
     }
   }
@@ -1152,7 +991,7 @@ class MainActivity :
     }
   }
 
-  private fun onCameraClick(destination: MainNavigationListLocation, isForQuickRestore: Boolean) {
+  private fun onCameraClick(destination: MainListRoute, isForQuickRestore: Boolean) {
     val onGranted = {
       if (isForQuickRestore) {
         startActivity(MediaSendLauncher.cameraForQuickRestore(context = this@MainActivity))
@@ -1160,7 +999,7 @@ class MainActivity :
         startActivity(
           MediaSendLauncher.camera(
             context = this@MainActivity,
-            isStory = destination == MainNavigationListLocation.STORIES
+            isStory = destination == MainListRoute.Stories
           )
         )
       }
@@ -1230,11 +1069,11 @@ class MainActivity :
     }
 
     override fun onStoryPrivacyClick() {
-      mainNavigationViewModel.goTo(MainNavigationDetailLocation.Stories.PrivacySettings)
+      mainNavigationViewModel.goTo(MainDetailRoute.Stories.PrivacySettings)
     }
 
     override fun onStoryArchiveClick() {
-      mainNavigationViewModel.goTo(MainNavigationDetailLocation.Stories.Archive)
+      mainNavigationViewModel.goTo(MainDetailRoute.Stories.Archive)
     }
 
     override fun onCloseSearchClick() {
@@ -1281,7 +1120,7 @@ class MainActivity :
       startActivity(NewCallActivity.createIntent(this@MainActivity))
     }
 
-    override fun onCameraClick(destination: MainNavigationListLocation) {
+    override fun onCameraClick(destination: MainListRoute) {
       onCameraClick(destination, false)
     }
 
@@ -1327,17 +1166,18 @@ class MainActivity :
     }
   }
 
-  private inner class MainNavigationCallback : (MainNavigationListLocation) -> Unit {
-    override fun invoke(location: MainNavigationListLocation) {
+  private inner class MainNavigationCallback : (MainListRoute) -> Unit {
+    override fun invoke(location: MainListRoute) {
       when (location) {
-        MainNavigationListLocation.CHATS -> mainNavigationViewModel.onChatsSelected()
-        MainNavigationListLocation.CALLS -> mainNavigationViewModel.onCallsSelected()
-        MainNavigationListLocation.STORIES -> mainNavigationViewModel.onStoriesSelected()
-        MainNavigationListLocation.ARCHIVE -> mainNavigationViewModel.onArchiveSelected()
+        MainListRoute.Chats -> mainNavigationViewModel.onChatsSelected()
+        MainListRoute.Calls -> mainNavigationViewModel.onCallsSelected()
+        MainListRoute.Stories -> mainNavigationViewModel.onStoriesSelected()
+        MainListRoute.Archive -> mainNavigationViewModel.onArchiveSelected()
       }
     }
   }
 
-  override fun goTo(location: MainNavigationListLocation) = mainNavigationViewModel.goTo(location)
-  override fun goTo(location: MainNavigationDetailLocation) = mainNavigationViewModel.goTo(location)
+  override fun goTo(location: MainListRoute) = mainNavigationViewModel.goTo(location)
+  override fun goTo(location: MainDetailRoute) = mainNavigationViewModel.goTo(location)
+  override fun exitDetailLocation() = mainNavigationViewModel.exitDetailLocation()
 }

app/src/main/java/org/thoughtcrime/securesms/MainNavigator.java

@@ -10,7 +10,7 @@ import org.signal.core.util.concurrent.LifecycleDisposable;
 import org.thoughtcrime.securesms.components.settings.app.AppSettingsActivity;
 import org.thoughtcrime.securesms.conversation.ConversationIntents;
 import org.thoughtcrime.securesms.groups.ui.creategroup.CreateGroupActivity;
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation;
+import org.thoughtcrime.securesms.main.MainDetailRoute;
 import org.thoughtcrime.securesms.main.MainNavigationViewModel;
 import org.thoughtcrime.securesms.recipients.RecipientId;
 
@@ -50,7 +50,7 @@ public class MainNavigator {
                                                                       .withStartingPosition(startingPosition)
                                                                       .asIncognito(incognito)
                                                                       .toConversationArgs())
-                                               .subscribe(args -> viewModel.goTo(new MainNavigationDetailLocation.Conversation(args)));
+                                               .subscribe(args -> viewModel.goTo(new MainDetailRoute.Conversation(args)));
 
     lifecycleDisposable.add(disposable);
   }

app/src/main/java/org/thoughtcrime/securesms/calls/CallsBackStack.kt

@@ -1,64 +0,0 @@
-/*
- * Copyright 2026 Signal Messenger, LLC
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-package org.thoughtcrime.securesms.calls
-
-import androidx.compose.runtime.mutableStateListOf
-import androidx.compose.runtime.saveable.Saver
-import androidx.compose.runtime.snapshots.SnapshotStateList
-import androidx.lifecycle.SavedStateHandle
-import androidx.lifecycle.viewmodel.compose.SavedStateHandleSaveableApi
-import androidx.lifecycle.viewmodel.compose.saveable
-import org.thoughtcrime.securesms.calls.log.CallLogRow
-import org.thoughtcrime.securesms.main.MainDetailBackStack
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-
-/**
- * Controls the navigation stack used by the calls screen.
- */
-@OptIn(SavedStateHandleSaveableApi::class)
-class CallsBackStack(savedStateHandle: SavedStateHandle) : MainDetailBackStack {
-
-  companion object {
-    private const val KEY = "calls_back_stack"
-
-    val saver: Saver<SnapshotStateList<MainNavigationDetailLocation>, ArrayList<MainNavigationDetailLocation>> = Saver(
-      save = { ArrayList(it) },
-      restore = { mutableStateListOf(*it.toTypedArray()) }
-    )
-  }
-
-  override val entries: SnapshotStateList<MainNavigationDetailLocation> = savedStateHandle.saveable(
-    key = KEY,
-    saver = saver
-  ) {
-    mutableStateListOf(MainNavigationDetailLocation.Empty)
-  }
-
-  val activeCallId: CallLogRow.Id?
-    get() = entries.asReversed().firstNotNullOfOrNull { location ->
-      when (location) {
-        is MainNavigationDetailLocation.Calls -> location.controllerKey
-        is MainNavigationDetailLocation.CallLinkDetails -> location.controllerKey
-        else -> null
-      }
-    }
-
-  /**
-   * Pushes an entry onto the stack.
-   */
-  override fun push(location: MainNavigationDetailLocation) {
-    when {
-      location is MainNavigationDetailLocation.Empty || location == entries.lastOrNull() -> Unit
-
-      location.isContentRoot -> {
-        entries.removeAll { it !is MainNavigationDetailLocation.Empty }
-        entries.add(location)
-      }
-
-      else -> entries.add(location)
-    }
-  }
-}

app/src/main/java/org/thoughtcrime/securesms/calls/CallsNavigation.kt

@@ -8,45 +8,55 @@ package org.thoughtcrime.securesms.calls
 import androidx.activity.compose.LocalActivity
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.ui.Modifier
+import androidx.fragment.compose.AndroidFragment
+import androidx.fragment.compose.rememberFragmentState
 import androidx.navigation3.runtime.EntryProviderScope
 import androidx.navigation3.runtime.NavKey
+import org.signal.core.ui.compose.split.detailEntry
 import org.signal.core.ui.navigation.TransitionSpecs
 import org.thoughtcrime.securesms.MainNavigator
 import org.thoughtcrime.securesms.calls.links.EditCallLinkNameScreen
 import org.thoughtcrime.securesms.calls.links.details.CallLinkDetailsScreen
-import org.thoughtcrime.securesms.main.EmptyDetailScreen
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
+import org.thoughtcrime.securesms.calls.log.CallLogFragment
+import org.thoughtcrime.securesms.main.MainDetailRoute
 
-fun EntryProviderScope<NavKey>.callsNavEntries(isSplitPane: Boolean) {
-  entry<MainNavigationDetailLocation.Empty> {
-    NoCallSelectedEntry()
-  }
-
-  entry<MainNavigationDetailLocation.CallLinkDetails>(
+/**
+ * Registers the routes utilized for the main screen calls tab.
+ */
+fun EntryProviderScope<NavKey>.registerCallsTabDetailRoutes(isSplitPane: Boolean) {
+  detailEntry<MainDetailRoute.CallLinkDetails>(
     metadata = if (isSplitPane) TransitionSpecs.None.metadata else emptyMap()
   ) { route ->
     CallLinkDetailsEntry(route)
   }
 
-  entry<MainNavigationDetailLocation.Calls.CallLinks.EditCallLinkName> { route ->
+  detailEntry<MainDetailRoute.Calls.CallLinks.EditCallLinkName> { route ->
     EditCallLinkNameEntry(route)
   }
 }
 
+/**
+ * List pane content for the calls tab.
+ */
 @Composable
-private fun NoCallSelectedEntry() {
-  EmptyDetailScreen()
+fun CallsListPane(modifier: Modifier = Modifier) {
+  AndroidFragment(
+    clazz = CallLogFragment::class.java,
+    fragmentState = rememberFragmentState(),
+    modifier = modifier
+  )
 }
 
 @Composable
-private fun CallLinkDetailsEntry(route: MainNavigationDetailLocation.CallLinkDetails) {
+private fun CallLinkDetailsEntry(route: MainDetailRoute.CallLinkDetails) {
   informNavigatorWeAreReady()
 
   CallLinkDetailsScreen(roomId = route.callLinkRoomId)
 }
 
 @Composable
-private fun EditCallLinkNameEntry(route: MainNavigationDetailLocation.Calls.CallLinks.EditCallLinkName) {
+private fun EditCallLinkNameEntry(route: MainDetailRoute.Calls.CallLinks.EditCallLinkName) {
   informNavigatorWeAreReady()
 
   EditCallLinkNameScreen(

app/src/main/java/org/thoughtcrime/securesms/calls/links/details/CallLinkDetailsScreen.kt

@@ -46,8 +46,8 @@ import org.thoughtcrime.securesms.calls.YouAreAlreadyInACallSnackbar.YouAreAlrea
 import org.thoughtcrime.securesms.calls.links.CallLinks
 import org.thoughtcrime.securesms.calls.links.SignalCallRow
 import org.thoughtcrime.securesms.database.CallLinkTable
+import org.thoughtcrime.securesms.main.MainDetailRoute
 import org.thoughtcrime.securesms.main.MainNavigationCallDetailRouter
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
 import org.thoughtcrime.securesms.main.MainNavigationViewModel
 import org.thoughtcrime.securesms.recipients.RecipientId
 import org.thoughtcrime.securesms.service.webrtc.links.CallLinkCredentials
@@ -114,7 +114,7 @@ class DefaultCallLinkDetailsCallback(
 
   override fun onEditNameClicked() {
     router.goToCallDetail(
-      MainNavigationDetailLocation.Calls.CallLinks.EditCallLinkName(
+      MainDetailRoute.Calls.CallLinks.EditCallLinkName(
         callLinkRoomId = viewModel.recipientSnapshot!!.requireCallLinkRoomId(),
         currentName = viewModel.nameSnapshot
       )

app/src/main/java/org/thoughtcrime/securesms/calls/log/CallLogFragment.kt

@@ -49,8 +49,8 @@ import org.thoughtcrime.securesms.conversationlist.chatfilter.FilterLerp
 import org.thoughtcrime.securesms.conversationlist.chatfilter.FilterPullState
 import org.thoughtcrime.securesms.databinding.CallLogFragmentBinding
 import org.thoughtcrime.securesms.dependencies.AppDependencies
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-import org.thoughtcrime.securesms.main.MainNavigationListLocation
+import org.thoughtcrime.securesms.main.MainDetailRoute
+import org.thoughtcrime.securesms.main.MainListRoute
 import org.thoughtcrime.securesms.main.MainNavigationViewModel
 import org.thoughtcrime.securesms.main.MainSnackbarHostKey
 import org.thoughtcrime.securesms.main.MainToolbarMode
@@ -210,7 +210,7 @@ class CallLogFragment : Fragment(R.layout.call_log_fragment), CallLogAdapter.Cal
 
   private fun initializeTapToScrollToTop(scrollToPositionDelegate: ScrollToPositionDelegate) {
     disposables += mainNavigationViewModel.tabClickEventsObservable
-      .filter { it == MainNavigationListLocation.CALLS }
+      .filter { it == MainListRoute.Calls }
       .subscribeBy(onNext = {
         scrollToPositionDelegate.resetScrollPosition()
       })
@@ -335,7 +335,7 @@ class CallLogFragment : Fragment(R.layout.call_log_fragment), CallLogAdapter.Cal
     if (viewModel.selectionStateSnapshot.isNotEmpty(binding.recycler.adapter!!.itemCount)) {
       viewModel.toggleSelected(callLogRow.id)
     } else {
-      mainNavigationViewModel.goTo(MainNavigationDetailLocation.CallLinkDetails(callLogRow.record.roomId))
+      mainNavigationViewModel.goTo(MainDetailRoute.CallLinkDetails(callLogRow.record.roomId))
     }
   }
 
@@ -390,7 +390,7 @@ class CallLogFragment : Fragment(R.layout.call_log_fragment), CallLogAdapter.Cal
   }
 
   override fun goToCallLinkDetails(roomId: CallLinkRoomId) {
-    mainNavigationViewModel.goTo(MainNavigationDetailLocation.CallLinkDetails(roomId))
+    mainNavigationViewModel.goTo(MainDetailRoute.CallLinkDetails(roomId))
   }
 
   override fun deleteCall(call: CallLogRow) {

app/src/main/java/org/thoughtcrime/securesms/calls/new/NewCallActivity.kt

@@ -137,7 +137,6 @@ private fun NewCallScreenUi(
 
   RecipientPickerScaffold(
     title = stringResource(R.string.NewCallActivity__new_call),
-    forceSplitPane = uiState.forceSplitPane,
     onNavigateUpClick = callbacks::onBackPressed,
     topAppBarActions = { TopAppBarActions(callbacks) },
     snackbarHostState = snackbarHostState,
@@ -245,9 +244,7 @@ private fun UserMessagesHost(
 private fun NewCallScreenPreview() {
   Previews.Preview {
     NewCallScreenUi(
-      uiState = NewCallUiState(
-        forceSplitPane = false
-      ),
+      uiState = NewCallUiState(),
       callbacks = NewCallUiCallbacks.Empty
     )
   }

app/src/main/java/org/thoughtcrime/securesms/calls/new/NewCallViewModel.kt

@@ -151,7 +151,6 @@ class NewCallViewModel : ViewModel() {
 }
 
 data class NewCallUiState(
-  val forceSplitPane: Boolean = SignalStore.internal.forceSplitPane,
   val searchQuery: String = "",
   val isLookingUpRecipient: Boolean = false,
   val isRefreshingContacts: Boolean = false,

app/src/main/java/org/thoughtcrime/securesms/chats/ChatsBackStack.kt

@@ -1,64 +0,0 @@
-/*
- * Copyright 2026 Signal Messenger, LLC
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-package org.thoughtcrime.securesms.chats
-
-import androidx.compose.runtime.mutableStateListOf
-import androidx.compose.runtime.saveable.Saver
-import androidx.compose.runtime.snapshots.SnapshotStateList
-import androidx.lifecycle.SavedStateHandle
-import androidx.lifecycle.viewmodel.compose.SavedStateHandleSaveableApi
-import androidx.lifecycle.viewmodel.compose.saveable
-import org.thoughtcrime.securesms.main.MainDetailBackStack
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-import org.thoughtcrime.securesms.recipients.RecipientId
-
-/**
- * Controls the navigation stack used by the chats screen.
- */
-@OptIn(SavedStateHandleSaveableApi::class)
-class ChatsBackStack(savedStateHandle: SavedStateHandle) : MainDetailBackStack {
-
-  companion object {
-    private const val KEY = "chats_back_stack"
-
-    val saver: Saver<SnapshotStateList<MainNavigationDetailLocation>, ArrayList<MainNavigationDetailLocation>> = Saver(
-      save = { ArrayList(it) },
-      restore = { mutableStateListOf(*it.toTypedArray()) }
-    )
-  }
-
-  override val entries: SnapshotStateList<MainNavigationDetailLocation> = savedStateHandle.saveable(
-    key = KEY,
-    saver = saver
-  ) {
-    mutableStateListOf(MainNavigationDetailLocation.Empty)
-  }
-
-  val activeRecipientId: RecipientId?
-    get() = entries.asReversed().firstNotNullOfOrNull {
-      when (it) {
-        is MainNavigationDetailLocation.Conversation -> it.conversationArgs.recipientId
-        is MainNavigationDetailLocation.Chats -> it.controllerKey
-        else -> null
-      }
-    }
-
-  /**
-   * Pushes an entry onto the stack.
-   */
-  override fun push(location: MainNavigationDetailLocation) {
-    when (location) {
-      is MainNavigationDetailLocation.Empty, entries.lastOrNull() -> Unit
-
-      is MainNavigationDetailLocation.Conversation -> {
-        entries.removeAll { it !is MainNavigationDetailLocation.Empty }
-        entries.add(location)
-      }
-
-      else -> entries.add(location)
-    }
-  }
-}

app/src/main/java/org/thoughtcrime/securesms/chats/ChatsNavigation.kt

@@ -27,6 +27,7 @@ import androidx.navigation3.runtime.NavKey
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.launch
+import org.signal.core.ui.compose.split.detailEntry
 import org.signal.core.ui.navigation.TransitionSpecs
 import org.thoughtcrime.securesms.MainNavigator
 import org.thoughtcrime.securesms.components.settings.conversation.ConversationSettingsNavHostFragment
@@ -35,43 +36,62 @@ import org.thoughtcrime.securesms.compose.FragmentBackHandler
 import org.thoughtcrime.securesms.compose.FragmentBackPressedState
 import org.thoughtcrime.securesms.conversation.ConversationIntents
 import org.thoughtcrime.securesms.conversation.v2.ConversationFragment
-import org.thoughtcrime.securesms.main.EmptyDetailScreen
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
+import org.thoughtcrime.securesms.conversationlist.ConversationListArchiveFragment
+import org.thoughtcrime.securesms.conversationlist.ConversationListFragment
+import org.thoughtcrime.securesms.main.MainDetailRoute
 import org.thoughtcrime.securesms.messagedetails.MessageDetailsFragment
 
-fun EntryProviderScope<NavKey>.chatsNavEntries(
+/**
+ * Registers the routes available in the chats tab of the main screen.
+ */
+fun EntryProviderScope<NavKey>.registerChatsTabDetailRoutes(
   transitionState: ConversationTransitionState
 ) {
-  entry<MainNavigationDetailLocation.Empty> {
-    NoConvoSelectedEntry()
-  }
-
-  entry<MainNavigationDetailLocation.Conversation>(
-    // disable slide animation - it's unnecessary in split pane mode and is handled by ConversationLoadingMask for single pane mode.
-    metadata = TransitionSpecs.None.metadata
+  detailEntry<MainDetailRoute.Conversation>(
+    // Since we can't do delayed entry transitions yet, we disable this one and fake our own with the transition state.
+    metadata = TransitionSpecs.suppressEnterMetadata
   ) { route ->
     ConversationEntry(route, transitionState)
   }
 
-  entry<MainNavigationDetailLocation.Chats.MessageDetails> { route ->
+  detailEntry<MainDetailRoute.Chats.MessageDetails> { route ->
     MessageDetailsEntry(route)
   }
 
-  entry<MainNavigationDetailLocation.Chats.ConversationSettings>(
+  detailEntry<MainDetailRoute.Chats.ConversationSettings>(
     metadata = TransitionSpecs.FadeScale.metadata
   ) { route ->
     ConversationSettingsEntry(route)
   }
 }
 
+/**
+ * List pane content for the chats tab.
+ */
 @Composable
-private fun NoConvoSelectedEntry() {
-  EmptyDetailScreen()
+fun ChatsListPane(modifier: Modifier = Modifier) {
+  AndroidFragment(
+    clazz = ConversationListFragment::class.java,
+    fragmentState = rememberFragmentState(),
+    modifier = modifier
+  )
+}
+
+/**
+ * List pane content for the archive, which is displayed within the chats tab.
+ */
+@Composable
+fun ArchiveListPane(modifier: Modifier = Modifier) {
+  AndroidFragment(
+    clazz = ConversationListArchiveFragment::class.java,
+    fragmentState = rememberFragmentState(),
+    modifier = modifier
+  )
 }
 
 @Composable
 private fun ConversationEntry(
-  route: MainNavigationDetailLocation.Conversation,
+  route: MainDetailRoute.Conversation,
   transitionState: ConversationTransitionState
 ) {
   val context = LocalContext.current
@@ -110,7 +130,7 @@ private fun ConversationEntry(
 }
 
 @Composable
-private fun MessageDetailsEntry(route: MainNavigationDetailLocation.Chats.MessageDetails) {
+private fun MessageDetailsEntry(route: MainDetailRoute.Chats.MessageDetails) {
   val navigatorProvider = LocalContext.current as? MainNavigator.NavigatorProvider
   val fragmentState = key(route) { rememberFragmentState() }
 
@@ -129,7 +149,7 @@ private fun MessageDetailsEntry(route: MainNavigationDetailLocation.Chats.Messag
 }
 
 @Composable
-private fun ConversationSettingsEntry(route: MainNavigationDetailLocation.Chats.ConversationSettings) {
+private fun ConversationSettingsEntry(route: MainDetailRoute.Chats.ConversationSettings) {
   val navigatorProvider = LocalContext.current as? MainNavigator.NavigatorProvider
   val fragmentState = key(route) { rememberFragmentState() }
   val arguments: Bundle? by produceState(null, route.recipientId) {

app/src/main/java/org/thoughtcrime/securesms/components/settings/app/internal/InternalSettingsFragment.kt

@@ -324,23 +324,6 @@ class InternalSettingsFragment : DSLSettingsFragment(R.string.preferences__inter
 
       sectionHeaderPref(DSLSettingsText.from("App UI"))
 
-      switchPref(
-        title = DSLSettingsText.from("Force split pane UI on phones."),
-        isEnabled = !state.forceSinglePane,
-        isChecked = state.forceSplitPane,
-        onClick = {
-          viewModel.setForceSplitPane(!state.forceSplitPane)
-        }
-      )
-
-      switchPref(
-        title = DSLSettingsText.from("Force single-pane on newer devices."),
-        isChecked = state.forceSinglePane,
-        onClick = {
-          viewModel.setForceSinglePane(!state.forceSinglePane)
-        }
-      )
-
       clickPref(
         title = DSLSettingsText.from("Display enable permission sheet"),
         onClick = {

app/src/main/java/org/thoughtcrime/securesms/components/settings/app/internal/InternalSettingsState.kt

@@ -35,8 +35,6 @@ data class InternalSettingsState(
   val pnpInitialized: Boolean,
   val useConversationItemV2ForMedia: Boolean,
   val hasPendingOneTimeDonation: Boolean,
-  val forceSplitPane: Boolean,
-  val forceSinglePane: Boolean,
   val disableInternalUser: Boolean,
   val searchQuery: String = ""
 )

app/src/main/java/org/thoughtcrime/securesms/components/settings/app/internal/InternalSettingsViewModel.kt

@@ -274,8 +274,6 @@ class InternalSettingsViewModel(private val repository: InternalSettingsReposito
     pnpInitialized = SignalStore.misc.hasPniInitializedDevices,
     useConversationItemV2ForMedia = SignalStore.internal.useConversationItemV2Media,
     hasPendingOneTimeDonation = SignalStore.inAppPayments.getPendingOneTimeDonation() != null,
-    forceSplitPane = SignalStore.internal.forceSplitPane,
-    forceSinglePane = SignalStore.internal.forceSinglePane,
     disableInternalUser = RemoteConfig.internalUserDisabled
   )
 
@@ -292,16 +290,6 @@ class InternalSettingsViewModel(private val repository: InternalSettingsReposito
     refresh()
   }
 
-  fun setForceSplitPane(forceSplitPane: Boolean) {
-    SignalStore.internal.forceSplitPane = forceSplitPane
-    refresh()
-  }
-
-  fun setForceSinglePane(forceSinglePane: Boolean) {
-    SignalStore.internal.forceSinglePane = forceSinglePane
-    refresh()
-  }
-
   private fun buildSearchGroups(items: MappingModelList): List<SearchGroup> {
     val groups = mutableListOf<SearchGroup>()
     var divider: DividerPreference? = null

app/src/main/java/org/thoughtcrime/securesms/components/settings/conversation/ConversationSettingsFragment.kt

@@ -65,7 +65,6 @@ import org.thoughtcrime.securesms.groups.ui.managegroup.dialogs.GroupInviteSentD
 import org.thoughtcrime.securesms.groups.ui.managegroup.dialogs.GroupsLearnMoreBottomSheetDialogFragment
 import org.thoughtcrime.securesms.jobs.AttachmentDownloadJob
 import org.thoughtcrime.securesms.main.MainNavigationChatDetailRouter
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
 import org.thoughtcrime.securesms.mediaoverview.MediaOverviewActivity
 import org.thoughtcrime.securesms.mediapreview.MediaIntentFactory
 import org.thoughtcrime.securesms.mediapreview.MediaPreviewCache
@@ -562,7 +561,7 @@ class ConversationSettingsFragment : ComposeFragment() {
     if (chatRouter != null) {
       chatRouter?.exitDetailLocation()
     } else {
-      startActivity(MainActivity.clearTopAndOpenDetail(requireContext(), MainNavigationDetailLocation.Empty))
+      startActivity(MainActivity.clearTopAndExitDetail(requireContext()))
     }
   }
 

app/src/main/java/org/thoughtcrime/securesms/components/settings/conversation/ConversationSettingsNavigator.kt

@@ -6,8 +6,8 @@
 package org.thoughtcrime.securesms.components.settings.conversation
 
 import androidx.fragment.app.FragmentActivity
+import org.thoughtcrime.securesms.main.MainDetailRoute
 import org.thoughtcrime.securesms.main.MainNavigationChatDetailRouter
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
 import org.thoughtcrime.securesms.recipients.Recipient
 
 /**
@@ -20,7 +20,7 @@ object ConversationSettingsNavigator {
     recipient: Recipient
   ) {
     if (activity is MainNavigationChatDetailRouter) {
-      activity.goToChatDetail(MainNavigationDetailLocation.Chats.ConversationSettings(recipient.id))
+      activity.goToChatDetail(MainDetailRoute.Chats.ConversationSettings(recipient.id))
       return
     }
 

app/src/main/java/org/thoughtcrime/securesms/conversation/NewConversationActivity.kt

@@ -195,7 +195,6 @@ private fun NewConversationScreenUi(
 
   RecipientPickerScaffold(
     title = stringResource(R.string.NewConversationActivity__new_message),
-    forceSplitPane = uiState.forceSplitPaneOnCompactLandscape,
     onNavigateUpClick = callbacks::onBackPressed,
     topAppBarActions = { TopAppBarActions(callbacks) },
     snackbarHostState = snackbarHostState,
@@ -405,9 +404,7 @@ private fun UserMessagesHost(
 private fun NewConversationScreenPreview() {
   Previews.Preview {
     NewConversationScreenUi(
-      uiState = NewConversationUiState(
-        forceSplitPaneOnCompactLandscape = false
-      ),
+      uiState = NewConversationUiState(),
       callbacks = NewConversationUiCallbacks.Empty
     )
   }

app/src/main/java/org/thoughtcrime/securesms/conversation/NewConversationViewModel.kt

@@ -179,7 +179,6 @@ class NewConversationViewModel : ViewModel() {
 }
 
 data class NewConversationUiState(
-  val forceSplitPaneOnCompactLandscape: Boolean = SignalStore.internal.forceSplitPane,
   val searchQuery: String = "",
   val isLookingUpRecipient: Boolean = false,
   val isRefreshingContacts: Boolean = false,

app/src/main/java/org/thoughtcrime/securesms/conversation/v2/ConversationActivity.kt

@@ -27,8 +27,8 @@ import org.thoughtcrime.securesms.components.voice.VoiceNoteMediaController
 import org.thoughtcrime.securesms.components.voice.VoiceNoteMediaControllerOwner
 import org.thoughtcrime.securesms.conversation.ConversationIntents
 import org.thoughtcrime.securesms.jobs.ConversationShortcutUpdateJob
+import org.thoughtcrime.securesms.main.MainDetailRoute
 import org.thoughtcrime.securesms.main.MainNavigationChatDetailRouter
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
 import org.thoughtcrime.securesms.messagedetails.MessageDetailsFragment
 import org.thoughtcrime.securesms.util.DynamicNoActionBarTheme
 import java.util.concurrent.TimeUnit
@@ -147,9 +147,9 @@ open class ConversationActivity : PassphraseRequiredActivity(), VoiceNoteMediaCo
     }
   }
 
-  override fun goToChatDetail(location: MainNavigationDetailLocation.Chats) {
+  override fun goToChatDetail(location: MainDetailRoute.Chats) {
     when (location) {
-      is MainNavigationDetailLocation.Chats.ConversationSettings -> {
+      is MainDetailRoute.Chats.ConversationSettings -> {
         lifecycleScope.launch {
           val args = ConversationSettingsNavHostFragment.createArgs(location.recipientId)
           supportFragmentManager
@@ -161,7 +161,7 @@ open class ConversationActivity : PassphraseRequiredActivity(), VoiceNoteMediaCo
         }
       }
 
-      is MainNavigationDetailLocation.Chats.MessageDetails -> {
+      is MainDetailRoute.Chats.MessageDetails -> {
         MessageDetailsFragment.create(location.messageId, location.recipientId)
           .show(supportFragmentManager, MESSAGE_DETAILS_FRAGMENT_TAG)
       }

app/src/main/java/org/thoughtcrime/securesms/conversation/v2/ConversationFragment.kt

@@ -296,9 +296,9 @@ import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.linkpreview.LinkPreview
 import org.thoughtcrime.securesms.linkpreview.LinkPreviewViewModelV2
 import org.thoughtcrime.securesms.longmessage.LongMessageFragment
+import org.thoughtcrime.securesms.main.MainDetailRoute
+import org.thoughtcrime.securesms.main.MainListRoute
 import org.thoughtcrime.securesms.main.MainNavigationChatDetailRouter
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-import org.thoughtcrime.securesms.main.MainNavigationListLocation
 import org.thoughtcrime.securesms.main.MainNavigationViewModel
 import org.thoughtcrime.securesms.main.MainSnackbarHostKey
 import org.thoughtcrime.securesms.mediaoverview.MediaOverviewActivity
@@ -3151,7 +3151,7 @@ class ConversationFragment :
 
   private fun handleDisplayDetails(conversationMessage: ConversationMessage) {
     val recipientSnapshot = viewModel.recipientSnapshot ?: return
-    chatRouter.goToChatDetail(MainNavigationDetailLocation.Chats.MessageDetails(recipientSnapshot.id, MessageId(conversationMessage.messageRecord.id)))
+    chatRouter.goToChatDetail(MainDetailRoute.Chats.MessageDetails(recipientSnapshot.id, MessageId(conversationMessage.messageRecord.id)))
   }
 
   private fun handleDeleteMessages(messageParts: Set<MultiselectPart>) {
@@ -3707,7 +3707,7 @@ class ConversationFragment :
       } else if (messageRecord.hasFailedWithNetworkFailures()) {
         ConversationDialogs.displayMessageCouldNotBeSentDialog(requireContext(), messageRecord)
       } else {
-        chatRouter.goToChatDetail(MainNavigationDetailLocation.Chats.MessageDetails(recipientId, MessageId(messageRecord.id)))
+        chatRouter.goToChatDetail(MainDetailRoute.Chats.MessageDetails(recipientId, MessageId(messageRecord.id)))
       }
     }
 
@@ -3877,7 +3877,7 @@ class ConversationFragment :
       when (action) {
         "gift_badge" -> checkoutLauncher.launch(InAppPaymentType.ONE_TIME_GIFT)
         "username_edit" -> startActivity(EditProfileActivity.getIntentForUsernameEdit(requireContext()))
-        "calls_tab" -> startActivity(MainActivity.clearTopAndOpenTab(requireContext(), MainNavigationListLocation.CALLS))
+        "calls_tab" -> startActivity(MainActivity.clearTopAndOpenTab(requireContext(), MainListRoute.Calls))
         "chat_folder" -> startActivity(AppSettingsActivity.chatFolders(requireContext()))
         "remote_backups" -> {
           if (SignalStore.backup.areBackupsEnabled) {
@@ -4388,7 +4388,7 @@ class ConversationFragment :
     override fun handleManageGroup() {
       viewModel.recipientSnapshot?.let { recipient ->
         container.hideKeyboard(composeText)
-        chatRouter.goToChatDetail(MainNavigationDetailLocation.Chats.ConversationSettings(recipient.id))
+        chatRouter.goToChatDetail(MainDetailRoute.Chats.ConversationSettings(recipient.id))
       }
     }
 
@@ -4426,7 +4426,7 @@ class ConversationFragment :
       viewModel.recipientSnapshot?.let { recipient ->
         if (!viewModel.hasMessageRequestState || recipient.isBlocked) {
           container.hideKeyboard(composeText)
-          chatRouter.goToChatDetail(MainNavigationDetailLocation.Chats.ConversationSettings(recipient.id))
+          chatRouter.goToChatDetail(MainDetailRoute.Chats.ConversationSettings(recipient.id))
         }
       }
     }

app/src/main/java/org/thoughtcrime/securesms/conversationlist/ConversationListArchiveFragment.java

@@ -33,7 +33,7 @@ import org.signal.core.ui.view.Stub;
 import org.thoughtcrime.securesms.R;
 import org.thoughtcrime.securesms.components.snackbars.SnackbarState;
 import org.thoughtcrime.securesms.database.SignalDatabase;
-import org.thoughtcrime.securesms.main.MainNavigationListLocation;
+import org.thoughtcrime.securesms.main.MainListRoute;
 import org.thoughtcrime.securesms.main.MainSnackbarHostKey;
 import org.thoughtcrime.securesms.util.ConversationUtil;
 
@@ -77,7 +77,7 @@ public class ConversationListArchiveFragment extends ConversationListFragment
     requireActivity().getOnBackPressedDispatcher().addCallback(getViewLifecycleOwner(), new OnBackPressedCallback(true) {
       @Override
       public void handleOnBackPressed() {
-        mainNavigationViewModel.goTo(MainNavigationListLocation.CHATS);
+        mainNavigationViewModel.goTo(MainListRoute.Chats);
       }
     });
   }

app/src/main/java/org/thoughtcrime/securesms/conversationlist/ConversationListFragment.java

@@ -143,7 +143,7 @@ import org.thoughtcrime.securesms.groups.SelectionLimits;
 import org.thoughtcrime.securesms.jobs.RefreshOwnProfileJob;
 import org.thoughtcrime.securesms.keyvalue.AccountValues;
 import org.thoughtcrime.securesms.keyvalue.SignalStore;
-import org.thoughtcrime.securesms.main.MainNavigationListLocation;
+import org.thoughtcrime.securesms.main.MainListRoute;
 import org.thoughtcrime.securesms.main.MainNavigationViewModel;
 import org.thoughtcrime.securesms.main.MainSnackbarHostKey;
 import org.thoughtcrime.securesms.main.MainToolbarMode;
@@ -465,7 +465,7 @@ public class ConversationListFragment extends MainFragment implements Conversati
     requireActivity().getOnBackPressedDispatcher().addCallback(getViewLifecycleOwner(), chatListBackHandler);
 
     lifecycleDisposable.bindTo(getViewLifecycleOwner());
-    lifecycleDisposable.add(mainNavigationViewModel.getTabClickEventsObservable().filter(tab -> tab == MainNavigationListLocation.CHATS)
+    lifecycleDisposable.add(mainNavigationViewModel.getTabClickEventsObservable().filter(tab -> tab == MainListRoute.Chats)
                                                    .subscribe(unused -> {
                                                      Log.d(TAG, "Scroll to top please");
                                                      LinearLayoutManager layoutManager            = (LinearLayoutManager) list.getLayoutManager();
@@ -702,7 +702,7 @@ public class ConversationListFragment extends MainFragment implements Conversati
   @Override
   public void onShowArchiveClick() {
     if (viewModel.currentSelectedConversations().isEmpty()) {
-      mainNavigationViewModel.goTo(MainNavigationListLocation.ARCHIVE);
+      mainNavigationViewModel.goTo(MainListRoute.Archive);
     }
   }
 
@@ -765,7 +765,7 @@ public class ConversationListFragment extends MainFragment implements Conversati
           } else if (event instanceof MainToolbarViewModel.Event.Chats.ClearFilter) {
             onClearFilterClick();
           } else if (event instanceof MainToolbarViewModel.Event.Chats.CloseArchive) {
-            mainNavigationViewModel.goTo(MainNavigationListLocation.CHATS);
+            mainNavigationViewModel.goTo(MainListRoute.Chats);
           }
         })
     );

app/src/main/java/org/thoughtcrime/securesms/conversationlist/ConversationListFragmentExtensions.kt

@@ -16,7 +16,7 @@ import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.launch
 import org.greenrobot.eventbus.EventBus
 import org.signal.core.ui.isSplitPane
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
+import org.thoughtcrime.securesms.main.MainDetailRoute
 
 /**
  * When the user searches for a conversation and then enters a message, we should clear
@@ -29,7 +29,7 @@ import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
  * On other screen types, since we are in a multi-pane mode, we can subscribe immediately.
  */
 fun Fragment.listenToEventBusWhileResumed(
-  detailLocation: Flow<MainNavigationDetailLocation>
+  detailLocation: Flow<MainDetailRoute?>
 ) {
   lifecycleScope.launch {
     detailLocation
@@ -37,8 +37,8 @@ fun Fragment.listenToEventBusWhileResumed(
       .collectLatest {
         if (!resources.isSplitPane()) {
           when (it) {
-            is MainNavigationDetailLocation.Conversation -> unsubscribe()
-            MainNavigationDetailLocation.Empty -> subscribe()
+            is MainDetailRoute.Conversation -> unsubscribe()
+            null -> subscribe()
             else -> Unit
           }
         } else {

app/src/main/java/org/thoughtcrime/securesms/dependencies/CoreUiDependenciesProvider.kt

@@ -7,7 +7,6 @@ package org.thoughtcrime.securesms.dependencies
 
 import org.signal.core.ui.CoreUiDependencies
 import org.thoughtcrime.securesms.BuildConfig
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.util.TextSecurePreferences
 
 object CoreUiDependenciesProvider : CoreUiDependencies.Provider {
@@ -22,8 +21,4 @@ object CoreUiDependenciesProvider : CoreUiDependencies.Provider {
   override fun provideIsScreenSecurityEnabled(): Boolean {
     return TextSecurePreferences.isScreenSecurityEnabled(AppDependencies.application)
   }
-
-  override fun provideForceSplitPane(): Boolean {
-    return SignalStore.internal.forceSplitPane
-  }
 }

app/src/main/java/org/thoughtcrime/securesms/groups/ui/addmembers/AddMembersActivity.kt

@@ -163,7 +163,6 @@ private fun AddMembersScreenUi(
 
   RecipientPickerScaffold(
     title = title,
-    forceSplitPane = uiState.forceSplitPane,
     onNavigateUpClick = callbacks::onBackPressed,
     topAppBarActions = {},
     snackbarHostState = remember { SnackbarHostState() },
@@ -318,7 +317,6 @@ private fun AddMembersScreenPreview() {
   Previews.Preview {
     AddMembersScreenUi(
       uiState = AddMembersUiState(
-        forceSplitPane = false,
         selectionLimits = SelectionLimits.NO_LIMITS
       ),
       callbacks = AddMembersUiCallbacks.Empty

app/src/main/java/org/thoughtcrime/securesms/groups/ui/addmembers/AddMembersViewModel.kt

@@ -20,7 +20,6 @@ import org.thoughtcrime.securesms.database.model.GroupRecord
 import org.thoughtcrime.securesms.groups.GroupId
 import org.thoughtcrime.securesms.groups.SelectionLimits
 import org.thoughtcrime.securesms.groups.ui.addmembers.AddMembersUiState.UserMessage
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.recipients.PhoneNumber
 import org.thoughtcrime.securesms.recipients.Recipient
 import org.thoughtcrime.securesms.recipients.RecipientId
@@ -134,7 +133,6 @@ class AddMembersViewModel(
 }
 
 data class AddMembersUiState(
-  val forceSplitPane: Boolean = SignalStore.internal.forceSplitPane,
   val searchQuery: String = "",
   val existingMembersMinusSelf: Set<RecipientId> = emptySet(),
   val selectionLimits: SelectionLimits,

app/src/main/java/org/thoughtcrime/securesms/groups/ui/addtogroup/AddToGroupsActivity.kt

@@ -129,7 +129,6 @@ private fun AddToGroupsScreenUi(
 
   RecipientPickerScaffold(
     title = title,
-    forceSplitPane = uiState.forceSplitPane,
     onNavigateUpClick = callbacks::onBackPressed,
     topAppBarActions = {},
     snackbarHostState = remember { SnackbarHostState() },
@@ -282,7 +281,6 @@ private fun AddToSingleGroupScreenPreview() {
   Previews.Preview {
     AddToGroupsScreenUi(
       uiState = AddToGroupsUiState(
-        forceSplitPane = false,
         selectionLimits = null
       ),
       callbacks = AddToGroupsUiCallbacks.Empty
@@ -296,7 +294,6 @@ private fun AddToMultipleGroupsScreenPreview() {
   Previews.Preview {
     AddToGroupsScreenUi(
       uiState = AddToGroupsUiState(
-        forceSplitPane = false,
         selectionLimits = SelectionLimits.NO_LIMITS
       ),
       callbacks = AddToGroupsUiCallbacks.Empty

app/src/main/java/org/thoughtcrime/securesms/groups/ui/addtogroup/AddToGroupsViewModel.kt

@@ -20,7 +20,6 @@ import org.thoughtcrime.securesms.groups.ui.GroupChangeFailureReason
 import org.thoughtcrime.securesms.groups.ui.addtogroup.AddToGroupsUiState.UserMessage
 import org.thoughtcrime.securesms.groups.v2.GroupAddMembersResult
 import org.thoughtcrime.securesms.groups.v2.GroupManagementRepository
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.recipients.Recipient
 import org.thoughtcrime.securesms.recipients.RecipientId
 
@@ -127,7 +126,6 @@ class AddToGroupsViewModel(
 }
 
 data class AddToGroupsUiState(
-  val forceSplitPane: Boolean = SignalStore.internal.forceSplitPane,
   val searchQuery: String = "",
   val existingGroupMemberships: Set<RecipientId> = emptySet(),
   val selectionLimits: SelectionLimits? = null,

app/src/main/java/org/thoughtcrime/securesms/groups/ui/creategroup/CreateGroupActivity.kt

@@ -162,7 +162,6 @@ private fun CreateGroupScreenUi(
 
   RecipientPickerScaffold(
     title = title,
-    forceSplitPane = uiState.forceSplitPane,
     onNavigateUpClick = callbacks::onBackPressed,
     topAppBarActions = {},
     snackbarHostState = remember { SnackbarHostState() },
@@ -285,7 +284,6 @@ private fun CreateGroupScreenPreview() {
   Previews.Preview {
     CreateGroupScreenUi(
       uiState = CreateGroupUiState(
-        forceSplitPane = false,
         selectionLimits = SelectionLimits.NO_LIMITS
       ),
       callbacks = CreateGroupUiCallbacks.Empty

app/src/main/java/org/thoughtcrime/securesms/groups/ui/creategroup/CreateGroupViewModel.kt

@@ -17,7 +17,6 @@ import org.thoughtcrime.securesms.contacts.SelectedContact
 import org.thoughtcrime.securesms.groups.SelectionLimits
 import org.thoughtcrime.securesms.groups.ui.creategroup.CreateGroupUiState.NavTarget
 import org.thoughtcrime.securesms.groups.ui.creategroup.CreateGroupUiState.UserMessage
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.recipients.PhoneNumber
 import org.thoughtcrime.securesms.recipients.RecipientId
 import org.thoughtcrime.securesms.recipients.RecipientRepository
@@ -120,7 +119,6 @@ class CreateGroupViewModel : ViewModel() {
 }
 
 data class CreateGroupUiState(
-  val forceSplitPane: Boolean = SignalStore.internal.forceSplitPane,
   val searchQuery: String = "",
   val selectionLimits: SelectionLimits = RemoteConfig.groupLimits.excludingSelf(),
   val newSelections: List<SelectedContact> = emptyList(),

app/src/main/java/org/thoughtcrime/securesms/keyvalue/InternalValues.kt

@@ -38,8 +38,6 @@ class InternalValues internal constructor(store: KeyValueStore) : SignalStoreVal
     const val LAST_SCROLL_POSITION: String = "internal.last_scroll_position"
     const val CONVERSATION_ITEM_V2_MEDIA: String = "internal.conversation_item_v2_media"
     const val WEB_SOCKET_SHADOWING_STATS: String = "internal.web_socket_shadowing_stats"
-    const val FORCE_SPLIT_PANE_ON_COMPACT_LANDSCAPE: String = "internal.force.split.pane.on.compact.landscape.ui"
-    const val FORCE_SINGLE_PANE_ON_ALL_DEVICES: String = "internal.force_single_pane_on_all_devices"
     const val SHOW_ARCHIVE_STATE_HINT: String = "internal.show_archive_state_hint"
     const val INCLUDE_DEBUGLOG_IN_BACKUP: String = "internal.include_debuglog_in_backup"
     const val IMPORTED_BACKUP_DEBUG_INFO: String = "internal.imported_backup_debug_info"
@@ -52,16 +50,6 @@ class InternalValues internal constructor(store: KeyValueStore) : SignalStoreVal
 
   public override fun getKeysToIncludeInBackup(): List<String> = emptyList()
 
-  /**
-   * Force split-pane mode on compact landscape
-   */
-  var forceSplitPane by booleanValue(FORCE_SPLIT_PANE_ON_COMPACT_LANDSCAPE, false).falseForExternalUsers()
-
-  /**
-   * Force single-pane on all devices
-   */
-  var forceSinglePane by booleanValue(FORCE_SINGLE_PANE_ON_ALL_DEVICES, false).falseForExternalUsers()
-
   /**
    * Members will not be added directly to a GV2 even if they could be.
    */

app/src/main/java/org/thoughtcrime/securesms/main/MainBottomChrome.kt

@@ -15,23 +15,21 @@ import androidx.compose.material3.SnackbarHostState
 import androidx.compose.material3.SnackbarResult
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
-import androidx.compose.runtime.getValue
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalResources
+import org.signal.core.ui.NavigationType
 import org.signal.core.ui.compose.BreakpointPreviews
 import org.signal.core.ui.compose.Previews
 import org.signal.core.ui.compose.Snackbars
 import org.signal.core.ui.compose.showSnackbar
-import org.signal.core.ui.isSplitPane
 import org.signal.core.ui.rememberIsSplitPane
 import org.thoughtcrime.securesms.components.snackbars.SnackbarHostKey
 import org.thoughtcrime.securesms.components.snackbars.rememberSnackbarState
 import org.thoughtcrime.securesms.megaphone.Megaphone
 import org.thoughtcrime.securesms.megaphone.MegaphoneActionController
 import org.thoughtcrime.securesms.megaphone.Megaphones
-import org.thoughtcrime.securesms.window.NavigationType
 
 interface MainBottomChromeCallback : MainFloatingActionButtonsCallback {
   fun onMegaphoneVisible(megaphone: Megaphone)
@@ -40,14 +38,14 @@ interface MainBottomChromeCallback : MainFloatingActionButtonsCallback {
   object Empty : MainBottomChromeCallback {
     override fun onNewChatClick() = Unit
     override fun onNewCallClick() = Unit
-    override fun onCameraClick(destination: MainNavigationListLocation) = Unit
+    override fun onCameraClick(destination: MainListRoute) = Unit
     override fun onMegaphoneVisible(megaphone: Megaphone) = Unit
     override fun onSnackbarDismissed() = Unit
   }
 }
 
 data class MainBottomChromeState(
-  val destination: MainNavigationListLocation = MainNavigationListLocation.CHATS,
+  val destination: MainListRoute = MainListRoute.Chats,
   val megaphoneState: MainMegaphoneState = MainMegaphoneState(),
   val mainToolbarMode: MainToolbarMode = MainToolbarMode.FULL
 )

app/src/main/java/org/thoughtcrime/securesms/main/MainContentLayoutData.kt

@@ -1,118 +0,0 @@
-/*
- * Copyright 2025 Signal Messenger, LLC
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-package org.thoughtcrime.securesms.main
-
-import androidx.compose.foundation.shape.RoundedCornerShape
-import androidx.compose.material3.adaptive.currentWindowAdaptiveInfo
-import androidx.compose.runtime.Composable
-import androidx.compose.runtime.Immutable
-import androidx.compose.runtime.remember
-import androidx.compose.ui.graphics.RectangleShape
-import androidx.compose.ui.graphics.Shape
-import androidx.compose.ui.platform.LocalResources
-import androidx.compose.ui.unit.Dp
-import androidx.compose.ui.unit.dp
-import androidx.window.core.layout.WindowSizeClass
-import org.signal.core.ui.WindowBreakpoint
-import org.signal.core.ui.getWindowBreakpoint
-import org.signal.core.ui.rememberIsSplitPane
-
-private val MEDIUM_CONTENT_CORNERS = 18.dp
-private val EXTENDED_CONTENT_CORNERS = 14.dp
-
-/**
- * Describes metrics for the content layout (list and detail) of the main screen.
- *
- * @param shape The clipping shape of each of the list and detail fragments
- * @param navigationBarShape The clipping shape applied to the navigation bar, if present.
- * @param partitionWidth The width of the divider between list and detail
- * @param listPaddingStart The padding between the list pane and the navigation rail
- * @param detailPaddingEnd The padding at the end of the detail pane
- */
-@Immutable
-data class MainContentLayoutData(
-  val shape: Shape,
-  val navigationBarShape: Shape,
-  val partitionWidth: Dp,
-  val listPaddingStart: Dp,
-  val detailPaddingEnd: Dp
-) {
-  private val extraPadding: Dp = partitionWidth + listPaddingStart + detailPaddingEnd
-
-  /**
-   * Whether or not the WindowSizeClass supports drag handles.
-   */
-  @Composable
-  fun hasDragHandle(): Boolean {
-    return LocalResources.current.rememberIsSplitPane()
-  }
-
-  /**
-   * Calculates the default preferred width
-   */
-  @Composable
-  fun rememberDefaultPanePreferredWidth(maxWidth: Dp): Dp {
-    val isSplitPane = LocalResources.current.rememberIsSplitPane()
-    val windowSizeClass = currentWindowAdaptiveInfo().windowSizeClass
-
-    return remember(maxWidth, windowSizeClass, isSplitPane) {
-      when {
-        !isSplitPane -> maxWidth
-        windowSizeClass.isWidthAtLeastBreakpoint(WindowSizeClass.WIDTH_DP_EXPANDED_LOWER_BOUND) -> 416.dp
-        else -> (maxWidth - extraPadding) / 2f
-      }
-    }
-  }
-
-  companion object {
-    /**
-     * Uses the [WindowSizeClass] and [MainToolbarMode] to build out a MainContentLayoutData.
-     */
-    @Composable
-    fun rememberContentLayoutData(mode: MainToolbarMode): MainContentLayoutData {
-      val windowSizeClass = currentWindowAdaptiveInfo().windowSizeClass
-      val resources = LocalResources.current
-      val breakpoint = resources.getWindowBreakpoint()
-      val isSplitPane = resources.rememberIsSplitPane()
-
-      return remember(windowSizeClass, mode, breakpoint, isSplitPane) {
-        val isLargeWindowSize = breakpoint is WindowBreakpoint.Large
-
-        MainContentLayoutData(
-          shape = when {
-            !isSplitPane -> RectangleShape
-            isLargeWindowSize -> RoundedCornerShape(EXTENDED_CONTENT_CORNERS)
-            else -> RoundedCornerShape(MEDIUM_CONTENT_CORNERS)
-          },
-          navigationBarShape = when {
-            !isSplitPane -> RectangleShape
-            isLargeWindowSize -> RoundedCornerShape(0.dp, 0.dp, EXTENDED_CONTENT_CORNERS, EXTENDED_CONTENT_CORNERS)
-            else -> RoundedCornerShape(0.dp, 0.dp, MEDIUM_CONTENT_CORNERS, MEDIUM_CONTENT_CORNERS)
-          },
-          partitionWidth = when {
-            !isSplitPane -> 0.dp
-            isLargeWindowSize -> 24.dp
-            else -> 13.dp
-          },
-          listPaddingStart = when {
-            !isSplitPane -> 0.dp
-            else -> {
-              when (mode) {
-                MainToolbarMode.SEARCH -> 24.dp
-                else -> 0.dp
-              }
-            }
-          },
-          detailPaddingEnd = when {
-            !isSplitPane -> 0.dp
-            isLargeWindowSize -> 24.dp
-            else -> 12.dp
-          }
-        )
-      }
-    }
-  }
-}

app/src/main/java/org/thoughtcrime/securesms/main/MainDetailBackStack.kt

@@ -5,32 +5,44 @@
 
 package org.thoughtcrime.securesms.main
 
-import androidx.compose.runtime.snapshots.SnapshotStateList
+import org.signal.core.ui.compose.split.ListDetailBackStack
+import org.signal.core.ui.compose.split.detailLocation
+import org.signal.core.ui.compose.split.listLocation
+import org.thoughtcrime.securesms.calls.log.CallLogRow
+import org.thoughtcrime.securesms.recipients.RecipientId
 
-interface MainDetailBackStack {
-  val entries: SnapshotStateList<MainNavigationDetailLocation>
+/**
+ * The list currently being displayed.
+ */
+val ListDetailBackStack.listLocation: MainListRoute
+  get() = listLocation<MainListRoute>()
 
-  val isEmpty: Boolean
-    get() = entries.singleOrNull() is MainNavigationDetailLocation.Empty
+/**
+ * The detail content displayed above the current list, or null when the list is showing on its own.
+ */
+val ListDetailBackStack.detailLocation: MainDetailRoute?
+  get() = detailLocation<MainDetailRoute>()
 
-  fun push(location: MainNavigationDetailLocation)
-
-  /**
-   * Pops the top entry off the stack. Returns true if something was popped, false if the stack is already at its root.
-   */
-  fun pop(): Boolean {
-    if (entries.size <= 1) return false
-    entries.removeAt(entries.lastIndex)
-    return true
+/**
+ * The recipient whose content is displayed by the topmost entry that has one.
+ */
+val ListDetailBackStack.activeRecipientId: RecipientId?
+  get() = asReversed().firstNotNullOfOrNull {
+    when (it) {
+      is MainDetailRoute.Conversation -> it.conversationArgs.recipientId
+      is MainDetailRoute.Chats -> it.controllerKey
+      else -> null
+    }
   }
 
-  /**
-   * Resets the stack to its base empty state.
-   */
-  fun reset() {
-    entries.removeAll { it !is MainNavigationDetailLocation.Empty }
-    if (entries.isEmpty()) {
-      entries.add(MainNavigationDetailLocation.Empty)
+/**
+ * The call whose content is displayed by the topmost entry that has one.
+ */
+val ListDetailBackStack.activeCallId: CallLogRow.Id?
+  get() = asReversed().firstNotNullOfOrNull {
+    when (it) {
+      is MainDetailRoute.Calls -> it.controllerKey
+      is MainDetailRoute.CallLinkDetails -> it.controllerKey
+      else -> null
     }
   }
-}

app/src/main/java/org/thoughtcrime/securesms/main/MainDetailRoute.kt

@@ -6,11 +6,11 @@
 package org.thoughtcrime.securesms.main
 
 import android.os.Parcelable
-import androidx.navigation3.runtime.NavKey
 import kotlinx.parcelize.IgnoredOnParcel
 import kotlinx.parcelize.Parcelize
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.Transient
+import org.signal.core.ui.compose.split.DetailNavKey
 import org.thoughtcrime.securesms.calls.log.CallLogRow
 import org.thoughtcrime.securesms.conversation.ConversationArgs
 import org.thoughtcrime.securesms.database.model.MessageId
@@ -22,25 +22,18 @@ import org.thoughtcrime.securesms.service.webrtc.links.CallLinkRoomId
  */
 @Serializable
 @Parcelize
-sealed interface MainNavigationDetailLocation : Parcelable, NavKey {
+sealed interface MainDetailRoute : Parcelable, DetailNavKey {
 
   /**
-   * Flag utilized internally to determine whether the given route is displayed at the root
-   * of a task stack (or on top of Empty)
+   * Whether this is the bottom-most piece of detail content rather than something stacked on top of it.
+   * Pushing one replaces the detail already displayed above the current list; see [push].
    */
   @IgnoredOnParcel
-  val isContentRoot: Boolean
+  override val isContentRoot: Boolean
     get() = false
 
   @Serializable
-  data object Empty : MainNavigationDetailLocation {
-    @Transient
-    @IgnoredOnParcel
-    override val isContentRoot: Boolean = true
-  }
-
-  @Serializable
-  data class Conversation(val conversationArgs: ConversationArgs) : MainNavigationDetailLocation {
+  data class Conversation(val conversationArgs: ConversationArgs) : MainDetailRoute {
     @Transient
     @IgnoredOnParcel
     override val isContentRoot: Boolean = true
@@ -51,7 +44,7 @@ sealed interface MainNavigationDetailLocation : Parcelable, NavKey {
   }
 
   @Serializable
-  data class CallLinkDetails(val callLinkRoomId: CallLinkRoomId) : MainNavigationDetailLocation {
+  data class CallLinkDetails(val callLinkRoomId: CallLinkRoomId) : MainDetailRoute {
     @Transient
     @IgnoredOnParcel
     override val isContentRoot: Boolean = true
@@ -65,7 +58,7 @@ sealed interface MainNavigationDetailLocation : Parcelable, NavKey {
    * Subscreens that can be displayed within the chats tab.
    */
   @Parcelize
-  sealed interface Chats : MainNavigationDetailLocation {
+  sealed interface Chats : MainDetailRoute {
 
     val controllerKey: RecipientId
 
@@ -94,7 +87,7 @@ sealed interface MainNavigationDetailLocation : Parcelable, NavKey {
    * Subscreens that can be displayed within the calls tab.
    */
   @Parcelize
-  sealed interface Calls : MainNavigationDetailLocation {
+  sealed interface Calls : MainDetailRoute {
     val controllerKey: CallLogRow.Id
 
     @Parcelize
@@ -115,7 +108,7 @@ sealed interface MainNavigationDetailLocation : Parcelable, NavKey {
    * Subscreens that can be displayed within the stories tab.
    */
   @Parcelize
-  sealed class Stories : MainNavigationDetailLocation {
+  sealed class Stories : MainDetailRoute {
     @Transient
     @IgnoredOnParcel
     override val isContentRoot: Boolean = true

app/src/main/java/org/thoughtcrime/securesms/main/MainFloatingActionButtons.kt

@@ -36,12 +36,12 @@ import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.res.vectorResource
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
+import org.signal.core.ui.NavigationType
 import org.signal.core.ui.compose.DayNightPreviews
 import org.signal.core.ui.compose.Previews
 import org.signal.core.ui.compose.SignalIcons
 import org.signal.core.ui.compose.theme.SignalTheme
 import org.thoughtcrime.securesms.R
-import org.thoughtcrime.securesms.window.NavigationType
 import kotlin.math.roundToInt
 import org.signal.core.ui.R as CoreUiR
 
@@ -51,18 +51,18 @@ private val ACTION_BUTTON_SPACING = 16.dp
 interface MainFloatingActionButtonsCallback {
   fun onNewChatClick()
   fun onNewCallClick()
-  fun onCameraClick(destination: MainNavigationListLocation)
+  fun onCameraClick(destination: MainListRoute)
 
   object Empty : MainFloatingActionButtonsCallback {
     override fun onNewChatClick() = Unit
     override fun onNewCallClick() = Unit
-    override fun onCameraClick(destination: MainNavigationListLocation) = Unit
+    override fun onCameraClick(destination: MainListRoute) = Unit
   }
 }
 
 @Composable
 fun MainFloatingActionButtons(
-  destination: MainNavigationListLocation,
+  destination: MainListRoute,
   callback: MainFloatingActionButtonsCallback,
   modifier: Modifier = Modifier,
   navigationType: NavigationType = NavigationType.rememberNavigationType()
@@ -114,10 +114,10 @@ fun MainFloatingActionButtons(
 
 @Composable
 private fun BoxScope.SecondaryActionButton(
-  destination: MainNavigationListLocation,
+  destination: MainListRoute,
   boxHeightPx: Int,
   elevation: Dp,
-  onCameraClick: (MainNavigationListLocation) -> Unit
+  onCameraClick: (MainListRoute) -> Unit
 ) {
   val navigationType = NavigationType.rememberNavigationType()
   val secondaryButtonAlignment = remember(navigationType) {
@@ -139,7 +139,7 @@ private fun BoxScope.SecondaryActionButton(
   }
 
   AnimatedVisibility(
-    visible = destination == MainNavigationListLocation.CHATS || destination == MainNavigationListLocation.ARCHIVE,
+    visible = destination == MainListRoute.Chats || destination == MainListRoute.Archive,
     modifier = Modifier.align(secondaryButtonAlignment),
     enter = slideInVertically(initialOffsetY = offsetYProvider),
     exit = slideOutVertically(targetOffsetY = offsetYProvider)
@@ -155,7 +155,7 @@ private fun BoxScope.SecondaryActionButton(
         contentColor = MaterialTheme.colorScheme.onSurface
       ),
       onClick = {
-        onCameraClick(MainNavigationListLocation.CHATS)
+        onCameraClick(MainListRoute.Chats)
       },
       shadowElevation = animatedElevation
     )
@@ -164,18 +164,18 @@ private fun BoxScope.SecondaryActionButton(
 
 @Composable
 private fun PrimaryActionButton(
-  destination: MainNavigationListLocation,
+  destination: MainListRoute,
   elevation: Dp,
   onNewChatClick: () -> Unit = {},
-  onCameraClick: (MainNavigationListLocation) -> Unit = {},
+  onCameraClick: (MainListRoute) -> Unit = {},
   onNewCallClick: () -> Unit = {}
 ) {
   val onClick = remember(destination) {
     when (destination) {
-      MainNavigationListLocation.ARCHIVE -> onNewChatClick
-      MainNavigationListLocation.CHATS -> onNewChatClick
-      MainNavigationListLocation.CALLS -> onNewCallClick
-      MainNavigationListLocation.STORIES -> {
+      MainListRoute.Archive -> onNewChatClick
+      MainListRoute.Chats -> onNewChatClick
+      MainListRoute.Calls -> onNewCallClick
+      MainListRoute.Stories -> {
         { onCameraClick(destination) }
       }
     }
@@ -187,10 +187,10 @@ private fun PrimaryActionButton(
     icon = {
       AnimatedContent(destination) { targetState ->
         val (icon, contentDescriptionId) = when (targetState) {
-          MainNavigationListLocation.ARCHIVE -> CoreUiR.drawable.symbol_edit_24 to R.string.conversation_list_fragment__fab_content_description
-          MainNavigationListLocation.CHATS -> CoreUiR.drawable.symbol_edit_24 to R.string.conversation_list_fragment__fab_content_description
-          MainNavigationListLocation.CALLS -> R.drawable.symbol_phone_plus_24 to R.string.CallLogFragment__start_a_new_call
-          MainNavigationListLocation.STORIES -> CoreUiR.drawable.symbol_camera_24 to R.string.conversation_list_fragment__open_camera_description
+          MainListRoute.Archive -> CoreUiR.drawable.symbol_edit_24 to R.string.conversation_list_fragment__fab_content_description
+          MainListRoute.Chats -> CoreUiR.drawable.symbol_edit_24 to R.string.conversation_list_fragment__fab_content_description
+          MainListRoute.Calls -> R.drawable.symbol_phone_plus_24 to R.string.CallLogFragment__start_a_new_call
+          MainListRoute.Stories -> CoreUiR.drawable.symbol_camera_24 to R.string.conversation_list_fragment__open_camera_description
         }
 
         Icon(
@@ -247,19 +247,19 @@ private fun MainFloatingActionButton(
 @DayNightPreviews
 @Composable
 private fun MainFloatingActionButtonsNavigationRailPreview() {
-  var currentDestination by remember { mutableStateOf(MainNavigationListLocation.CHATS) }
+  var currentDestination by remember { mutableStateOf(MainListRoute.Chats) }
   val callback = remember {
     object : MainFloatingActionButtonsCallback {
-      override fun onCameraClick(destination: MainNavigationListLocation) {
-        currentDestination = MainNavigationListLocation.CALLS
+      override fun onCameraClick(destination: MainListRoute) {
+        currentDestination = MainListRoute.Calls
       }
 
       override fun onNewChatClick() {
-        currentDestination = MainNavigationListLocation.STORIES
+        currentDestination = MainListRoute.Stories
       }
 
       override fun onNewCallClick() {
-        currentDestination = MainNavigationListLocation.CHATS
+        currentDestination = MainListRoute.Chats
       }
     }
   }
@@ -276,19 +276,19 @@ private fun MainFloatingActionButtonsNavigationRailPreview() {
 @DayNightPreviews
 @Composable
 private fun MainFloatingActionButtonsNavigationBarPreview() {
-  var currentDestination by remember { mutableStateOf(MainNavigationListLocation.CHATS) }
+  var currentDestination by remember { mutableStateOf(MainListRoute.Chats) }
   val callback = remember {
     object : MainFloatingActionButtonsCallback {
-      override fun onCameraClick(destination: MainNavigationListLocation) {
-        currentDestination = MainNavigationListLocation.CALLS
+      override fun onCameraClick(destination: MainListRoute) {
+        currentDestination = MainListRoute.Calls
       }
 
       override fun onNewChatClick() {
-        currentDestination = MainNavigationListLocation.STORIES
+        currentDestination = MainListRoute.Stories
       }
 
       override fun onNewCallClick() {
-        currentDestination = MainNavigationListLocation.CHATS
+        currentDestination = MainListRoute.Chats
       }
     }
   }

app/src/main/java/org/thoughtcrime/securesms/main/MainListRoute.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+import androidx.annotation.RawRes
+import androidx.annotation.StringRes
+import kotlinx.serialization.Serializable
+import org.signal.core.ui.compose.split.ListNavKey
+import org.thoughtcrime.securesms.R
+
+/**
+ * Main activity tabs (chat, stories, calls) that the user can select between. This is basically tied to something we can show in the 'list'
+ * panel of the main activity.
+ */
+@Serializable
+enum class MainListRoute(
+  @get:StringRes val label: Int,
+  @get:RawRes val icon: Int,
+  @get:StringRes val contentDescription: Int = label
+) : ListNavKey {
+  Chats(
+    label = R.string.ConversationListTabs__chats,
+    icon = R.raw.chats_28
+  ),
+  Archive(
+    label = R.string.ConversationListTabs__chats,
+    icon = R.raw.chats_28
+  ),
+  Calls(
+    label = R.string.ConversationListTabs__calls,
+    icon = R.raw.calls_28
+  ),
+  Stories(
+    label = R.string.ConversationListTabs__stories,
+    icon = R.raw.stories_28
+  );
+
+  val isChatsTab: Boolean
+    get() = this == Chats || this == Archive
+
+  /**
+   * The tab this list location is displayed under. The archive is a list pushed onto the chats stack
+   * rather than a tab of its own.
+   */
+  val tab: MainListRoute
+    get() = if (isChatsTab) Chats else this
+}

app/src/main/java/org/thoughtcrime/securesms/main/MainNavigation.kt

@@ -1,389 +1,77 @@
 /*
- * Copyright 2025 Signal Messenger, LLC
+ * Copyright 2026 Signal Messenger, LLC
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
 package org.thoughtcrime.securesms.main
 
-import androidx.annotation.RawRes
-import androidx.annotation.StringRes
-import androidx.compose.animation.core.animateFloatAsState
-import androidx.compose.animation.core.tween
-import androidx.compose.foundation.background
-import androidx.compose.foundation.layout.Box
-import androidx.compose.foundation.layout.BoxScope
-import androidx.compose.foundation.layout.Spacer
-import androidx.compose.foundation.layout.WindowInsets
-import androidx.compose.foundation.layout.defaultMinSize
-import androidx.compose.foundation.layout.height
-import androidx.compose.foundation.layout.padding
-import androidx.compose.foundation.layout.size
-import androidx.compose.foundation.shape.RoundedCornerShape
-import androidx.compose.material3.MaterialTheme
-import androidx.compose.material3.NavigationBar
-import androidx.compose.material3.NavigationBarItem
-import androidx.compose.material3.NavigationRail
-import androidx.compose.material3.NavigationRailItem
-import androidx.compose.material3.Text
+import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.Composable
-import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.setValue
-import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.draw.drawWithContent
-import androidx.compose.ui.geometry.CornerRadius
-import androidx.compose.ui.geometry.Offset
-import androidx.compose.ui.geometry.Size
-import androidx.compose.ui.graphics.Color
-import androidx.compose.ui.layout.onSizeChanged
-import androidx.compose.ui.platform.LocalDensity
-import androidx.compose.ui.res.colorResource
-import androidx.compose.ui.res.stringResource
-import androidx.compose.ui.text.drawText
-import androidx.compose.ui.text.rememberTextMeasurer
-import androidx.compose.ui.tooling.preview.Preview
-import androidx.compose.ui.unit.IntSize
-import androidx.compose.ui.unit.dp
-import androidx.core.graphics.BlendModeColorFilterCompat
-import androidx.core.graphics.BlendModeCompat
-import com.airbnb.lottie.LottieProperty
-import com.airbnb.lottie.compose.LottieAnimation
-import com.airbnb.lottie.compose.LottieCompositionSpec
-import com.airbnb.lottie.compose.rememberLottieComposition
-import com.airbnb.lottie.compose.rememberLottieDynamicProperties
-import com.airbnb.lottie.compose.rememberLottieDynamicProperty
-import org.signal.core.ui.compose.DayNightPreviews
-import org.signal.core.ui.compose.Previews
-import org.signal.core.ui.compose.theme.SignalTheme
-import org.thoughtcrime.securesms.R
-
-private val LOTTIE_SIZE = 28.dp
-
-enum class MainNavigationListLocation(
-  @StringRes val label: Int,
-  @RawRes val icon: Int,
-  @StringRes val contentDescription: Int = label
-) {
-  CHATS(
-    label = R.string.ConversationListTabs__chats,
-    icon = R.raw.chats_28
-  ),
-  ARCHIVE(
-    label = R.string.ConversationListTabs__chats,
-    icon = R.raw.chats_28
-  ),
-  CALLS(
-    label = R.string.ConversationListTabs__calls,
-    icon = R.raw.calls_28
-  ),
-  STORIES(
-    label = R.string.ConversationListTabs__stories,
-    icon = R.raw.stories_28
-  );
-
-  val isChatsTab: Boolean
-    get() = this == CHATS || this == ARCHIVE
-}
-
-data class MainNavigationState(
-  val chatsCount: Int = 0,
-  val callsCount: Int = 0,
-  val storiesCount: Int = 0,
-  val storyFailure: Boolean = false,
-  val isStoriesFeatureEnabled: Boolean = true,
-  val currentListLocation: MainNavigationListLocation = MainNavigationListLocation.CHATS,
-  val compact: Boolean = false
-)
+import androidx.navigation3.runtime.EntryProviderScope
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.entryProvider
+import org.signal.core.ui.compose.split.listEntry
+import org.signal.core.ui.compose.split.rememberCurrentDecoratedNavEntries
+import org.thoughtcrime.securesms.calls.CallsListPane
+import org.thoughtcrime.securesms.calls.registerCallsTabDetailRoutes
+import org.thoughtcrime.securesms.chats.ArchiveListPane
+import org.thoughtcrime.securesms.chats.ChatsListPane
+import org.thoughtcrime.securesms.chats.ConversationTransitionState
+import org.thoughtcrime.securesms.chats.registerChatsTabDetailRoutes
+import org.thoughtcrime.securesms.stories.StoriesListPane
+import org.thoughtcrime.securesms.stories.registerStoriesTabDetailRoutes
 
 /**
- * Chats list bottom navigation bar.
+ * Builds our nav entries and decorates them so they can save state, handing back the entries of whichever
+ * tab is displayed.
+ *
+ * A new *backstack* on [MainNavigationViewModel] is picked up here for free; a new *screen* needs a route
+ * registering in [rememberMainNavEntryProvider].
  */
 @Composable
-fun MainNavigationBar(
-  state: MainNavigationState,
-  onDestinationSelected: (MainNavigationListLocation) -> Unit
-) {
-  NavigationBar(
-    containerColor = SignalTheme.colors.colorSurface2,
-    contentColor = MaterialTheme.colorScheme.onSurface,
-    modifier = Modifier.height(if (state.compact) 48.dp else 80.dp),
-    windowInsets = WindowInsets(0, 0, 0, 0)
-  ) {
-    val entries = remember(state.isStoriesFeatureEnabled) {
-      if (state.isStoriesFeatureEnabled) {
-        MainNavigationListLocation.entries.filterNot { it == MainNavigationListLocation.ARCHIVE }
-      } else {
-        MainNavigationListLocation.entries.filterNot { it == MainNavigationListLocation.STORIES || it == MainNavigationListLocation.ARCHIVE }
-      }
-    }
-
-    entries.forEach { destination ->
-
-      val badgeCount = when (destination) {
-        MainNavigationListLocation.ARCHIVE -> error("Not supported")
-        MainNavigationListLocation.CHATS -> state.chatsCount
-        MainNavigationListLocation.CALLS -> state.callsCount
-        MainNavigationListLocation.STORIES -> state.storiesCount
-      }
-
-      val selected = state.currentListLocation == destination
-      NavigationBarItem(
-        selected = selected,
-        icon = {
-          NavigationDestinationIcon(
-            destination = destination,
-            selected = selected
-          )
-        },
-        label = if (state.compact) null else {
-          { NavigationDestinationLabel(destination) }
-        },
-        onClick = {
-          onDestinationSelected(destination)
-        },
-        modifier = Modifier.drawNavigationBarBadge(count = badgeCount, compact = state.compact)
-      )
-    }
-  }
-}
-
-/**
- * Draws badge over navigation bar item. We do this since they're required to be inside a row,
- * and things get really funky or clip weird if we try to use a normal composable.
- */
-@Composable
-private fun Modifier.drawNavigationBarBadge(count: Int, compact: Boolean): Modifier {
-  return if (count <= 0) {
-    this
-  } else {
-    val formatted = formatCount(count)
-    val textMeasurer = rememberTextMeasurer()
-    val color = colorResource(R.color.ConversationListTabs__unread)
-    val textStyle = MaterialTheme.typography.labelMedium
-    val textLayoutResult = remember(formatted) {
-      textMeasurer.measure(formatted, textStyle)
-    }
-
-    var size by remember { mutableStateOf(IntSize.Zero) }
-
-    val padding = with(LocalDensity.current) {
-      4.dp.toPx()
-    }
-
-    val xOffsetExtra = with(LocalDensity.current) {
-      4.dp.toPx()
-    }
-
-    val yOffset = with(LocalDensity.current) {
-      if (compact) 6.dp.toPx() else 10.dp.toPx()
-    }
-
-    this
-      .onSizeChanged {
-        size = it
-      }
-      .drawWithContent {
-        drawContent()
-
-        val xOffset = size.width.toFloat() / 2f + xOffsetExtra
-        val yRadius = size.height.toFloat() / 2f
-
-        if (size != IntSize.Zero) {
-          drawRoundRect(
-            color = color,
-            topLeft = Offset(xOffset, yOffset),
-            size = Size(textLayoutResult.size.width.toFloat() + padding * 2, textLayoutResult.size.height.toFloat()),
-            cornerRadius = CornerRadius(yRadius, yRadius)
-          )
-
-          drawText(
-            textLayoutResult = textLayoutResult,
-            color = Color.White,
-            topLeft = Offset(xOffset + padding, yOffset)
-          )
-        }
-      }
-  }
-}
-
-/**
- * Navigation Rail for medium and large form factor devices.
- */
-@Composable
-fun MainNavigationRail(
-  state: MainNavigationState,
-  mainFloatingActionButtonsCallback: MainFloatingActionButtonsCallback,
-  onDestinationSelected: (MainNavigationListLocation) -> Unit
-) {
-  NavigationRail(
-    containerColor = SignalTheme.colors.colorSurface1
-  ) {
-    Spacer(modifier = Modifier.height(40.dp).weight(1f, fill = false))
-
-    MainFloatingActionButtons(
-      destination = state.currentListLocation,
-      callback = mainFloatingActionButtonsCallback,
-      modifier = Modifier.padding(vertical = 8.dp)
-    )
-
-    Spacer(modifier = Modifier.height(40.dp).weight(1f, fill = false))
-
-    val entries = remember(state.isStoriesFeatureEnabled) {
-      if (state.isStoriesFeatureEnabled) {
-        MainNavigationListLocation.entries.filterNot { it == MainNavigationListLocation.ARCHIVE }
-      } else {
-        MainNavigationListLocation.entries.filterNot { it == MainNavigationListLocation.STORIES || it == MainNavigationListLocation.ARCHIVE }
-      }
-    }
-
-    val selectedDestination = if (state.currentListLocation == MainNavigationListLocation.ARCHIVE) {
-      MainNavigationListLocation.CHATS
-    } else {
-      state.currentListLocation
-    }
-
-    entries.forEachIndexed { idx, destination ->
-      val selected = selectedDestination == destination
-
-      Box {
-        NavigationRailItem(
-          modifier = Modifier.padding(bottom = if (MainNavigationListLocation.entries.lastIndex == idx) 0.dp else 16.dp),
-          icon = {
-            NavigationDestinationIcon(
-              destination = destination,
-              selected = selected
-            )
-          },
-          label = {
-            NavigationDestinationLabel(destination)
-          },
-          selected = selected,
-          onClick = {
-            onDestinationSelected(destination)
-          }
-        )
-
-        NavigationRailCountIndicator(
-          state = state,
-          destination = destination
-        )
-      }
-    }
-  }
-}
-
-@Composable
-private fun BoxScope.NavigationRailCountIndicator(
-  state: MainNavigationState,
-  destination: MainNavigationListLocation
-) {
-  val count = remember(state, destination) {
-    when (destination) {
-      MainNavigationListLocation.ARCHIVE -> error("Not supported")
-      MainNavigationListLocation.CHATS -> state.chatsCount
-      MainNavigationListLocation.CALLS -> state.callsCount
-      MainNavigationListLocation.STORIES -> state.storiesCount
-    }
-  }
-
-  if (count > 0) {
-    Box(
-      modifier = Modifier
-        .padding(start = 42.dp)
-        .height(16.dp)
-        .defaultMinSize(minWidth = 16.dp)
-        .background(color = colorResource(R.color.ConversationListTabs__unread), shape = RoundedCornerShape(percent = 50))
-        .align(Alignment.TopStart)
-    ) {
-      Text(
-        text = formatCount(count),
-        style = MaterialTheme.typography.labelMedium,
-        color = Color.White,
-        modifier = Modifier
-          .align(Alignment.Center)
-          .padding(horizontal = 4.dp)
-      )
-    }
-  }
-}
-
-@Composable
-private fun NavigationDestinationIcon(
-  destination: MainNavigationListLocation,
-  selected: Boolean
-) {
-  val dynamicProperties = rememberLottieDynamicProperties(
-    rememberLottieDynamicProperty(
-      property = LottieProperty.COLOR_FILTER,
-      value = BlendModeColorFilterCompat.createBlendModeColorFilterCompat(
-        MaterialTheme.colorScheme.onSurface.hashCode(),
-        BlendModeCompat.SRC_ATOP
-      ),
-      keyPath = arrayOf("**")
-    )
-  )
-
-  val composition by rememberLottieComposition(LottieCompositionSpec.RawRes(destination.icon))
-  val progress by animateFloatAsState(targetValue = if (selected) 1f else 0f, animationSpec = tween(durationMillis = composition?.duration?.toInt() ?: 0))
-
-  LottieAnimation(
-    composition = composition,
-    progress = { if (selected) progress else 0f },
-    dynamicProperties = dynamicProperties,
-    modifier = Modifier.size(LOTTIE_SIZE)
+fun rememberDecoratedDetailEntries(
+  mainNavigationViewModel: MainNavigationViewModel,
+  convoTransitionState: ConversationTransitionState,
+  isSplitPane: Boolean
+): List<NavEntry<NavKey>> {
+  return rememberCurrentDecoratedNavEntries(
+    navigator = mainNavigationViewModel.navigator,
+    entryProvider = rememberMainNavEntryProvider(convoTransitionState, isSplitPane)
   )
 }
 
+/**
+ * The entry-provider for main screen. The methods which are called are where you'd add additional
+ * screens for different tabs.
+ */
 @Composable
-private fun NavigationDestinationLabel(destination: MainNavigationListLocation) {
-  Text(stringResource(destination.label))
-}
-
-@Composable
-private fun formatCount(count: Int): String {
-  if (count > 99) {
-    return stringResource(R.string.ConversationListTabs__99p)
-  }
-  return count.toString()
-}
-
-@DayNightPreviews
-@Preview(device = "spec:parent=pixel_7,orientation=landscape")
-@Composable
-private fun MainNavigationRailPreview() {
-  Previews.Preview {
-    var selected by remember { mutableStateOf(MainNavigationListLocation.CHATS) }
-
-    MainNavigationRail(
-      state = MainNavigationState(
-        chatsCount = 500,
-        callsCount = 10,
-        storiesCount = 5,
-        currentListLocation = selected
-      ),
-      mainFloatingActionButtonsCallback = MainFloatingActionButtonsCallback.Empty,
-      onDestinationSelected = { selected = it }
-    )
+private fun rememberMainNavEntryProvider(
+  convoTransitionState: ConversationTransitionState,
+  isSplitPane: Boolean
+): (NavKey) -> NavEntry<NavKey> {
+  return remember(convoTransitionState, isSplitPane) {
+    entryProvider {
+      registerMainScreenRoutes()
+      registerChatsTabDetailRoutes(convoTransitionState)
+      registerCallsTabDetailRoutes(isSplitPane)
+      registerStoriesTabDetailRoutes()
+    }
   }
 }
 
-@DayNightPreviews
-@Composable
-private fun MainNavigationBarPreview() {
-  Previews.Preview {
-    var selected by remember { mutableStateOf(MainNavigationListLocation.CHATS) }
-
-    MainNavigationBar(
-      state = MainNavigationState(
-        chatsCount = 500,
-        callsCount = 10,
-        storiesCount = 5,
-        currentListLocation = selected,
-        compact = false
-      ),
-      onDestinationSelected = { selected = it }
-    )
+/**
+ * Registers the main list of tab routes (list locations)
+ */
+private fun EntryProviderScope<NavKey>.registerMainScreenRoutes() {
+  listEntry<MainListRoute> { location ->
+    when (location) {
+      MainListRoute.Chats -> ChatsListPane(modifier = Modifier.fillMaxSize())
+      MainListRoute.Archive -> ArchiveListPane(modifier = Modifier.fillMaxSize())
+      MainListRoute.Calls -> CallsListPane(modifier = Modifier.fillMaxSize())
+      MainListRoute.Stories -> StoriesListPane(modifier = Modifier.fillMaxSize())
+    }
   }
 }

app/src/main/java/org/thoughtcrime/securesms/main/MainNavigationBar.kt

@@ -0,0 +1,334 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+import androidx.compose.animation.core.animateFloatAsState
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxScope
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.WindowInsets
+import androidx.compose.foundation.layout.defaultMinSize
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.NavigationBar
+import androidx.compose.material3.NavigationBarItem
+import androidx.compose.material3.NavigationRail
+import androidx.compose.material3.NavigationRailItem
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.drawWithContent
+import androidx.compose.ui.geometry.CornerRadius
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.layout.onSizeChanged
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.res.colorResource
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.text.drawText
+import androidx.compose.ui.text.rememberTextMeasurer
+import androidx.compose.ui.tooling.preview.Preview
+import androidx.compose.ui.unit.IntSize
+import androidx.compose.ui.unit.dp
+import androidx.core.graphics.BlendModeColorFilterCompat
+import androidx.core.graphics.BlendModeCompat
+import com.airbnb.lottie.LottieProperty
+import com.airbnb.lottie.compose.LottieAnimation
+import com.airbnb.lottie.compose.LottieCompositionSpec
+import com.airbnb.lottie.compose.rememberLottieComposition
+import com.airbnb.lottie.compose.rememberLottieDynamicProperties
+import com.airbnb.lottie.compose.rememberLottieDynamicProperty
+import org.signal.core.ui.compose.DayNightPreviews
+import org.signal.core.ui.compose.Previews
+import org.signal.core.ui.compose.theme.SignalTheme
+import org.thoughtcrime.securesms.R
+
+private val LOTTIE_SIZE = 28.dp
+
+/**
+ * Chats list bottom navigation bar.
+ */
+@Composable
+fun MainNavigationBar(
+  state: MainNavigationBarState,
+  onDestinationSelected: (MainListRoute) -> Unit
+) {
+  NavigationBar(
+    containerColor = SignalTheme.colors.colorSurface2,
+    contentColor = MaterialTheme.colorScheme.onSurface,
+    modifier = Modifier.height(if (state.compact) 48.dp else 80.dp),
+    windowInsets = WindowInsets(0, 0, 0, 0)
+  ) {
+    state.destinations.forEach { destination ->
+      val badgeCount = when (destination) {
+        MainListRoute.Archive -> error("Not supported")
+        MainListRoute.Chats -> state.chatsCount
+        MainListRoute.Calls -> state.callsCount
+        MainListRoute.Stories -> state.storiesCount
+      }
+
+      val selected = state.currentListLocation == destination
+      NavigationBarItem(
+        selected = selected,
+        icon = {
+          NavigationDestinationIcon(
+            destination = destination,
+            selected = selected
+          )
+        },
+        label = if (state.compact) null else {
+          { NavigationDestinationLabel(destination) }
+        },
+        onClick = {
+          onDestinationSelected(destination)
+        },
+        modifier = Modifier.drawNavigationBarBadge(count = badgeCount, compact = state.compact)
+      )
+    }
+  }
+}
+
+/**
+ * Draws badge over navigation bar item. We do this since they're required to be inside a row,
+ * and things get really funky or clip weird if we try to use a normal composable.
+ */
+@Composable
+private fun Modifier.drawNavigationBarBadge(count: Int, compact: Boolean): Modifier {
+  return if (count <= 0) {
+    this
+  } else {
+    val formatted = formatCount(count)
+    val textMeasurer = rememberTextMeasurer()
+    val color = colorResource(R.color.ConversationListTabs__unread)
+    val textStyle = MaterialTheme.typography.labelMedium
+    val textLayoutResult = remember(formatted) {
+      textMeasurer.measure(formatted, textStyle)
+    }
+
+    var size by remember { mutableStateOf(IntSize.Zero) }
+
+    val padding = with(LocalDensity.current) {
+      4.dp.toPx()
+    }
+
+    val xOffsetExtra = with(LocalDensity.current) {
+      4.dp.toPx()
+    }
+
+    val yOffset = with(LocalDensity.current) {
+      if (compact) 6.dp.toPx() else 10.dp.toPx()
+    }
+
+    this
+      .onSizeChanged {
+        size = it
+      }
+      .drawWithContent {
+        drawContent()
+
+        val xOffset = size.width.toFloat() / 2f + xOffsetExtra
+        val yRadius = size.height.toFloat() / 2f
+
+        if (size != IntSize.Zero) {
+          drawRoundRect(
+            color = color,
+            topLeft = Offset(xOffset, yOffset),
+            size = Size(textLayoutResult.size.width.toFloat() + padding * 2, textLayoutResult.size.height.toFloat()),
+            cornerRadius = CornerRadius(yRadius, yRadius)
+          )
+
+          drawText(
+            textLayoutResult = textLayoutResult,
+            color = Color.White,
+            topLeft = Offset(xOffset + padding, yOffset)
+          )
+        }
+      }
+  }
+}
+
+/**
+ * Navigation Rail for medium and large form factor devices.
+ */
+@Composable
+fun MainNavigationRail(
+  state: MainNavigationBarState,
+  mainFloatingActionButtonsCallback: MainFloatingActionButtonsCallback,
+  onDestinationSelected: (MainListRoute) -> Unit
+) {
+  NavigationRail(
+    containerColor = SignalTheme.colors.colorSurface1
+  ) {
+    Spacer(modifier = Modifier.height(40.dp).weight(1f, fill = false))
+
+    MainFloatingActionButtons(
+      destination = state.currentListLocation,
+      callback = mainFloatingActionButtonsCallback,
+      modifier = Modifier.padding(vertical = 8.dp)
+    )
+
+    Spacer(modifier = Modifier.height(40.dp).weight(1f, fill = false))
+
+    val selectedDestination = if (state.currentListLocation == MainListRoute.Archive) {
+      MainListRoute.Chats
+    } else {
+      state.currentListLocation
+    }
+
+    state.destinations.forEachIndexed { idx, destination ->
+      val selected = selectedDestination == destination
+
+      Box {
+        NavigationRailItem(
+          modifier = Modifier.padding(bottom = if (state.destinations.lastIndex == idx) 0.dp else 16.dp),
+          icon = {
+            NavigationDestinationIcon(
+              destination = destination,
+              selected = selected
+            )
+          },
+          label = {
+            NavigationDestinationLabel(destination)
+          },
+          selected = selected,
+          onClick = {
+            onDestinationSelected(destination)
+          }
+        )
+
+        NavigationRailCountIndicator(
+          state = state,
+          destination = destination
+        )
+      }
+    }
+  }
+}
+
+@Composable
+private fun BoxScope.NavigationRailCountIndicator(
+  state: MainNavigationBarState,
+  destination: MainListRoute
+) {
+  val count = remember(state, destination) {
+    when (destination) {
+      MainListRoute.Archive -> error("Not supported")
+      MainListRoute.Chats -> state.chatsCount
+      MainListRoute.Calls -> state.callsCount
+      MainListRoute.Stories -> state.storiesCount
+    }
+  }
+
+  if (count > 0) {
+    Box(
+      modifier = Modifier
+        .padding(start = 42.dp)
+        .height(16.dp)
+        .defaultMinSize(minWidth = 16.dp)
+        .background(color = colorResource(R.color.ConversationListTabs__unread), shape = RoundedCornerShape(percent = 50))
+        .align(Alignment.TopStart)
+    ) {
+      Text(
+        text = formatCount(count),
+        style = MaterialTheme.typography.labelMedium,
+        color = Color.White,
+        modifier = Modifier
+          .align(Alignment.Center)
+          .padding(horizontal = 4.dp)
+      )
+    }
+  }
+}
+
+@Composable
+private fun NavigationDestinationIcon(
+  destination: MainListRoute,
+  selected: Boolean
+) {
+  val dynamicProperties = rememberLottieDynamicProperties(
+    rememberLottieDynamicProperty(
+      property = LottieProperty.COLOR_FILTER,
+      value = BlendModeColorFilterCompat.createBlendModeColorFilterCompat(
+        MaterialTheme.colorScheme.onSurface.hashCode(),
+        BlendModeCompat.SRC_ATOP
+      ),
+      keyPath = arrayOf("**")
+    )
+  )
+
+  val composition by rememberLottieComposition(LottieCompositionSpec.RawRes(destination.icon))
+  val progress by animateFloatAsState(targetValue = if (selected) 1f else 0f, animationSpec = tween(durationMillis = composition?.duration?.toInt() ?: 0))
+
+  LottieAnimation(
+    composition = composition,
+    progress = { if (selected) progress else 0f },
+    dynamicProperties = dynamicProperties,
+    modifier = Modifier.size(LOTTIE_SIZE)
+  )
+}
+
+@Composable
+private fun NavigationDestinationLabel(destination: MainListRoute) {
+  Text(stringResource(destination.label))
+}
+
+@Composable
+private fun formatCount(count: Int): String {
+  if (count > 99) {
+    return stringResource(R.string.ConversationListTabs__99p)
+  }
+  return count.toString()
+}
+
+@DayNightPreviews
+@Preview(device = "spec:parent=pixel_7,orientation=landscape")
+@Composable
+private fun MainNavigationRailPreview() {
+  Previews.Preview {
+    var selected by remember { mutableStateOf(MainListRoute.Chats) }
+
+    MainNavigationRail(
+      state = MainNavigationBarState(
+        chatsCount = 500,
+        callsCount = 10,
+        storiesCount = 5,
+        currentListLocation = selected
+      ),
+      mainFloatingActionButtonsCallback = MainFloatingActionButtonsCallback.Empty,
+      onDestinationSelected = { selected = it }
+    )
+  }
+}
+
+@DayNightPreviews
+@Composable
+private fun MainNavigationBarPreview() {
+  Previews.Preview {
+    var selected by remember { mutableStateOf(MainListRoute.Chats) }
+
+    MainNavigationBar(
+      state = MainNavigationBarState(
+        chatsCount = 500,
+        callsCount = 10,
+        storiesCount = 5,
+        currentListLocation = selected,
+        compact = false
+      ),
+      onDestinationSelected = { selected = it }
+    )
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/main/MainNavigationBarState.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+/**
+ * State of the main navigation bar or rail.
+ *
+ * @param destinations the tabs to display, in order. Whoever builds this state decides which ones the user
+ *   gets; the bar and the rail display exactly what they are handed.
+ */
+data class MainNavigationBarState(
+  val chatsCount: Int = 0,
+  val callsCount: Int = 0,
+  val storiesCount: Int = 0,
+  val storyFailure: Boolean = false,
+  val destinations: List<MainListRoute> = ALL_DESTINATIONS,
+  val currentListLocation: MainListRoute = MainListRoute.Chats,
+  val compact: Boolean = false
+) {
+  companion object {
+    /**
+     * Every tab there is, in display order. The archive is not among them: it is a list within chats
+     * rather than a tab of its own.
+     */
+    val ALL_DESTINATIONS = listOf(MainListRoute.Chats, MainListRoute.Calls, MainListRoute.Stories)
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/main/MainNavigationRouter.kt

@@ -10,7 +10,7 @@ package org.thoughtcrime.securesms.main
  */
 interface MainNavigationChatDetailRouter {
   fun exitDetailLocation()
-  fun goToChatDetail(location: MainNavigationDetailLocation.Chats)
+  fun goToChatDetail(location: MainDetailRoute.Chats)
 }
 
 /**
@@ -18,17 +18,16 @@ interface MainNavigationChatDetailRouter {
  */
 interface MainNavigationCallDetailRouter {
   fun exitDetailLocation()
-  fun goToCallDetail(location: MainNavigationDetailLocation.Calls)
+  fun goToCallDetail(location: MainDetailRoute.Calls)
 }
 
 /**
- * Handles navigation to all [MainNavigationListLocation]s and [MainNavigationDetailLocation]s, including the top-level roots.
+ * Handles navigation to all [MainListRoute]s and [MainDetailRoute]s, including the top-level roots.
  */
 interface MainNavigationRouter : MainNavigationChatDetailRouter, MainNavigationCallDetailRouter {
-  fun goTo(location: MainNavigationListLocation)
-  fun goTo(location: MainNavigationDetailLocation)
+  fun goTo(location: MainListRoute)
+  fun goTo(location: MainDetailRoute)
 
-  override fun goToChatDetail(location: MainNavigationDetailLocation.Chats) = goTo(location)
-  override fun goToCallDetail(location: MainNavigationDetailLocation.Calls) = goTo(location)
-  override fun exitDetailLocation() = goTo(MainNavigationDetailLocation.Empty)
+  override fun goToChatDetail(location: MainDetailRoute.Chats) = goTo(location)
+  override fun goToCallDetail(location: MainDetailRoute.Calls) = goTo(location)
 }

app/src/main/java/org/thoughtcrime/securesms/main/MainNavigationViewModel.kt

@@ -5,12 +5,6 @@
 
 package org.thoughtcrime.securesms.main
 
-import androidx.compose.material3.adaptive.ExperimentalMaterial3AdaptiveApi
-import androidx.compose.material3.adaptive.layout.ThreePaneScaffoldRole
-import androidx.compose.material3.adaptive.navigation.BackNavigationBehavior
-import androidx.compose.material3.adaptive.navigation.ThreePaneScaffoldNavigator
-import androidx.compose.runtime.snapshotFlow
-import androidx.compose.runtime.snapshots.SnapshotStateList
 import androidx.lifecycle.SavedStateHandle
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.ViewModelProvider
@@ -18,31 +12,27 @@ import androidx.lifecycle.createSavedStateHandle
 import androidx.lifecycle.viewModelScope
 import androidx.lifecycle.viewmodel.CreationExtras
 import io.reactivex.rxjava3.core.Observable
-import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
-import kotlinx.coroutines.flow.SharedFlow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.flow.combine
-import kotlinx.coroutines.flow.flatMapLatest
-import kotlinx.coroutines.flow.flowOf
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.flow.update
-import kotlinx.coroutines.isActive
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.reactive.asFlow
 import kotlinx.coroutines.rx3.asObservable
 import kotlinx.coroutines.withContext
 import kotlinx.coroutines.withTimeoutOrNull
+import org.signal.core.ui.compose.split.ListDetailEvents
+import org.signal.core.ui.compose.split.ListDetailNavigator
+import org.signal.core.ui.compose.split.PaneAnchor
+import org.signal.core.ui.compose.split.exitDetail
 import org.signal.core.util.logging.Log
-import org.thoughtcrime.securesms.calls.CallsBackStack
 import org.thoughtcrime.securesms.calls.log.CallLogRow
-import org.thoughtcrime.securesms.chats.ChatsBackStack
 import org.thoughtcrime.securesms.components.settings.app.notifications.profiles.NotificationProfilesRepository
 import org.thoughtcrime.securesms.components.snackbars.SnackbarStateConsumerRegistry
 import org.thoughtcrime.securesms.dependencies.AppDependencies
@@ -53,26 +43,24 @@ import org.thoughtcrime.securesms.notifications.profiles.NotificationProfile
 import org.thoughtcrime.securesms.recipients.Recipient
 import org.thoughtcrime.securesms.recipients.RecipientId
 import org.thoughtcrime.securesms.stories.Stories
-import org.thoughtcrime.securesms.stories.StoriesBackStack
-import org.thoughtcrime.securesms.util.delegate
-import org.thoughtcrime.securesms.window.AppScaffoldNavigator
 import java.util.Optional
 import kotlin.time.Duration.Companion.milliseconds
 
-@OptIn(ExperimentalMaterial3AdaptiveApi::class)
 class MainNavigationViewModel(
   savedStateHandle: SavedStateHandle,
-  initialListLocation: MainNavigationListLocation = MainNavigationListLocation.CHATS
+  initialListLocation: MainListRoute = MainListRoute.Chats
 ) : ViewModel(), MainNavigationRouter {
 
   companion object {
     private val TAG = Log.tag(MainNavigationViewModel::class)
-    private const val LOCK_PANE_TO_SECONDARY = "lock_pane_to_secondary"
     private const val NAV_PREFETCH_TIMEOUT_MS = 250L
+    private const val CHATS_BACK_STACK_KEY = "chats_back_stack_v2"
+    private const val CALLS_BACK_STACK_KEY = "calls_back_stack_v2"
+    private const val STORIES_BACK_STACK_KEY = "stories_back_stack_v2"
   }
 
   class Factory(
-    private val initialListLocation: MainNavigationListLocation = MainNavigationListLocation.CHATS
+    private val initialListLocation: MainListRoute = MainListRoute.Chats
   ) : ViewModelProvider.Factory {
     override fun <T : ViewModel> create(modelClass: Class<T>, extras: CreationExtras): T {
       val savedStateHandle = extras.createSavedStateHandle()
@@ -83,45 +71,39 @@ class MainNavigationViewModel(
 
   private val megaphoneRepository = AppDependencies.megaphoneRepository
 
-  private var navigator: AppScaffoldNavigator<Any>? = null
-  private var navigatorScope: CoroutineScope? = null
-
   private var captureChatListSnapshot: (suspend () -> Unit)? = null
-  private var isSplitPane: Boolean = false
 
-  private val chatsBackStack: ChatsBackStack = ChatsBackStack(savedStateHandle)
-  val chatsBackStackEntries: SnapshotStateList<MainNavigationDetailLocation>
-    get() = chatsBackStack.entries
+  /**
+   * The stacks behind the main window, one per tab. The archive gets no stack of its own: it is a list
+   * pushed onto the chats stack, so that opening it keeps whatever chat was open beside it.
+   */
+  val navigator = ListDetailNavigator<MainListRoute, MainDetailRoute>(
+    savedStateHandle = savedStateHandle,
+    scope = viewModelScope,
+    stackKeys = mapOf(
+      MainListRoute.Chats to CHATS_BACK_STACK_KEY,
+      MainListRoute.Calls to CALLS_BACK_STACK_KEY,
+      MainListRoute.Stories to STORIES_BACK_STACK_KEY
+    ),
+    initialRoot = initialListLocation.tab
+  )
 
-  private val callsBackStack: CallsBackStack = CallsBackStack(savedStateHandle)
-  val callsBackStackEntries: SnapshotStateList<MainNavigationDetailLocation>
-    get() = callsBackStack.entries
+  /** The currently selected tab. */
+  val currentTab: StateFlow<MainListRoute> = navigator.currentRoot
 
-  private val storiesBackStack: StoriesBackStack = StoriesBackStack(savedStateHandle)
-  val storiesBackStackEntries: SnapshotStateList<MainNavigationDetailLocation>
-    get() = storiesBackStack.entries
+  /** How a split-pane window currently divides the list and detail panes. */
+  val paneAnchor: StateFlow<PaneAnchor> = navigator.paneAnchor
 
-  private val currentTabBackStack: MainDetailBackStack?
-    get() {
-      val currentListLocation = internalMainNavigationState.value.currentListLocation
-      return when {
-        currentListLocation.isChatsTab -> chatsBackStack
-        currentListLocation == MainNavigationListLocation.CALLS -> callsBackStack
-        currentListLocation == MainNavigationListLocation.STORIES -> storiesBackStack
-        else -> null
-      }
-    }
-
-  private val internalIsFullScreenPane = MutableStateFlow(false)
-  val isFullScreenPane: StateFlow<Boolean> = internalIsFullScreenPane
+  /** Whether one pane currently occupies the whole window. Only meaningful in split-pane layouts. */
+  val isFullScreenPane: StateFlow<Boolean> = navigator.isFullScreenPane
 
   val observableActiveRecipientId: Observable<Optional<out RecipientId>> =
-    snapshotFlow { chatsBackStack.activeRecipientId }
+    navigator.snapshotsOf(MainListRoute.Chats) { activeRecipientId }
       .combine(isFullScreenPane) { id, expanded -> if (expanded) Optional.ofNullable(null) else Optional.ofNullable(id) }
       .asObservable()
 
   val observableActiveCallId: Observable<Optional<out CallLogRow.Id>> =
-    snapshotFlow { callsBackStack.activeCallId }
+    navigator.snapshotsOf(MainListRoute.Calls) { activeCallId }
       .combine(isFullScreenPane) { id, expanded -> if (expanded) Optional.ofNullable(null) else Optional.ofNullable(id) }
       .asObservable()
 
@@ -133,56 +115,26 @@ class MainNavigationViewModel(
 
   private val notificationProfilesRepository: NotificationProfilesRepository = NotificationProfilesRepository()
 
-  private val internalMainNavigationState = MutableStateFlow(MainNavigationState(currentListLocation = initialListLocation))
-  val mainNavigationState: StateFlow<MainNavigationState> = internalMainNavigationState
+  private val internalMainNavigationBarState = MutableStateFlow(MainNavigationBarState(currentListLocation = initialListLocation))
+  val mainNavigationBarState: StateFlow<MainNavigationBarState> = combine(internalMainNavigationBarState, navigator.displayedList) { state, listLocation ->
+    state.copy(currentListLocation = listLocation)
+  }.stateIn(viewModelScope, SharingStarted.Eagerly, MainNavigationBarState(currentListLocation = initialListLocation))
 
-  @OptIn(ExperimentalCoroutinesApi::class)
-  val detailLocation: StateFlow<MainNavigationDetailLocation> = mainNavigationState.flatMapLatest { state ->
-    when {
-      state.currentListLocation.isChatsTab -> {
-        snapshotFlow { chatsBackStack.entries.lastOrNull() ?: MainNavigationDetailLocation.Empty }
-      }
+  /**
+   * The detail content displayed above the current list, or null when the list is showing on its own.
+   */
+  val detailLocation: StateFlow<MainDetailRoute?> = navigator.detail
 
-      state.currentListLocation == MainNavigationListLocation.CALLS -> {
-        snapshotFlow { callsBackStack.entries.lastOrNull() ?: MainNavigationDetailLocation.Empty }
-      }
-
-      state.currentListLocation == MainNavigationListLocation.STORIES -> {
-        snapshotFlow { storiesBackStack.entries.lastOrNull() ?: MainNavigationDetailLocation.Empty }
-      }
-
-      else -> flowOf(MainNavigationDetailLocation.Empty)
-    }
-  }.stateIn(viewModelScope, SharingStarted.Eagerly, MainNavigationDetailLocation.Empty)
+  /**
+   * Whether the current tab is displaying detail content.
+   */
+  val hasDetailContent: StateFlow<Boolean> = navigator.hasDetail
 
   /**
    * This is Rx because these are still accessed from Java.
    */
-  private val internalTabClickEvents: MutableSharedFlow<MainNavigationListLocation> = MutableSharedFlow()
-  val tabClickEventsObservable: Observable<MainNavigationListLocation> = internalTabClickEvents.asObservable()
-
-  private var earlyNavigationListLocationRequested: MainNavigationListLocation? = null
-
-  private val internalPaneFocusRequests = MutableSharedFlow<ThreePaneScaffoldRole?>()
-  val paneFocusRequests: SharedFlow<ThreePaneScaffoldRole?> = internalPaneFocusRequests
-
-  private var earlyFocusedPaneRequested: ThreePaneScaffoldRole? = null
-
-  /**
-   * The navigator and its scope are owned by the MainActivity composition. That composition is disposed (and its scope
-   * cancelled) whenever the activity is recreated, but this view-model survives, so we can be left holding a navigator
-   * that can no longer do anything. Requests that arrive in that window have to be deferred until [wrapNavigator]
-   * hands us a live one, otherwise they're silently dropped.
-   */
-  private val hasLiveNavigator: Boolean
-    get() = navigator != null && navigatorScope?.isActive == true
-
-  /**
-   * Which pane we display to the user at a given time should be driven solely by user intention. There are cases
-   * where the user can change configurations (such as opening a foldable) and we will restore state and errantly
-   * take them back into a PRIMARY pane. This boolean helps avoid these cases.
-   */
-  private var lockPaneToSecondary: Boolean by savedStateHandle.delegate(LOCK_PANE_TO_SECONDARY, true)
+  private val internalTabClickEvents: MutableSharedFlow<MainListRoute> = MutableSharedFlow()
+  val tabClickEventsObservable: Observable<MainListRoute> = internalTabClickEvents.asObservable()
 
   val snackbarRegistry = SnackbarStateConsumerRegistry()
 
@@ -204,83 +156,21 @@ class MainNavigationViewModel(
     }
   }
 
-  fun onPaneAnchorChanged(isFullScreenPane: Boolean) {
-    internalIsFullScreenPane.update { isFullScreenPane }
+  /**
+   * The user dragged the pane divider to [anchor].
+   */
+  fun onPaneAnchorSelected(anchor: PaneAnchor) {
+    navigator.processEvent(ListDetailEvents.AnchorSelected(anchor))
   }
 
-  fun setChatListSnapshotCaptureProvider(capture: suspend () -> Unit) {
+  /** Set from the MainActivity composition, and cleared when it is disposed. */
+  fun setChatListSnapshotCaptureProvider(capture: (suspend () -> Unit)?) {
     captureChatListSnapshot = capture
   }
 
-  fun onSplitPaneChanged(isSplitPane: Boolean) {
-    this@MainNavigationViewModel.isSplitPane = isSplitPane
+  override fun goTo(location: MainDetailRoute) = setDetailLocation(location)
 
-    if (!isSplitPane) {
-      if (currentTabBackStack?.isEmpty == true) {
-        lockPaneToSecondary = true
-        setFocusedPane(ThreePaneScaffoldRole.Secondary)
-      }
-    }
-  }
-
-  /**
-   * Sets the navigator on the view-model. This wraps the given navigator in our own delegating implementation
-   * such that we can react to navigateTo/Back signals and maintain proper state for internalDetailLocation.
-   */
-  fun wrapNavigator(composeScope: CoroutineScope, threePaneScaffoldNavigator: ThreePaneScaffoldNavigator<Any>): AppScaffoldNavigator<Any> {
-    this.navigatorScope = composeScope
-    this.navigator = Nav(threePaneScaffoldNavigator)
-
-    val pendingFocus = earlyFocusedPaneRequested
-    earlyFocusedPaneRequested = null
-
-    earlyNavigationListLocationRequested?.let {
-      goTo(it)
-    }
-
-    earlyNavigationListLocationRequested = null
-
-    pendingFocus?.let { role ->
-      if (role == ThreePaneScaffoldRole.Primary) {
-        lockPaneToSecondary = false
-      }
-      setFocusedPane(role)
-    }
-
-    return this.navigator!!
-  }
-
-  fun setFocusedPane(role: ThreePaneScaffoldRole) {
-    val roleToGoTo = if (lockPaneToSecondary) {
-      ThreePaneScaffoldRole.Secondary
-    } else {
-      role
-    }
-
-    if (!hasLiveNavigator) {
-      earlyFocusedPaneRequested = roleToGoTo
-      return
-    }
-
-    navigatorScope?.launch {
-      navigator?.navigateTo(roleToGoTo)
-    }
-
-    viewModelScope.launch {
-      internalPaneFocusRequests.emit(roleToGoTo)
-    }
-  }
-
-  /**
-   * Navigates to the requested location. If the navigator is not present, this functionally sets our
-   * "default" location to that specified, and we will route the user there when the navigator is set.
-   *
-   * This does not update what panel is currently focused, so that we can perform actions (such as first
-   * render) *before* swapping panes. This helps to prevent flashing / duplicate loads.
-   */
-  override fun goTo(location: MainNavigationDetailLocation) = setDetailLocation(location)
-
-  private suspend fun MainNavigationDetailLocation.Conversation.withPreloadedWallpaper(): MainNavigationDetailLocation.Conversation {
+  private suspend fun MainDetailRoute.Conversation.withPreloadedWallpaper(): MainDetailRoute.Conversation {
     val args = conversationArgs
     val liveRecipient = Recipient.live(args.recipientId)
     val recipientSnapshot = liveRecipient.get()
@@ -306,26 +196,26 @@ class MainNavigationViewModel(
     return copy(conversationArgs = updatedArgs)
   }
 
-  private fun setDetailLocation(location: MainNavigationDetailLocation) {
-    lockPaneToSecondary = false
-    val currentListLocation = internalMainNavigationState.value.currentListLocation
-
+  private fun setDetailLocation(location: MainDetailRoute) {
     when (location) {
-      is MainNavigationDetailLocation.Empty if currentListLocation.isChatsTab -> clearDetailLocation(chatsBackStack)
-      is MainNavigationDetailLocation.Empty if currentListLocation == MainNavigationListLocation.CALLS -> clearDetailLocation(callsBackStack)
-      is MainNavigationDetailLocation.Empty if currentListLocation == MainNavigationListLocation.STORIES -> clearDetailLocation(storiesBackStack)
-      is MainNavigationDetailLocation.Chats -> pushChatsDetailLocation(location)
-      is MainNavigationDetailLocation.Conversation -> goToConversation(location)
-      is MainNavigationDetailLocation.Calls, is MainNavigationDetailLocation.CallLinkDetails -> pushCallsDetailLocation(location)
-      is MainNavigationDetailLocation.Stories -> pushStoriesDetailLocation(location)
-      is MainNavigationDetailLocation.Empty -> Unit
+      is MainDetailRoute.Chats -> pushChatsDetailLocation(location)
+      is MainDetailRoute.Conversation -> goToConversation(location)
+      is MainDetailRoute.Calls, is MainDetailRoute.CallLinkDetails -> pushCallsDetailLocation(location)
+      is MainDetailRoute.Stories -> pushStoriesDetailLocation(location)
     }
   }
 
-  private fun goToConversation(location: MainNavigationDetailLocation.Conversation) {
+  /**
+   * Drops the detail content above the current list, leaving that list displayed on its own.
+   */
+  override fun exitDetailLocation() {
+    navigator.processEvent(ListDetailEvents.ExitDetail)
+  }
+
+  private fun goToConversation(location: MainDetailRoute.Conversation) {
     val captureSnapshot = captureChatListSnapshot
 
-    if (captureSnapshot == null || !hasLiveNavigator) {
+    if (captureSnapshot == null) {
       // share intent or process restore - push synchronously, since there's no chat-list snapshot to capture and no need to preload a wallpaper
       pushChatsDetailLocation(location)
     } else {
@@ -336,77 +226,42 @@ class MainNavigationViewModel(
     }
   }
 
-  private fun pushChatsDetailLocation(location: MainNavigationDetailLocation) {
-    if (location is MainNavigationDetailLocation.Chats && chatsBackStack.activeRecipientId != location.controllerKey) {
-      chatsBackStack.reset()
+  private fun pushChatsDetailLocation(location: MainDetailRoute) {
+    val chatsBackStack = navigator[MainListRoute.Chats]
+    if (location is MainDetailRoute.Chats && chatsBackStack.activeRecipientId != location.controllerKey) {
+      chatsBackStack.exitDetail()
     }
 
-    chatsBackStack.push(location)
-    setFocusedPane(ThreePaneScaffoldRole.Primary)
+    navigator.processEvent(ListDetailEvents.Push(location, MainListRoute.Chats))
   }
 
-  fun popChatsDetailLocation() = popDetailLocation(chatsBackStack)
-
-  private fun pushCallsDetailLocation(location: MainNavigationDetailLocation) {
-    if (location is MainNavigationDetailLocation.Calls && callsBackStack.activeCallId != location.controllerKey) {
-      callsBackStack.reset()
+  private fun pushCallsDetailLocation(location: MainDetailRoute) {
+    val callsBackStack = navigator[MainListRoute.Calls]
+    if (location is MainDetailRoute.Calls && callsBackStack.activeCallId != location.controllerKey) {
+      callsBackStack.exitDetail()
     }
 
-    callsBackStack.push(location)
-    setFocusedPane(ThreePaneScaffoldRole.Primary)
+    navigator.processEvent(ListDetailEvents.Push(location, MainListRoute.Calls))
   }
 
-  fun popCallsDetailLocation() = popDetailLocation(callsBackStack)
-
-  private fun pushStoriesDetailLocation(location: MainNavigationDetailLocation) {
-    storiesBackStack.push(location)
-    setFocusedPane(ThreePaneScaffoldRole.Primary)
+  private fun pushStoriesDetailLocation(location: MainDetailRoute) {
+    navigator.processEvent(ListDetailEvents.Push(location, MainListRoute.Stories))
   }
 
-  fun popStoriesDetailLocation() = popDetailLocation(storiesBackStack)
-
-  private fun popDetailLocation(backStack: MainDetailBackStack) {
-    backStack.pop()
-
-    if (backStack.isEmpty) {
-      lockPaneToSecondary = true
-      popDetailPane()
-    }
+  /** Pops the stack belonging to whichever tab the user is currently on. */
+  fun popCurrentDetailLocation() {
+    navigator.processEvent(ListDetailEvents.Back)
   }
 
-  private fun clearDetailLocation(backStack: MainDetailBackStack) {
-    backStack.reset()
-    if (!isSplitPane) {
-      lockPaneToSecondary = true
-      popDetailPane()
-    }
-  }
-
-  private fun popDetailPane() {
-    navigatorScope?.launch {
-      navigator?.let { scaffoldNavigator ->
-        if (scaffoldNavigator.canNavigateBack()) {
-          scaffoldNavigator.navigateBack()
-        }
-      }
-    }
-
-    viewModelScope.launch {
-      internalPaneFocusRequests.emit(ThreePaneScaffoldRole.Secondary)
-    }
-  }
-
-  override fun goTo(location: MainNavigationListLocation) {
-    lockPaneToSecondary = true
-
-    if (navigator == null) {
-      earlyNavigationListLocationRequested = location
-      return
-    }
-
-    internalMainNavigationState.update {
-      it.copy(currentListLocation = location)
-    }
+  /** Switching tabs only changes which stack is displayed; each tab comes back to whatever it had open. */
+  override fun goTo(location: MainListRoute) {
+    navigator.processEvent(
+      ListDetailEvents.GoToList(
+        listRoute = location,
+        root = location.tab,
+        push = location == MainListRoute.Archive
+      )
+    )
   }
 
   fun goToCameraFirstStoryCapture() {
@@ -436,7 +291,14 @@ class MainNavigationViewModel(
   }
 
   fun refreshNavigationBarState() {
-    internalMainNavigationState.update { it.copy(compact = SignalStore.settings.useCompactNavigationBar, isStoriesFeatureEnabled = Stories.isFeatureEnabled()) }
+    internalMainNavigationBarState.update {
+      it.copy(
+        compact = SignalStore.settings.useCompactNavigationBar,
+        destinations = MainNavigationBarState.ALL_DESTINATIONS.filter { destination ->
+          destination != MainListRoute.Stories || Stories.isFeatureEnabled()
+        }
+      )
+    }
   }
 
   fun getNotificationProfiles(): Flow<List<NotificationProfile>> {
@@ -444,38 +306,37 @@ class MainNavigationViewModel(
   }
 
   fun onChatsSelected() {
-    onTabSelected(MainNavigationListLocation.CHATS)
+    onTabSelected(MainListRoute.Chats)
   }
 
   fun onArchiveSelected() {
-    onTabSelected(MainNavigationListLocation.ARCHIVE)
+    onTabSelected(MainListRoute.Archive)
   }
 
   fun onCallsSelected() {
-    onTabSelected(MainNavigationListLocation.CALLS)
+    onTabSelected(MainListRoute.Calls)
   }
 
   fun onStoriesSelected() {
-    onTabSelected(MainNavigationListLocation.STORIES)
+    onTabSelected(MainListRoute.Stories)
   }
 
-  private fun onTabSelected(destination: MainNavigationListLocation) {
+  private fun onTabSelected(destination: MainListRoute) {
     viewModelScope.launch {
-      val currentTab = internalMainNavigationState.value.currentListLocation
-      if (currentTab == destination) {
-        internalPaneFocusRequests.emit(ThreePaneScaffoldRole.Secondary)
+      val displayed = navigator.displayedList.value
+      if (displayed == destination) {
+        navigator.processEvent(ListDetailEvents.RevealList)
         internalTabClickEvents.emit(destination)
       } else {
-        setFocusedPane(ThreePaneScaffoldRole.Secondary)
         goTo(destination)
       }
     }
   }
 
-  private fun <T : Any> performStoreUpdate(flow: Flow<T>, fn: (T, MainNavigationState) -> MainNavigationState) {
+  private fun <T : Any> performStoreUpdate(flow: Flow<T>, fn: (T, MainNavigationBarState) -> MainNavigationBarState) {
     viewModelScope.launch {
       flow.collectLatest { item ->
-        internalMainNavigationState.update { state -> fn(item, state) }
+        internalMainNavigationBarState.update { state -> fn(item, state) }
       }
     }
   }
@@ -483,18 +344,4 @@ class MainNavigationViewModel(
   enum class NavigationEvent {
     STORY_CAMERA_FIRST
   }
-
-  /**
-   * Ensures that when the user navigates back from the PRIMARY to SECONDARY pane, we lock our pane until they choose another primary
-   * piece of content via [goTo].
-   */
-  private inner class Nav<T>(delegate: ThreePaneScaffoldNavigator<T>) : AppScaffoldNavigator<T>(delegate) {
-    override suspend fun navigateBack(backNavigationBehavior: BackNavigationBehavior): Boolean {
-      val result = super.navigateBack(backNavigationBehavior)
-      if (result) {
-        lockPaneToSecondary = true
-      }
-      return result
-    }
-  }
 }

app/src/main/java/org/thoughtcrime/securesms/main/MainToolbar.kt

@@ -67,6 +67,7 @@ import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.res.vectorResource
 import androidx.compose.ui.semantics.contentDescription
 import androidx.compose.ui.semantics.semantics
+import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import org.signal.core.ui.compose.DayNightPreviews
 import org.signal.core.ui.compose.DropdownMenus
@@ -152,13 +153,19 @@ enum class MainToolbarMode(val crossFadeKey: CrossFadeKey) {
     FULL,
     BASIC
   }
+
+  /**
+   * The search bar is inset from the start edge of the list pane, so the pane it sits in has to be too.
+   */
+  val listPaddingStart: Dp
+    get() = if (this == SEARCH) 24.dp else 0.dp
 }
 
 data class MainToolbarState(
   val toolbarColor: Color? = null,
   val self: Recipient = Recipient.UNKNOWN,
   val mode: MainToolbarMode = MainToolbarMode.FULL,
-  val destination: MainNavigationListLocation = MainNavigationListLocation.CHATS,
+  val destination: MainListRoute = MainListRoute.Chats,
   val chatFilter: ConversationFilter = ConversationFilter.OFF,
   val callFilter: CallLogFilter = CallLogFilter.ALL,
   val hasUnreadPayments: Boolean = false,
@@ -435,7 +442,7 @@ private fun PrimaryToolbar(
       NotificationProfileAction(state, callback)
       ProxyAction(state, callback)
 
-      if (state.destination == MainNavigationListLocation.STORIES && SignalStore.labs.storyArchive) {
+      if (state.destination == MainListRoute.Stories && SignalStore.labs.storyArchive) {
         IconButtons.IconButton(
           onClick = callback::onStoryArchiveClick
         ) {
@@ -471,10 +478,10 @@ private fun PrimaryToolbar(
         controller = controller
       ) {
         when (state.destination) {
-          MainNavigationListLocation.ARCHIVE -> Unit
-          MainNavigationListLocation.CHATS -> ChatDropdownItems(state, callback, dismiss)
-          MainNavigationListLocation.CALLS -> CallDropdownItems(state.callFilter, callback, dismiss)
-          MainNavigationListLocation.STORIES -> StoryDropDownItems(callback, dismiss)
+          MainListRoute.Archive -> Unit
+          MainListRoute.Chats -> ChatDropdownItems(state, callback, dismiss)
+          MainListRoute.Calls -> CallDropdownItems(state.callFilter, callback, dismiss)
+          MainListRoute.Stories -> StoryDropDownItems(callback, dismiss)
         }
       }
     }
@@ -790,7 +797,7 @@ private fun FullMainToolbarPreview() {
       state = MainToolbarState(
         self = Recipient(isResolving = false),
         mode = mode,
-        destination = MainNavigationListLocation.CHATS,
+        destination = MainListRoute.Chats,
         hasEnabledNotificationProfile = true,
         proxyState = MainToolbarState.ProxyState.CONNECTED,
         hasFailedBackups = true,

app/src/main/java/org/thoughtcrime/securesms/main/MainToolbarViewModel.kt

@@ -83,19 +83,19 @@ class MainToolbarViewModel : ViewModel() {
   fun isInActionMode(): Boolean = state.value.mode == MainToolbarMode.ACTION_MODE
 
   fun presentToolbarForConversationListFragment() {
-    setToolbarMode(MainToolbarMode.FULL, destination = MainNavigationListLocation.CHATS, overwriteExtraMode = false)
+    setToolbarMode(MainToolbarMode.FULL, destination = MainListRoute.Chats, overwriteExtraMode = false)
   }
 
   fun presentToolbarForConversationListArchiveFragment() {
-    setToolbarMode(MainToolbarMode.BASIC, destination = MainNavigationListLocation.CHATS)
+    setToolbarMode(MainToolbarMode.BASIC, destination = MainListRoute.Chats)
   }
 
   fun presentToolbarForStoriesLandingFragment() {
-    setToolbarMode(MainToolbarMode.FULL, destination = MainNavigationListLocation.STORIES)
+    setToolbarMode(MainToolbarMode.FULL, destination = MainListRoute.Stories)
   }
 
   fun presentToolbarForCallLogFragment() {
-    setToolbarMode(MainToolbarMode.FULL, destination = MainNavigationListLocation.CALLS)
+    setToolbarMode(MainToolbarMode.FULL, destination = MainListRoute.Calls)
   }
 
   fun presentToolbarForMultiselect() {
@@ -104,7 +104,7 @@ class MainToolbarViewModel : ViewModel() {
 
   fun presentToolbarForCurrentDestination() {
     when (state.value.destination) {
-      MainNavigationListLocation.ARCHIVE -> setToolbarMode(MainToolbarMode.BASIC)
+      MainListRoute.Archive -> setToolbarMode(MainToolbarMode.BASIC)
       else -> setToolbarMode(MainToolbarMode.FULL)
     }
   }
@@ -112,7 +112,7 @@ class MainToolbarViewModel : ViewModel() {
   @JvmOverloads
   fun setToolbarMode(
     mode: MainToolbarMode,
-    destination: MainNavigationListLocation? = null,
+    destination: MainListRoute? = null,
     overwriteExtraMode: Boolean = true
   ) {
     val previousMode = internalStateFlow.value.mode

app/src/main/java/org/thoughtcrime/securesms/recipients/ui/RecipientPickerScaffold.kt

@@ -48,14 +48,13 @@ import org.thoughtcrime.securesms.window.rememberAppScaffoldNavigator
 @Composable
 fun RecipientPickerScaffold(
   title: String,
-  forceSplitPane: Boolean,
   onNavigateUpClick: () -> Unit,
   topAppBarActions: @Composable () -> Unit,
   snackbarHostState: SnackbarHostState,
   primaryContent: @Composable () -> Unit,
   floatingActionButton: (@Composable () -> Unit)? = null
 ) {
-  val isSplitPane = LocalResources.current.rememberIsSplitPane(forceSplitPane)
+  val isSplitPane = LocalResources.current.rememberIsSplitPane()
   val windowSizeClass = currentWindowAdaptiveInfo().windowSizeClass
 
   AppScaffold(
@@ -133,7 +132,6 @@ private fun RecipientPickerScaffoldPreview() {
   Previews.Preview {
     RecipientPickerScaffold(
       title = "Screen Title",
-      forceSplitPane = false,
       onNavigateUpClick = {},
       topAppBarActions = {},
       snackbarHostState = SnackbarHostState(),

app/src/main/java/org/thoughtcrime/securesms/stories/StoriesBackStack.kt

@@ -1,51 +0,0 @@
-/*
- * Copyright 2026 Signal Messenger, LLC
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-package org.thoughtcrime.securesms.stories
-
-import androidx.compose.runtime.mutableStateListOf
-import androidx.compose.runtime.saveable.Saver
-import androidx.compose.runtime.snapshots.SnapshotStateList
-import androidx.lifecycle.SavedStateHandle
-import androidx.lifecycle.viewmodel.compose.SavedStateHandleSaveableApi
-import androidx.lifecycle.viewmodel.compose.saveable
-import org.thoughtcrime.securesms.main.MainDetailBackStack
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-
-/**
- * Controls the navigation stack used by the stories screen.
- */
-@OptIn(SavedStateHandleSaveableApi::class)
-class StoriesBackStack(savedStateHandle: SavedStateHandle) : MainDetailBackStack {
-
-  companion object {
-    private const val KEY = "stories_back_stack"
-
-    val saver: Saver<SnapshotStateList<MainNavigationDetailLocation>, ArrayList<MainNavigationDetailLocation>> = Saver(
-      save = { ArrayList(it) },
-      restore = { mutableStateListOf(*it.toTypedArray()) }
-    )
-  }
-
-  override val entries: SnapshotStateList<MainNavigationDetailLocation> = savedStateHandle.saveable(
-    key = KEY,
-    saver = saver
-  ) {
-    mutableStateListOf(MainNavigationDetailLocation.Empty)
-  }
-
-  override fun push(location: MainNavigationDetailLocation) {
-    when {
-      location is MainNavigationDetailLocation.Empty || location == entries.lastOrNull() -> Unit
-
-      location.isContentRoot -> {
-        entries.removeAll { it !is MainNavigationDetailLocation.Empty }
-        entries.add(location)
-      }
-
-      else -> entries.add(location)
-    }
-  }
-}

app/src/main/java/org/thoughtcrime/securesms/stories/StoriesNavigation.kt

@@ -16,47 +16,52 @@ import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.key
 import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
 import androidx.fragment.compose.AndroidFragment
 import androidx.fragment.compose.rememberFragmentState
 import androidx.navigation3.runtime.EntryProviderScope
 import androidx.navigation3.runtime.NavKey
+import org.signal.core.ui.compose.split.detailEntry
 import org.signal.core.ui.navigation.TransitionSpecs
 import org.thoughtcrime.securesms.MainNavigator
 import org.thoughtcrime.securesms.compose.FragmentBackHandler
 import org.thoughtcrime.securesms.compose.FragmentBackPressedState
-import org.thoughtcrime.securesms.main.EmptyDetailScreen
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
+import org.thoughtcrime.securesms.main.MainDetailRoute
 import org.thoughtcrime.securesms.stories.archive.StoryArchiveScreen
+import org.thoughtcrime.securesms.stories.landing.StoriesLandingFragment
 import org.thoughtcrime.securesms.stories.my.MyStoriesFragment
 import org.thoughtcrime.securesms.stories.settings.StorySettingsNavHostFragment
 
-fun EntryProviderScope<NavKey>.storiesNavEntries() {
-  entry<MainNavigationDetailLocation.Empty> {
-    NoStorySelectedEntry()
-  }
-
-  entry<MainNavigationDetailLocation.Stories.Archive>(
+fun EntryProviderScope<NavKey>.registerStoriesTabDetailRoutes() {
+  detailEntry<MainDetailRoute.Stories.Archive>(
     metadata = TransitionSpecs.None.metadata
   ) {
     StoryArchiveEntry()
   }
 
-  entry<MainNavigationDetailLocation.Stories.MyStories>(
+  detailEntry<MainDetailRoute.Stories.MyStories>(
     metadata = TransitionSpecs.None.metadata
   ) {
     MyStoriesEntry()
   }
 
-  entry<MainNavigationDetailLocation.Stories.PrivacySettings>(
+  detailEntry<MainDetailRoute.Stories.PrivacySettings>(
     metadata = TransitionSpecs.None.metadata
   ) {
     StoryPrivacySettingsEntry()
   }
 }
 
+/**
+ * List pane content for the stories tab.
+ */
 @Composable
-private fun NoStorySelectedEntry() {
-  EmptyDetailScreen()
+fun StoriesListPane(modifier: Modifier = Modifier) {
+  AndroidFragment(
+    clazz = StoriesLandingFragment::class.java,
+    fragmentState = rememberFragmentState(),
+    modifier = modifier
+  )
 }
 
 @Composable
@@ -71,28 +76,26 @@ private fun StoryArchiveEntry() {
 
 @Composable
 private fun MyStoriesEntry() {
-  val fragmentState = key(MainNavigationDetailLocation.Stories.MyStories) { rememberFragmentState() }
-  val backPressedState = remember { FragmentBackPressedState() }
-  FragmentBackHandler(backPressedState)
+  val fragmentState = key(MainDetailRoute.Stories.MyStories) { rememberFragmentState() }
 
   informNavigatorWeAreReady()
 
+  // No FragmentBackHandler: the fragment has no back state of its own, so back belongs to the display
+  // that put this entry on the stack.
   AndroidFragment(
     clazz = MyStoriesFragment::class.java,
     fragmentState = fragmentState,
-    modifier = androidx.compose.ui.Modifier
+    modifier = Modifier
       .fillMaxSize()
       .background(MaterialTheme.colorScheme.background)
       .statusBarsPadding()
       .navigationBarsPadding()
-  ) { fragment ->
-    backPressedState.attach(fragment)
-  }
+  )
 }
 
 @Composable
 private fun StoryPrivacySettingsEntry() {
-  val fragmentState = key(MainNavigationDetailLocation.Stories.PrivacySettings) { rememberFragmentState() }
+  val fragmentState = key(MainDetailRoute.Stories.PrivacySettings) { rememberFragmentState() }
   val backPressedState = remember { FragmentBackPressedState() }
   FragmentBackHandler(backPressedState)
 
@@ -101,7 +104,7 @@ private fun StoryPrivacySettingsEntry() {
   AndroidFragment(
     clazz = StorySettingsNavHostFragment::class.java,
     fragmentState = fragmentState,
-    modifier = androidx.compose.ui.Modifier
+    modifier = Modifier
       .fillMaxSize()
       .background(MaterialTheme.colorScheme.background)
       .statusBarsPadding()

app/src/main/java/org/thoughtcrime/securesms/stories/landing/StoriesLandingFragment.kt

@@ -4,7 +4,6 @@ import android.content.Intent
 import android.net.Uri
 import android.os.Bundle
 import android.view.View
-import androidx.activity.OnBackPressedCallback
 import androidx.compose.ui.platform.ComposeView
 import androidx.core.app.ActivityOptionsCompat
 import androidx.core.view.ViewCompat
@@ -33,11 +32,10 @@ import org.thoughtcrime.securesms.conversation.mutiselect.forward.MultiselectFor
 import org.thoughtcrime.securesms.database.model.MmsMessageRecord
 import org.thoughtcrime.securesms.database.model.StoryViewState
 import org.thoughtcrime.securesms.dependencies.AppDependencies
-import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
-import org.thoughtcrime.securesms.main.MainNavigationListLocation
+import org.thoughtcrime.securesms.main.MainDetailRoute
+import org.thoughtcrime.securesms.main.MainListRoute
 import org.thoughtcrime.securesms.main.MainNavigationViewModel
 import org.thoughtcrime.securesms.main.MainSnackbarHostKey
-import org.thoughtcrime.securesms.main.MainToolbarMode
 import org.thoughtcrime.securesms.main.MainToolbarViewModel
 import org.thoughtcrime.securesms.main.Material3OnScrollHelperBinder
 import org.thoughtcrime.securesms.safety.SafetyNumberBottomSheet
@@ -149,23 +147,8 @@ class StoriesLandingFragment : DSLSettingsFragment(layoutId = R.layout.stories_l
       }
     }
 
-    requireActivity().onBackPressedDispatcher.addCallback(
-      viewLifecycleOwner,
-      object : OnBackPressedCallback(true) {
-        override fun handleOnBackPressed() {
-          if (!closeSearchIfOpen()) {
-            if (mainNavigationViewModel.storiesBackStackEntries.last() != MainNavigationDetailLocation.Empty) {
-              mainNavigationViewModel.popStoriesDetailLocation()
-            } else {
-              mainNavigationViewModel.onChatsSelected()
-            }
-          }
-        }
-      }
-    )
-
     lifecycleDisposable += mainNavigationViewModel.tabClickEventsObservable
-      .filter { it == MainNavigationListLocation.STORIES }
+      .filter { it == MainListRoute.Stories }
       .subscribeBy(onNext = {
         val layoutManager = recyclerView?.layoutManager as? LinearLayoutManager ?: return@subscribeBy
         if (layoutManager.findFirstVisibleItemPosition() <= LIST_SMOOTH_SCROLL_TO_TOP_THRESHOLD) {
@@ -288,7 +271,7 @@ class StoriesLandingFragment : DSLSettingsFragment(layoutId = R.layout.stories_l
 
   private fun openStoryViewer(model: StoriesLandingItem.Model, preview: View, isFromInfoContextMenuAction: Boolean) {
     if (model.data.storyRecipient.isMyStory) {
-      mainNavigationViewModel.goTo(MainNavigationDetailLocation.Stories.MyStories)
+      mainNavigationViewModel.goTo(MainDetailRoute.Stories.MyStories)
     } else if (model.data.primaryStory.messageRecord.isOutgoing && model.data.primaryStory.messageRecord.isFailed) {
       if (model.data.primaryStory.messageRecord.isIdentityMismatchFailure) {
         SafetyNumberBottomSheet
@@ -360,20 +343,4 @@ class StoriesLandingFragment : DSLSettingsFragment(layoutId = R.layout.stories_l
     viewModel.isTransitioningToAnotherScreen = true
     startActivity(intent, options)
   }
-
-  private fun isSearchOpen(): Boolean {
-    return isSearchVisible()
-  }
-
-  private fun isSearchVisible(): Boolean {
-    return mainToolbarViewModel.state.value.mode == MainToolbarMode.SEARCH
-  }
-
-  private fun closeSearchIfOpen(): Boolean {
-    if (isSearchOpen()) {
-      mainToolbarViewModel.setToolbarMode(MainToolbarMode.FULL)
-      return true
-    }
-    return false
-  }
 }

app/src/main/java/org/thoughtcrime/securesms/stories/my/MyStoriesFragment.kt

@@ -2,10 +2,8 @@ package org.thoughtcrime.securesms.stories.my
 
 import android.net.Uri
 import android.view.View
-import androidx.activity.OnBackPressedCallback
 import androidx.core.app.ActivityOptionsCompat
 import androidx.core.view.ViewCompat
-import androidx.fragment.app.activityViewModels
 import androidx.fragment.app.viewModels
 import androidx.lifecycle.lifecycleScope
 import kotlinx.coroutines.launch
@@ -19,7 +17,6 @@ import org.thoughtcrime.securesms.components.settings.configure
 import org.thoughtcrime.securesms.conversation.mutiselect.forward.MultiselectForwardFragment
 import org.thoughtcrime.securesms.conversation.mutiselect.forward.MultiselectForwardFragmentArgs
 import org.thoughtcrime.securesms.database.model.MmsMessageRecord
-import org.thoughtcrime.securesms.main.MainNavigationViewModel
 import org.thoughtcrime.securesms.recipients.Recipient
 import org.thoughtcrime.securesms.safety.SafetyNumberBottomSheet
 import org.thoughtcrime.securesms.stories.StoryTextPostModel
@@ -37,8 +34,6 @@ class MyStoriesFragment : DSLSettingsFragment(
 
   private val lifecycleDisposable = LifecycleDisposable()
 
-  private val mainNavigationViewModel: MainNavigationViewModel by activityViewModels()
-
   private val viewModel: MyStoriesViewModel by viewModels(
     factoryProducer = {
       MyStoriesViewModel.Factory(MyStoriesRepository(requireContext()))
@@ -48,15 +43,6 @@ class MyStoriesFragment : DSLSettingsFragment(
   override fun bindAdapter(adapter: MappingAdapter) {
     MyStoriesItem.register(adapter)
 
-    requireActivity().onBackPressedDispatcher.addCallback(
-      viewLifecycleOwner,
-      object : OnBackPressedCallback(true) {
-        override fun handleOnBackPressed() {
-          mainNavigationViewModel.popStoriesDetailLocation()
-        }
-      }
-    )
-
     val emptyNotice = requireView().findViewById<View>(R.id.empty_notice)
     lifecycleDisposable.bindTo(viewLifecycleOwner)
     viewModel.state.observe(viewLifecycleOwner) {

app/src/main/java/org/thoughtcrime/securesms/window/AppScaffold.kt

@@ -48,8 +48,6 @@ import androidx.compose.ui.draw.drawWithContent
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.layout.layout
-import androidx.compose.ui.platform.LocalConfiguration
-import androidx.compose.ui.platform.LocalInspectionMode
 import androidx.compose.ui.platform.LocalLayoutDirection
 import androidx.compose.ui.platform.LocalResources
 import androidx.compose.ui.text.style.TextAlign
@@ -57,38 +55,16 @@ import androidx.compose.ui.unit.LayoutDirection
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.zIndex
 import kotlinx.coroutines.launch
-import org.signal.core.ui.WindowBreakpoint
+import org.signal.core.ui.NavigationType
 import org.signal.core.ui.compose.BreakpointPreviews
 import org.signal.core.ui.compose.Previews
-import org.signal.core.ui.getWindowBreakpoint
 import org.signal.core.ui.rememberIsSplitPane
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.main.MainFloatingActionButtonsCallback
 import org.thoughtcrime.securesms.main.MainNavigationBar
+import org.thoughtcrime.securesms.main.MainNavigationBarState
 import org.thoughtcrime.securesms.main.MainNavigationRail
-import org.thoughtcrime.securesms.main.MainNavigationState
 import kotlin.math.max
 
-enum class NavigationType {
-  RAIL,
-  BAR;
-
-  companion object {
-    @Composable
-    fun rememberNavigationType(): NavigationType {
-      val resources = LocalResources.current
-      val config = LocalConfiguration.current
-      val windowBreakpoint = remember(config) { resources.getWindowBreakpoint() }
-
-      return when (windowBreakpoint) {
-        is WindowBreakpoint.Small -> BAR
-        is WindowBreakpoint.Medium -> if (windowBreakpoint.isWidthExpanded) RAIL else BAR
-        is WindowBreakpoint.Large -> RAIL
-      }
-    }
-  }
-}
-
 /**
  * A top-level scaffold that automatically adapts its layout based on the device's window size class. It is a generic container designed to handle the
  * arrangement of navigation rails, top/bottom bars, and list-detail pane management for both compact and large screens.
@@ -127,13 +103,7 @@ fun AppScaffold(
   contentWindowInsets: WindowInsets = WindowInsets.systemBars,
   animatorFactory: AppScaffoldAnimationStateFactory = AppScaffoldAnimationStateFactory.Default
 ) {
-  val isForceSinglePane = if (LocalInspectionMode.current) {
-    false
-  } else {
-    SignalStore.internal.forceSinglePane
-  }
-
-  val useSimpleScaffold = isForceSinglePane || (navigator.scaffoldDirective.maxHorizontalPartitions == 1 && Build.VERSION.SDK_INT < 33)
+  val useSimpleScaffold = navigator.scaffoldDirective.maxHorizontalPartitions == 1 && Build.VERSION.SDK_INT < 33
   if (useSimpleScaffold && LocalLayoutDirection.current != LayoutDirection.Rtl) {
     SinglePaneAppScaffold(
       navigator = navigator,
@@ -404,7 +374,7 @@ private fun ListAndNavigation(
 private fun AppScaffoldPreview() {
   Previews.Preview {
     val windowSizeClass = currentWindowAdaptiveInfo().windowSizeClass
-    val isSplitPane = LocalResources.current.rememberIsSplitPane(false)
+    val isSplitPane = LocalResources.current.rememberIsSplitPane()
 
     AppScaffold(
       navigator = rememberAppScaffoldNavigator(
@@ -440,14 +410,14 @@ private fun AppScaffoldPreview() {
       },
       navRailContent = {
         MainNavigationRail(
-          state = MainNavigationState(),
+          state = MainNavigationBarState(),
           mainFloatingActionButtonsCallback = MainFloatingActionButtonsCallback.Empty,
           onDestinationSelected = {}
         )
       },
       bottomNavContent = {
         MainNavigationBar(
-          state = MainNavigationState(),
+          state = MainNavigationBarState(),
           onDestinationSelected = {}
         )
       },

app/src/main/java/org/thoughtcrime/securesms/window/AppScaffoldNavigator.kt

@@ -19,7 +19,6 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
-import androidx.compose.ui.platform.LocalInspectionMode
 import androidx.compose.ui.platform.LocalResources
 import androidx.compose.ui.unit.Dp
 import androidx.window.core.layout.WindowSizeClass
@@ -27,7 +26,6 @@ import org.signal.core.ui.horizontalPartitionDefaultSpacerSize
 import org.signal.core.ui.isSplitPane
 import org.signal.core.ui.listPaneDefaultPreferredWidth
 import org.signal.core.ui.rememberIsSplitPane
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 
 /**
  * AppScaffoldNavigator wraps a delegate navigator (such as the value returned by [rememberThreePaneScaffoldNavigatorDelegate]
@@ -105,9 +103,7 @@ open class AppScaffoldNavigator<T> @RememberInComposition constructor(private va
 @Composable
 fun rememberAppScaffoldNavigator(
   windowSizeClass: WindowSizeClass = currentWindowAdaptiveInfo().windowSizeClass,
-  isSplitPane: Boolean = LocalResources.current.rememberIsSplitPane(
-    forceSplitPane = if (LocalInspectionMode.current) false else SignalStore.internal.forceSplitPane
-  ),
+  isSplitPane: Boolean = LocalResources.current.rememberIsSplitPane(),
   horizontalPartitionSpacerSize: Dp = windowSizeClass.horizontalPartitionDefaultSpacerSize,
   defaultPanePreferredWidth: Dp = windowSizeClass.listPaneDefaultPreferredWidth
 ): AppScaffoldNavigator<Any> {

app/src/test/java/org/thoughtcrime/securesms/main/MainDetailBackStackPersistenceTest.kt

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+import android.app.Application
+import androidx.lifecycle.SavedStateHandle
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import org.signal.core.ui.backStack
+import org.signal.core.ui.compose.split.ListDetailBackStack
+import org.signal.core.ui.compose.split.push
+import org.thoughtcrime.securesms.recipients.RecipientId
+import org.thoughtcrime.securesms.service.webrtc.links.CallLinkRoomId
+
+/**
+ * That a stack survives process death at all is covered by `ListDetailBackStackPersistenceTest` in
+ * `core:ui`. What is left here is that *these* routes survive it: every key is serialized by its concrete
+ * class, so each route class has to carry a serializer that round-trips its own arguments.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class)
+class MainDetailBackStackPersistenceTest {
+
+  private class Host(savedStateHandle: SavedStateHandle) {
+    val backStack: ListDetailBackStack by savedStateHandle.backStack("test_back_stack", MainListRoute.Chats)
+  }
+
+  /** Saves and restores the handle the way a process death would. */
+  private fun SavedStateHandle.roundTrip(): SavedStateHandle {
+    return SavedStateHandle.createHandle(savedStateProvider().saveState(), null)
+  }
+
+  @Test
+  fun `given a stacked detail location, when restored, then the whole stack comes back`() {
+    val handle = SavedStateHandle()
+    val roomId = CallLinkRoomId.fromBytes(byteArrayOf(7, 8, 9))
+    Host(handle).backStack.apply {
+      push(MainDetailRoute.CallLinkDetails(roomId))
+      push(MainDetailRoute.Calls.CallLinks.EditCallLinkName(roomId, "movie night"))
+    }
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(
+      listOf(
+        MainListRoute.Chats,
+        MainDetailRoute.CallLinkDetails(roomId),
+        MainDetailRoute.Calls.CallLinks.EditCallLinkName(roomId, "movie night")
+      ),
+      restored.toList()
+    )
+  }
+
+  @Test
+  fun `given a chats sub screen, when restored, then its arguments come back`() {
+    val handle = SavedStateHandle()
+    val recipientId = RecipientId.from(12)
+    Host(handle).backStack.push(MainDetailRoute.Chats.ConversationSettings(recipientId))
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(
+      MainDetailRoute.Chats.ConversationSettings(recipientId),
+      restored.last()
+    )
+  }
+
+  /**
+   * [MainListRoute] is an enum, which `@Parcelize` cannot handle — this is what lets the
+   * list locations sit at the root of a stack once they move into it.
+   */
+  @Test
+  fun `given a list location on the stack, when restored, then the enum entry comes back`() {
+    val handle = SavedStateHandle()
+    Host(handle).backStack.push(MainListRoute.Archive)
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(MainListRoute.Archive, restored.last())
+  }
+}

app/src/test/java/org/thoughtcrime/securesms/main/MainNavigationViewModelTest.kt

@@ -0,0 +1,295 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+import android.app.Application
+import androidx.lifecycle.SavedStateHandle
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.mockkObject
+import io.mockk.unmockkObject
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.flow.emptyFlow
+import kotlinx.coroutines.test.StandardTestDispatcher
+import kotlinx.coroutines.test.resetMain
+import kotlinx.coroutines.test.setMain
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Before
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import org.signal.core.ui.compose.split.PaneAnchor
+import org.thoughtcrime.securesms.database.SignalDatabase
+import org.thoughtcrime.securesms.database.model.MessageId
+import org.thoughtcrime.securesms.recipients.RecipientId
+import org.thoughtcrime.securesms.service.webrtc.links.CallLinkRoomId
+import org.thoughtcrime.securesms.testutil.MockAppDependenciesRule
+
+/**
+ * Covers the view model as the sole owner of the tab back stacks: everything a screen can ask for goes
+ * through [MainNavigationRouter] or [MainNavigationViewModel.popCurrentDetailLocation], and the stacks it
+ * hands the display are what those calls leave behind.
+ */
+@OptIn(ExperimentalCoroutinesApi::class)
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class)
+class MainNavigationViewModelTest {
+
+  @get:Rule
+  val appDependencies = MockAppDependenciesRule()
+
+  private val testDispatcher = StandardTestDispatcher()
+
+  private val conversationSettings = MainDetailRoute.Chats.ConversationSettings(RecipientId.from(1))
+  private val messageDetails = MainDetailRoute.Chats.MessageDetails(RecipientId.from(1), MessageId(2))
+  private val callLinkDetails = MainDetailRoute.CallLinkDetails(CallLinkRoomId.fromBytes(byteArrayOf(1)))
+
+  private lateinit var viewModel: MainNavigationViewModel
+
+  @Before
+  fun setUp() {
+    Dispatchers.setMain(testDispatcher)
+
+    // Reached through NotificationProfilesRepository, which the view model builds but this suite never
+    // exercises.
+    mockkObject(SignalDatabase.Companion)
+    every { SignalDatabase.notificationProfiles } returns mockk()
+
+    mockkObject(MainNavigationRepository)
+    every { MainNavigationRepository.getNumberOfUnreadMessages() } returns emptyFlow()
+    every { MainNavigationRepository.getNumberOfUnseenCalls() } returns emptyFlow()
+    every { MainNavigationRepository.getNumberOfUnseenStories() } returns emptyFlow()
+    every { MainNavigationRepository.getHasFailedOutgoingStories() } returns emptyFlow()
+
+    viewModel = MainNavigationViewModel(SavedStateHandle())
+    testDispatcher.scheduler.advanceUntilIdle()
+  }
+
+  @After
+  fun tearDown() {
+    unmockkObject(MainNavigationRepository)
+    unmockkObject(SignalDatabase.Companion)
+    Dispatchers.resetMain()
+  }
+
+  @Test
+  fun `given a new view model, then chats is displayed with no detail`() {
+    assertEquals(MainListRoute.Chats, viewModel.currentTab.value)
+    assertEquals(listOf(MainListRoute.Chats), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `when going to a detail location, then it is pushed onto the displayed tab`() {
+    viewModel.goTo(conversationSettings)
+
+    assertEquals(listOf(MainListRoute.Chats, conversationSettings), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `given stacked detail, when popping, then only the top of the stack is dropped`() {
+    viewModel.goTo(conversationSettings)
+    viewModel.goTo(messageDetails)
+
+    viewModel.popCurrentDetailLocation()
+
+    assertEquals(listOf(MainListRoute.Chats, conversationSettings), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `given stacked detail, when exiting detail, then all of it is dropped`() {
+    viewModel.goTo(conversationSettings)
+    viewModel.goTo(messageDetails)
+
+    viewModel.exitDetailLocation()
+
+    assertEquals(listOf(MainListRoute.Chats), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `given a stack at its root, when popping, then nothing is dropped`() {
+    viewModel.popCurrentDetailLocation()
+
+    assertEquals(listOf(MainListRoute.Chats), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `when going to the archive, then it is pushed onto the chats stack rather than becoming a tab`() {
+    viewModel.goTo(MainListRoute.Archive)
+
+    assertEquals(MainListRoute.Chats, viewModel.currentTab.value)
+    assertEquals(
+      listOf(MainListRoute.Chats, MainListRoute.Archive),
+      viewModel.navigator[MainListRoute.Chats]
+    )
+  }
+
+  @Test
+  fun `given the archive is displayed, when going back to chats, then the archive is popped`() {
+    viewModel.goTo(MainListRoute.Archive)
+
+    viewModel.goTo(MainListRoute.Chats)
+
+    assertEquals(listOf(MainListRoute.Chats), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  /**
+   * Leaving a conversation opened from the archive must not also close the archive, which is why detail
+   * content stacks above the displayed list rather than above the root.
+   */
+  @Test
+  fun `given detail opened from the archive, when exiting detail, then the archive stays displayed`() {
+    viewModel.goTo(MainListRoute.Archive)
+    viewModel.goTo(conversationSettings)
+
+    viewModel.exitDetailLocation()
+
+    assertEquals(
+      listOf(MainListRoute.Chats, MainListRoute.Archive),
+      viewModel.navigator[MainListRoute.Chats]
+    )
+  }
+
+  @Test
+  fun `given detail is open, when opening the archive, then the detail stays displayed above it`() {
+    viewModel.goTo(conversationSettings)
+
+    viewModel.goTo(MainListRoute.Archive)
+
+    assertEquals(
+      listOf(MainListRoute.Chats, MainListRoute.Archive, conversationSettings),
+      viewModel.navigator[MainListRoute.Chats]
+    )
+  }
+
+  @Test
+  fun `when going to a calls destination, then it is pushed onto the calls stack`() {
+    viewModel.goTo(MainListRoute.Calls)
+    viewModel.goTo(callLinkDetails)
+
+    assertEquals(MainListRoute.Calls, viewModel.currentTab.value)
+    assertEquals(listOf(MainListRoute.Calls, callLinkDetails), viewModel.navigator[MainListRoute.Calls])
+    assertEquals(listOf(MainListRoute.Chats), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  /**
+   * Each tab keeps its own stack, so a tab that had detail content open comes back to it.
+   */
+  @Test
+  fun `given detail open on another tab, when switching away and back, then that stack is unchanged`() {
+    viewModel.goTo(MainListRoute.Calls)
+    viewModel.goTo(callLinkDetails)
+
+    viewModel.goTo(MainListRoute.Chats)
+    viewModel.goTo(MainListRoute.Calls)
+
+    assertEquals(listOf(MainListRoute.Calls, callLinkDetails), viewModel.navigator[MainListRoute.Calls])
+  }
+
+  /**
+   * Popping and exiting act on whichever tab is displayed, not on a fixed one, so both are exercised from
+   * a tab other than chats.
+   */
+  @Test
+  fun `given detail open on both tabs, when popping from calls, then only the calls stack is affected`() {
+    viewModel.goTo(MainListRoute.Chats)
+    viewModel.goTo(conversationSettings)
+    viewModel.goTo(MainListRoute.Calls)
+    viewModel.goTo(callLinkDetails)
+
+    viewModel.popCurrentDetailLocation()
+
+    assertEquals(listOf(MainListRoute.Calls), viewModel.navigator[MainListRoute.Calls])
+    assertEquals(listOf(MainListRoute.Chats, conversationSettings), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `given detail open on both tabs, when exiting detail from calls, then only the calls stack is affected`() {
+    viewModel.goTo(MainListRoute.Chats)
+    viewModel.goTo(conversationSettings)
+    viewModel.goTo(MainListRoute.Calls)
+    viewModel.goTo(callLinkDetails)
+
+    viewModel.exitDetailLocation()
+
+    assertEquals(listOf(MainListRoute.Calls), viewModel.navigator[MainListRoute.Calls])
+    assertEquals(listOf(MainListRoute.Chats, conversationSettings), viewModel.navigator[MainListRoute.Chats])
+  }
+
+  @Test
+  fun `given detail open on another tab, when popping from chats, then only the chats stack is affected`() {
+    viewModel.goTo(MainListRoute.Calls)
+    viewModel.goTo(callLinkDetails)
+    viewModel.goTo(MainListRoute.Chats)
+    viewModel.goTo(conversationSettings)
+
+    viewModel.popCurrentDetailLocation()
+
+    assertEquals(listOf(MainListRoute.Chats), viewModel.navigator[MainListRoute.Chats])
+    assertEquals(listOf(MainListRoute.Calls, callLinkDetails), viewModel.navigator[MainListRoute.Calls])
+  }
+
+  @Test
+  fun `when going to a stories destination, then it is pushed onto the stories stack`() {
+    viewModel.goTo(MainListRoute.Stories)
+    viewModel.goTo(MainDetailRoute.Stories.MyStories)
+
+    assertEquals(
+      listOf(MainListRoute.Stories, MainDetailRoute.Stories.MyStories),
+      viewModel.navigator[MainListRoute.Stories]
+    )
+  }
+
+  /**
+   * Stories destinations are content roots, so opening one replaces whatever is already displayed rather
+   * than stacking on it.
+   */
+  @Test
+  fun `given a stories destination is open, when opening another, then it replaces the first`() {
+    viewModel.goTo(MainListRoute.Stories)
+    viewModel.goTo(MainDetailRoute.Stories.MyStories)
+
+    viewModel.goTo(MainDetailRoute.Stories.PrivacySettings)
+
+    assertEquals(
+      listOf(MainListRoute.Stories, MainDetailRoute.Stories.PrivacySettings),
+      viewModel.navigator[MainListRoute.Stories]
+    )
+  }
+
+  @Test
+  fun `given the list fills the window, when detail content opens, then the detail is revealed`() {
+    viewModel.onPaneAnchorSelected(PaneAnchor.LIST_ONLY)
+
+    viewModel.goTo(conversationSettings)
+
+    assertEquals(PaneAnchor.DETAIL_ONLY, viewModel.paneAnchor.value)
+  }
+
+  @Test
+  fun `given the detail fills the window, when the last detail is popped, then the list is revealed`() {
+    viewModel.goTo(conversationSettings)
+    viewModel.onPaneAnchorSelected(PaneAnchor.DETAIL_ONLY)
+
+    viewModel.popCurrentDetailLocation()
+
+    assertEquals(PaneAnchor.LIST_ONLY, viewModel.paneAnchor.value)
+  }
+
+  @Test
+  fun `given stacked detail, when the top is popped, then the detail pane stays revealed`() {
+    viewModel.goTo(conversationSettings)
+    viewModel.goTo(messageDetails)
+    viewModel.onPaneAnchorSelected(PaneAnchor.DETAIL_ONLY)
+
+    viewModel.popCurrentDetailLocation()
+
+    assertEquals(PaneAnchor.DETAIL_ONLY, viewModel.paneAnchor.value)
+  }
+}

core/ui/build.gradle.kts

@@ -30,6 +30,8 @@ dependencies {
   api(libs.androidx.compose.material3.adaptive.navigation)
   implementation(libs.androidx.navigation3.ui)
   implementation(libs.androidx.navigation3.runtime)
+  implementation(libs.androidx.lifecycle.viewmodel.navigation3)
+  implementation(libs.androidx.lifecycle.runtime.compose)
   api(libs.androidx.compose.ui.tooling.preview)
   api(libs.androidx.activity.compose)
   debugApi(libs.androidx.compose.ui.tooling.core)
@@ -40,6 +42,10 @@ dependencies {
   api(libs.androidx.window.window)
   api(libs.accompanist.permissions)
 
+  testImplementation(testLibs.junit.junit)
+  testImplementation(testLibs.kotlinx.coroutines.test)
+  testImplementation(testLibs.robolectric.robolectric)
+
   // JUnit is used by test fixtures
   testFixturesImplementation(testLibs.junit.junit)
 }

core/ui/src/main/java/org/signal/core/ui/CoreUiDependencies.kt

@@ -33,13 +33,9 @@ object CoreUiDependencies {
   val isScreenSecurityEnabled: Boolean
     get() = _provider.provideIsScreenSecurityEnabled()
 
-  val forceSplitPane: Boolean
-    get() = _provider.provideForceSplitPane()
-
   interface Provider {
     fun providePackageId(): String
     fun provideIsIncognitoKeyboardEnabled(): Boolean
     fun provideIsScreenSecurityEnabled(): Boolean
-    fun provideForceSplitPane(): Boolean
   }
 }

core/ui/src/main/java/org/signal/core/ui/NavigationType.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui
+
+import androidx.compose.runtime.Composable
+
+/**
+ * Where a window's top-level navigation is placed: along the start edge, or across the bottom.
+ */
+enum class NavigationType {
+  RAIL,
+  BAR;
+
+  companion object {
+    @Composable
+    fun rememberNavigationType(): NavigationType = rememberWindowBreakpoint().navigationType
+  }
+}
+
+/**
+ * A window gets a rail once it is wide enough for one to sit beside the content rather than eat into it.
+ */
+val WindowBreakpoint.navigationType: NavigationType
+  get() = when (this) {
+    is WindowBreakpoint.Small -> NavigationType.BAR
+    is WindowBreakpoint.Medium -> if (isWidthExpanded) NavigationType.RAIL else NavigationType.BAR
+    is WindowBreakpoint.Large -> NavigationType.RAIL
+  }

core/ui/src/main/java/org/signal/core/ui/SavedStateHandleExtensions.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui
+
+import androidx.lifecycle.SavedStateHandle
+import androidx.lifecycle.serialization.saved
+import androidx.navigation3.runtime.NavBackStack
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.serialization.NavBackStackSerializer
+import androidx.navigation3.runtime.serialization.NavKeySerializer
+import kotlin.properties.ReadWriteProperty
+
+/**
+ * Creates a persistable backstack that can be owned by a ViewModel.
+ */
+fun SavedStateHandle.backStack(
+  key: String,
+  root: NavKey
+): ReadWriteProperty<Any?, NavBackStack<NavKey>> {
+  return saved(
+    serializer = NavBackStackSerializer(NavKeySerializer()),
+    key = key
+  ) {
+    NavBackStack(root)
+  }
+}
+
+/**
+ * The same persistable backstack as [backStack], for an owner that holds several of them and so cannot
+ * name each one as a property.
+ */
+fun SavedStateHandle.createBackStack(key: String, root: NavKey): NavBackStack<NavKey> {
+  return BackStackHolder(this, key, root).backStack
+}
+
+private class BackStackHolder(savedStateHandle: SavedStateHandle, key: String, root: NavKey) {
+  val backStack: NavBackStack<NavKey> by savedStateHandle.backStack(key, root)
+}

core/ui/src/main/java/org/signal/core/ui/WindowSizeClassExtensions.kt

@@ -126,25 +126,16 @@ val WindowBreakpoint.assumedFormFactor: FormFactor
   }
 
 @Composable
-fun Resources.rememberIsSplitPane(
-  forceSplitPane: Boolean = CoreUiDependencies.forceSplitPane
-): Boolean {
-  return remember(this, forceSplitPane) {
-    isSplitPane(forceSplitPane)
+fun Resources.rememberIsSplitPane(): Boolean {
+  return remember(this) {
+    isSplitPane()
   }
 }
 
 /**
  * Determines whether the UI should display in split-pane mode based on available screen space.
  */
-@JvmOverloads
-fun Resources.isSplitPane(
-  forceSplitPane: Boolean = CoreUiDependencies.forceSplitPane
-): Boolean {
-  if (forceSplitPane) {
-    return true
-  }
-
+fun Resources.isSplitPane(): Boolean {
   return when (val breakpoint = getWindowBreakpoint()) {
     is WindowBreakpoint.Small -> false
     is WindowBreakpoint.Medium -> true

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailBackStack.kt

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.navigation3.runtime.NavBackStack
+import androidx.navigation3.runtime.NavKey
+
+/**
+ * Describes a location that would sit in the "List" portion of a list-detail scaffold.
+ */
+interface ListNavKey : NavKey
+
+/**
+ * Describes a location that would sit in the "Detail" portion of a list-detail scaffold.
+ */
+interface DetailNavKey : NavKey {
+  val isContentRoot: Boolean
+}
+
+/**
+ * A backstack for a list-detail scaffold. Allows us to decorate the type with a bunch of helpful extensions.
+ */
+typealias ListDetailBackStack = NavBackStack<NavKey>
+
+/**
+ * Index of the list location currently being displayed. Everything above it is detail content.
+ */
+@PublishedApi
+internal val ListDetailBackStack.listIndex: Int
+  get() = indexOfLast { it is ListNavKey }
+
+/**
+ * The list location currently being displayed, which is the last one on the stack, as [L]. Throws if the
+ * displayed list is not an [L].
+ */
+inline fun <reified L : ListNavKey> ListDetailBackStack.listLocation(): L {
+  return this[listIndex] as L
+}
+
+/**
+ * The detail content displayed above the current list as [D], or null when the list is showing on its own.
+ */
+inline fun <reified D : DetailNavKey> ListDetailBackStack.detailLocation(): D? {
+  return lastOrNull() as? D
+}
+
+/**
+ * Whether detail content is displayed above the current list.
+ */
+val ListDetailBackStack.hasDetail: Boolean
+  get() = listIndex < lastIndex
+
+/**
+ * Pushes [location] into the place its kind belongs
+ */
+fun ListDetailBackStack.push(location: NavKey) {
+  when (location) {
+    lastOrNull() -> Unit
+    is ListNavKey -> {
+      if (this[listIndex] != location) {
+        add(listIndex + 1, location)
+      }
+    }
+
+    is DetailNavKey if location.isContentRoot -> {
+      exitDetail()
+      add(location)
+    }
+
+    else -> add(location)
+  }
+}
+
+/**
+ * Drops the list locations stacked above [location] so that it becomes the displayed list, leaving the
+ * detail content above them in place. Does nothing if [location] is not on the stack.
+ */
+internal fun ListDetailBackStack.popToList(location: ListNavKey) {
+  if (!contains(location)) {
+    return
+  }
+
+  while (this[listIndex] != location) {
+    removeAt(listIndex)
+  }
+}
+
+/**
+ * Pops the top entry, whether that is detail content or a pushed list. Returns false at the root; an
+ * empty stack cannot be displayed.
+ */
+internal fun ListDetailBackStack.pop(): Boolean {
+  if (size <= 1) {
+    return false
+  }
+
+  removeAt(lastIndex)
+  return true
+}
+
+/**
+ * Drops everything above the current list location, leaving that list displayed with no detail. Any
+ * pushed list beneath it stays.
+ */
+fun ListDetailBackStack.exitDetail() {
+  while (size > listIndex + 1) {
+    removeAt(lastIndex)
+  }
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailEvents.kt

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.navigation3.runtime.NavKey
+
+/**
+ * What a screen can ask of a [ListDetailNavigator]: the moves that are navigation and nothing else.
+ */
+sealed interface ListDetailEvents {
+
+  /** Push [location] onto [root]'s stack, or onto the displayed one when [root] is null. */
+  data class Push(val location: NavKey, val root: ListNavKey? = null) : ListDetailEvents
+
+  /** Display [listRoute], on [root]'s stack. See [ListDetailNavigator.goToList]. */
+  data class GoToList(
+    val listRoute: ListNavKey,
+    val root: ListNavKey = listRoute,
+    val push: Boolean = false
+  ) : ListDetailEvents
+
+  /** Back, which drops the top of the displayed stack. */
+  data object Back : ListDetailEvents
+
+  /** Drop all the detail above the displayed list, leaving that list on its own. */
+  data object ExitDetail : ListDetailEvents
+
+  /** The user dragged the pane divider. */
+  data class AnchorSelected(val anchor: PaneAnchor) : ListDetailEvents
+
+  /** Reveal the list pane if the detail was filling the window, leaving the stacks alone. */
+  data object RevealList : ListDetailEvents
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailNavDisplay.kt

@@ -0,0 +1,120 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.activity.compose.BackHandler
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.key
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import androidx.lifecycle.viewmodel.navigation3.rememberViewModelStoreNavEntryDecorator
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.rememberDecoratedNavEntries
+import androidx.navigation3.runtime.rememberSaveableStateHolderNavEntryDecorator
+import androidx.navigation3.ui.NavDisplay
+import org.signal.core.ui.navigation.TransitionSpecs
+
+/**
+ * The decorated entries of whichever stack [navigator] is currently displaying, which is what [entries]
+ * wants.
+ *
+ * @param navigator the navigator whose stacks are displayed, from the view model that owns it.
+ * @param entryProvider builds the [NavEntry] for a key, from `entryProvider { }`.
+ */
+@Composable
+fun rememberCurrentDecoratedNavEntries(
+  navigator: ListDetailNavigator<*, *>,
+  entryProvider: (NavKey) -> NavEntry<NavKey>
+): List<NavEntry<NavKey>> {
+  val currentRoot by navigator.currentRoot.collectAsStateWithLifecycle()
+
+  var currentEntries: List<NavEntry<NavKey>>? = null
+  for ((root, stack) in navigator.stacks) {
+    // Each stack needs decorators of its own: they are where its state is kept, and sharing them would
+    // mean sharing that state.
+    val entries = key(root) {
+      rememberDecoratedNavEntries(
+        backStack = stack,
+        entryDecorators = listOf(
+          rememberSaveableStateHolderNavEntryDecorator(),
+          rememberViewModelStoreNavEntryDecorator()
+        ),
+        entryProvider = entryProvider
+      )
+    }
+
+    if (root == currentRoot) {
+      currentEntries = entries
+    }
+  }
+
+  return checkNotNull(currentEntries) { "$currentRoot has no stack of its own." }
+}
+
+/**
+ * Boilerplate code that displays [entries] as a list/detail split, or as a single pane when [layout] is null.
+ *
+ * Wires up the scene strategy, the pane transitions, the geometry and chrome the scene reads, and the
+ * back handler for a detail pane filling the window.
+ *
+ * @param entries the stack to display. Which pane an entry lands in comes from how it was registered:
+ *   [listEntry] for the list, [detailEntry] for the detail, and a plain `entry` for a screen that takes
+ *   the whole window.
+ * @param isSplitPane whether the window is wide enough to show both panes. When false the entries are
+ *   displayed one at a time, whatever [layout] says.
+ * @param paneAnchor where the divider currently sits, which decides whether back exits the detail pane.
+ * @param onBack called when the user wishes to navigate back, which should go through your view model.
+ * @param onExitDetail called when the user wishes to exit the detail pane, such as when the pane covers the whole screen.
+ * @param modifier passed through to NavDisplay
+ * @param layout the split geometry, from [rememberListDetailPaneLayout]. Null shows the list on its own.
+ * @param listPaneChrome extra content around the list that is static, it will not animate as the list changes.
+ * @param emptyDetailContent empty content when there's no detail specified.
+ */
+@Composable
+fun ListDetailNavDisplay(
+  entries: List<NavEntry<NavKey>>,
+  isSplitPane: Boolean,
+  paneAnchor: PaneAnchor,
+  onBack: () -> Unit,
+  onExitDetail: () -> Unit,
+  modifier: Modifier = Modifier,
+  layout: ListDetailPaneLayout? = null,
+  listPaneChrome: ListPaneChrome? = null,
+  emptyDetailContent: @Composable () -> Unit = {}
+) {
+  // A full-screen entry is above the detail pane rather than in it, so back pops it and leaves the anchor
+  // beneath alone.
+  val isFullScreen = entries.lastOrNull()?.isFullScreen == true
+
+  BackHandler(!isFullScreen && isSplitPane && paneAnchor == PaneAnchor.DETAIL_ONLY) {
+    onExitDetail()
+  }
+
+  val sceneStrategy = remember(isSplitPane) { ListDetailSceneStrategy(isSplitPane) }
+
+  val paneShift = TransitionSpecs.paneShift()
+  val paneShiftPop = TransitionSpecs.paneShift(pop = true)
+
+  CompositionLocalProvider(
+    LocalListDetailPaneLayout provides layout,
+    LocalListPaneChrome provides listPaneChrome,
+    LocalEmptyDetailContent provides emptyDetailContent
+  ) {
+    NavDisplay(
+      entries = entries,
+      sceneStrategies = listOf(sceneStrategy),
+      transitionSpec = { paneShift },
+      popTransitionSpec = { paneShiftPop },
+      predictivePopTransitionSpec = { paneShiftPop },
+      onBack = { onBack() },
+      modifier = modifier
+    )
+  }
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailNavigator.kt

@@ -0,0 +1,184 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.runtime.snapshotFlow
+import androidx.lifecycle.SavedStateHandle
+import androidx.navigation3.runtime.NavKey
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.stateIn
+import kotlinx.coroutines.flow.update
+import org.signal.core.ui.createBackStack
+
+/**
+ * This is the navigation's brain, and allows for managing multiple backstacks for a single nav display.
+ *
+ * Intended to be held by a view model, which keeps its own screens' rules and delegates the stack and
+ * pane bookkeeping here.
+ *
+ * @param savedStateHandle of the view-model who owns this object.
+ * @param scope of the view-model who owns this object.
+ * @param stackKeys one saved-state key per root, which is also the key the root's stack starts at. The
+ *   keys are persisted, so changing one drops the stack it named.
+ * @param initialRoot the root displayed before anything navigates. Unlike the stacks themselves this is not
+ *   persisted, so a restored navigator comes back here with each root's stack intact.
+ */
+class ListDetailNavigator<L : ListNavKey, D : DetailNavKey>(
+  savedStateHandle: SavedStateHandle,
+  scope: CoroutineScope,
+  stackKeys: Map<L, String>,
+  initialRoot: L
+) {
+
+  /**
+   * Every root's stack, keyed by root. Created up front: allocating a stack inside [snapshotsOf]'s
+   * read-only snapshot would throw.
+   */
+  val stacks: Map<L, ListDetailBackStack> = stackKeys.mapValues { (root, key) ->
+    savedStateHandle.createBackStack(key, root)
+  }
+
+  private val paneAnchorController = PaneAnchorController(savedStateHandle)
+
+  private val internalCurrentRoot = MutableStateFlow(initialRoot)
+
+  /** The root whose stack is currently displayed. */
+  val currentRoot: StateFlow<L> = internalCurrentRoot.asStateFlow()
+
+  /** How a split-pane window currently divides the two panes. Persisted. */
+  val paneAnchor: StateFlow<PaneAnchor> = paneAnchorController.anchor
+
+  /**
+   * Whether one pane currently occupies the whole window. Only meaningful in split-pane layouts;
+   * consumers that care about the single-pane case check the window size themselves.
+   */
+  val isFullScreenPane: StateFlow<Boolean> = paneAnchorController.isFullScreenPane
+    .stateIn(scope, SharingStarted.Eagerly, false)
+
+  /** The list displayed by the current root's stack. */
+  @OptIn(ExperimentalCoroutinesApi::class)
+  val displayedList: StateFlow<L> = internalCurrentRoot
+    .flatMapLatest { root -> snapshotsOf(root) { listLocation() } }
+    .stateIn(scope, SharingStarted.Eagerly, currentStack.listLocation())
+
+  /** The detail content displayed above the current list, or null when the list is showing on its own. */
+  @OptIn(ExperimentalCoroutinesApi::class)
+  val detail: StateFlow<D?> = internalCurrentRoot
+    .flatMapLatest { root -> snapshotsOf(root) { detailLocation() } }
+    .stateIn(scope, SharingStarted.Eagerly, currentStack.detailLocation())
+
+  /** Whether the current root's stack is displaying detail content. */
+  val hasDetail: StateFlow<Boolean> = detail
+    .map { it != null }
+    .stateIn(scope, SharingStarted.Eagerly, false)
+
+  /** The stack belonging to [root]. */
+  operator fun get(root: L): ListDetailBackStack = stacks.getValue(root)
+
+  /** The stack currently displayed. */
+  private val currentStack: ListDetailBackStack
+    get() = this[internalCurrentRoot.value]
+
+  /**
+   * Observes [read] against [root]'s stack.
+   */
+  fun <T> snapshotsOf(root: L, read: ListDetailBackStack.() -> T): Flow<T> {
+    val stack = this[root]
+    return snapshotFlow { stack.read() }
+  }
+
+  /**
+   * Applies [event] to these stacks.
+   */
+  @Suppress("UNCHECKED_CAST")
+  fun processEvent(event: ListDetailEvents) {
+    when (event) {
+      is ListDetailEvents.Push -> push((event.root as L?) ?: internalCurrentRoot.value, event.location)
+      is ListDetailEvents.GoToList -> goToList(event.listRoute as L, event.root as L, event.push)
+      ListDetailEvents.Back -> popCurrentDetail()
+      ListDetailEvents.ExitDetail -> exitDetail()
+      is ListDetailEvents.AnchorSelected -> onAnchorSelected(event.anchor)
+      ListDetailEvents.RevealList -> revealList()
+    }
+  }
+
+  /** The user dragged the pane divider to [anchor]. */
+  private fun onAnchorSelected(anchor: PaneAnchor) {
+    paneAnchorController.onAnchorSelected(anchor)
+  }
+
+  /** Reveals the list pane if the detail was filling the window, leaving the stacks as they are. */
+  private fun revealList() {
+    paneAnchorController.revealListPane()
+  }
+
+  /**
+   * Pushes [location] onto [root]'s stack, wherever its kind belongs there.
+   */
+  private fun push(root: L, location: NavKey) {
+    this[root].push(location)
+
+    if (location is DetailNavKey) {
+      paneAnchorController.revealDetailPane()
+    }
+  }
+
+  /**
+   * Drops the detail content above the current list, leaving that list displayed on its own.
+   */
+  private fun exitDetail() {
+    currentStack.exitDetail()
+    paneAnchorController.revealListPane()
+  }
+
+  /**
+   * Pops the stack belonging to whichever root is displayed, revealing the list once the detail it was
+   * covering is gone.
+   */
+  private fun popCurrentDetail() {
+    val stack = currentStack
+    stack.pop()
+
+    if (!stack.hasDetail) {
+      paneAnchorController.revealListPane()
+    }
+  }
+
+  /**
+   * Displays [listRoute], on the stack rooted at [root].
+   *
+   * @param listRoute the list to display.
+   * @param root the stack it lives on. Defaults to [listRoute] itself, which is the case for a root list.
+   * @param push whether to stack [listRoute] above the current list rather than returning to one already
+   *   beneath it. Either way the detail content above stays where it is.
+   */
+  private fun goToList(listRoute: L, root: L = listRoute, push: Boolean = false) {
+    val stack = this[root]
+
+    if (push) {
+      stack.push(listRoute)
+    } else {
+      stack.popToList(listRoute)
+    }
+
+    internalCurrentRoot.update { root }
+    paneAnchorController.revealListPane()
+  }
+
+  @Suppress("UNCHECKED_CAST")
+  private fun ListDetailBackStack.listLocation(): L = this[listIndex] as L
+
+  @Suppress("UNCHECKED_CAST")
+  private fun ListDetailBackStack.detailLocation(): D? = lastOrNull() as? D
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailPaneLayout.kt

@@ -0,0 +1,116 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.Stable
+import androidx.compose.runtime.compositionLocalOf
+import androidx.compose.runtime.remember
+import androidx.compose.ui.graphics.Shape
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.platform.LocalResources
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.signal.core.ui.rememberIsSplitPane
+
+/**
+ * Geometry for the list/detail split, provided to [ListDetailScene] through [LocalListDetailPaneLayout].
+ */
+@Stable
+class ListDetailPaneLayout internal constructor(
+  internal val expansion: PaneExpansionState,
+  internal val partitionWidth: Dp,
+  internal val listPaddingStart: Dp,
+  internal val detailPaddingEnd: Dp,
+  internal val shape: Shape,
+  internal val hasDragHandle: Boolean,
+  internal val minPaneContentWidth: Dp
+)
+
+/**
+ * The current split geometry. A null layout means the display has no geometry to split with, and the
+ * scene falls back to showing the list on its own.
+ */
+internal val LocalListDetailPaneLayout = compositionLocalOf<ListDetailPaneLayout?> { null }
+
+/**
+ * Builds the geometry for a list/detail split and keeps it following [paneAnchor], reporting a dragged
+ * divider back through [onAnchorSelected].
+ *
+ * Everything but the anchors comes from [metrics], so the usual call passes a [paneAnchor], a [maxWidth]
+ * and nothing else. Override [metrics] when a screen needs to depart from the baseline.
+ *
+ * @param paneAnchor the anchor the divider settles at.
+ * @param maxWidth the width available to both panes, from the `BoxWithConstraints` around them.
+ * @param onAnchorSelected the anchor a drag settled on, or that an accessibility action asked for.
+ *   Note: The caller is expected to feed it back in through [paneAnchor], which is what
+ *   actually moves the divider.
+ * @param metrics the baseline geometry, which decides the widths, the gutter and the corners.
+ * @param collapsedListWidth what is left of the list pane once the detail fills the window. Zero unless
+ *   something inside the list stays on screen, such as a navigation rail.
+ */
+@Composable
+fun rememberListDetailPaneLayout(
+  paneAnchor: PaneAnchor,
+  maxWidth: Dp,
+  onAnchorSelected: (PaneAnchor) -> Unit,
+  metrics: ListDetailPaneMetrics = rememberListDetailPaneMetrics(),
+  collapsedListWidth: Dp = 0.dp
+): ListDetailPaneLayout {
+  val isSplitPane = LocalResources.current.rememberIsSplitPane()
+  val splitWidth = metrics.rememberSplitListPaneWidth(maxWidth)
+
+  val anchorWidths = remember(collapsedListWidth, splitWidth, maxWidth, metrics) {
+    PaneAnchorWidths(
+      detailOnly = collapsedListWidth + metrics.listPaddingStart,
+      split = splitWidth,
+      listOnly = maxWidth - metrics.detailPaddingEnd
+    )
+  }
+
+  val density = LocalDensity.current
+  val anchorOffsets = remember(density, anchorWidths) { anchorWidths.toOffsets(density) }
+
+  val expansion = rememberPaneExpansionState(
+    initialOffsetPx = anchorOffsets.getValue(paneAnchor),
+    onAnchorSelected = onAnchorSelected
+  )
+
+  LaunchedEffect(anchorOffsets, paneAnchor) {
+    expansion.updateAnchors(anchorOffsets)
+    expansion.goTo(paneAnchor)
+  }
+
+  return remember(expansion, metrics, isSplitPane, splitWidth) {
+    ListDetailPaneLayout(
+      expansion = expansion,
+      partitionWidth = metrics.partitionWidth,
+      listPaddingStart = metrics.listPaddingStart,
+      detailPaddingEnd = metrics.detailPaddingEnd,
+      shape = metrics.shape,
+      hasDragHandle = isSplitPane,
+      minPaneContentWidth = splitWidth
+    )
+  }
+}
+
+/**
+ * Wraps list content in the static chrome that belongs to the list pane: navigation rail or bar, toolbar, and
+ * anything layered over them.
+ *
+ * Supplied through a composition local so that a scene can place it *around* the list entry, keeping one
+ * chrome instance alive while the list content swaps underneath.
+ */
+typealias ListPaneChrome = @Composable (content: @Composable () -> Unit) -> Unit
+
+internal val LocalListPaneChrome = compositionLocalOf<ListPaneChrome?> { null }
+
+/**
+ * Fills the detail pane when there is no detail content to show. Defaults to nothing, leaving the pane's
+ * own background.
+ */
+internal val LocalEmptyDetailContent = compositionLocalOf<@Composable () -> Unit> { {} }

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailPaneMetrics.kt

@@ -0,0 +1,106 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.adaptive.currentWindowAdaptiveInfo
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.Immutable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.graphics.RectangleShape
+import androidx.compose.ui.graphics.Shape
+import androidx.compose.ui.platform.LocalResources
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.signal.core.ui.WindowBreakpoint
+import org.signal.core.ui.getWindowBreakpoint
+import org.signal.core.ui.isWidthExpanded
+import org.signal.core.ui.rememberIsSplitPane
+
+private val MEDIUM_CONTENT_CORNERS = 18.dp
+private val EXTENDED_CONTENT_CORNERS = 14.dp
+private val EXPANDED_LIST_PANE_WIDTH = 416.dp
+
+/**
+ * The baseline geometry of a list/detail window: the values every Signal list/detail screen starts from,
+ * and which you hand to [rememberListDetailPaneLayout].
+ *
+ * A single pane gets square corners and no padding, since there is no second pane to separate it from.
+ *
+ * @property shape the shape both panes are clipped to.
+ * @property navigationBarShape the shape a navigation bar sitting at the bottom of the list pane is clipped
+ *   to, which is [shape] with its top corners squared off.
+ * @property partitionWidth the gap between the two panes, where the drag handle sits.
+ * @property listPaddingStart padding between the window's start edge and the list pane.
+ * @property detailPaddingEnd padding between the detail pane and the window's end edge.
+ */
+@Immutable
+data class ListDetailPaneMetrics(
+  val shape: Shape,
+  val navigationBarShape: Shape,
+  val partitionWidth: Dp,
+  val listPaddingStart: Dp,
+  val detailPaddingEnd: Dp
+) {
+  private val extraPadding: Dp = partitionWidth + listPaddingStart + detailPaddingEnd
+
+  /**
+   * The list pane's width while both panes are visible, which is the [PaneAnchorWidths.split] anchor.
+   *
+   * A window wide enough gives the list a fixed width and lets the detail take the rest; a narrower one
+   * splits what is left over evenly. A single pane fills the window.
+   *
+   * @param maxWidth the width available to both panes, from the `BoxWithConstraints` around them.
+   */
+  @Composable
+  fun rememberSplitListPaneWidth(maxWidth: Dp): Dp {
+    val isSplitPane = LocalResources.current.rememberIsSplitPane()
+    val windowSizeClass = currentWindowAdaptiveInfo().windowSizeClass
+
+    return remember(maxWidth, windowSizeClass, isSplitPane) {
+      when {
+        !isSplitPane -> maxWidth
+        windowSizeClass.isWidthExpanded -> EXPANDED_LIST_PANE_WIDTH
+        else -> (maxWidth - extraPadding) / 2f
+      }
+    }
+  }
+}
+
+/**
+ * The baseline metrics for the current window.
+ *
+ * @param listPaddingStart overrides the padding at the start of the list pane, for a caller that needs to
+ *   inset the list itself. Ignored in a single-pane window, which has no padding.
+ */
+@Composable
+fun rememberListDetailPaneMetrics(listPaddingStart: Dp = 0.dp): ListDetailPaneMetrics {
+  val resources = LocalResources.current
+  val breakpoint = resources.getWindowBreakpoint()
+  val isSplitPane = resources.rememberIsSplitPane()
+
+  return remember(breakpoint, isSplitPane, listPaddingStart) {
+    val corners = if (breakpoint is WindowBreakpoint.Large) EXTENDED_CONTENT_CORNERS else MEDIUM_CONTENT_CORNERS
+
+    if (!isSplitPane) {
+      ListDetailPaneMetrics(
+        shape = RectangleShape,
+        navigationBarShape = RectangleShape,
+        partitionWidth = 0.dp,
+        listPaddingStart = 0.dp,
+        detailPaddingEnd = 0.dp
+      )
+    } else {
+      ListDetailPaneMetrics(
+        shape = RoundedCornerShape(corners),
+        navigationBarShape = RoundedCornerShape(0.dp, 0.dp, corners, corners),
+        partitionWidth = if (breakpoint is WindowBreakpoint.Large) 24.dp else 13.dp,
+        listPaddingStart = listPaddingStart,
+        detailPaddingEnd = if (breakpoint is WindowBreakpoint.Large) 24.dp else 12.dp
+      )
+    }
+  }
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/ListDetailScene.kt

@@ -0,0 +1,304 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.gestures.Orientation
+import androidx.compose.foundation.gestures.draggable
+import androidx.compose.foundation.gestures.rememberDraggableState
+import androidx.compose.foundation.interaction.MutableInteractionSource
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxScope
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxHeight
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.systemGestureExclusion
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.Immutable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.layout.layout
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.semantics.contentDescription
+import androidx.compose.ui.semantics.onClick
+import androidx.compose.ui.semantics.semantics
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.IntOffset
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.scene.Scene
+import androidx.navigation3.scene.SceneStrategy
+import androidx.navigation3.scene.SceneStrategyScope
+import org.signal.core.ui.R
+import kotlin.math.max
+
+/**
+ * Renders a list entry and, when there is one, the detail entry above it side by side.
+ *
+ * There is some custom equality code in here to make sure animations are correct.
+ */
+@Immutable
+internal class ListDetailScene(
+  override val key: Any,
+  override val previousEntries: List<NavEntry<NavKey>>,
+  private val listEntry: NavEntry<NavKey>,
+  private val detailEntry: NavEntry<NavKey>?
+) : Scene<NavKey> {
+
+  override val entries: List<NavEntry<NavKey>> = listOfNotNull(listEntry, detailEntry)
+
+  override val content: @Composable () -> Unit = {
+    val layout = LocalListDetailPaneLayout.current
+
+    if (layout == null) {
+      listEntry.Content()
+    } else {
+      Box(modifier = Modifier.fillMaxSize()) {
+        Row(modifier = Modifier.fillMaxSize()) {
+          Box(
+            modifier = Modifier
+              .width(layout.expansion.listWidth)
+              .fillMaxHeight()
+              .padding(start = layout.listPaddingStart)
+              .clip(layout.shape)
+              .minContentWidth(layout.minPaneContentWidth)
+          ) {
+            ListPaneContent { listEntry.Content() }
+          }
+
+          Spacer(modifier = Modifier.width(layout.partitionWidth))
+
+          Box(
+            modifier = Modifier
+              .weight(1f)
+              .fillMaxHeight()
+              .padding(end = layout.detailPaddingEnd)
+              .clip(layout.shape)
+              .background(color = MaterialTheme.colorScheme.surface)
+              .minContentWidth(layout.minPaneContentWidth)
+          ) {
+            if (detailEntry != null) {
+              detailEntry.Content()
+            } else {
+              LocalEmptyDetailContent.current()
+            }
+          }
+        }
+
+        if (layout.hasDragHandle) {
+          PaneDragHandle(layout = layout)
+        }
+      }
+    }
+  }
+
+  override fun equals(other: Any?): Boolean {
+    if (this === other) return true
+    if (other !is ListDetailScene) return false
+
+    return key == other.key &&
+      listEntry == other.listEntry &&
+      detailEntry == other.detailEntry &&
+      previousEntries == other.previousEntries
+  }
+
+  override fun hashCode(): Int {
+    var result = key.hashCode()
+    result = 31 * result + listEntry.hashCode()
+    result = 31 * result + (detailEntry?.hashCode() ?: 0)
+    result = 31 * result + previousEntries.hashCode()
+    return result
+  }
+}
+
+/**
+ * Pane divider modeled after the one from the material libraries that allows the user to change the size of the list or detail pane.
+ */
+@Composable
+private fun BoxScope.PaneDragHandle(layout: ListDetailPaneLayout) {
+  val interactionSource = remember { MutableInteractionSource() }
+  val density = LocalDensity.current
+
+  val dividerDescription = stringResource(R.string.ListDetailPane__accessibility_pane_divider)
+  val moveLabel = stringResource(R.string.ListDetailPane__accessibility_move_pane_divider)
+
+  val draggableState = rememberDraggableState { delta -> layout.expansion.dragBy(delta) }
+
+  val splitCenter = layout.expansion.listWidth + (layout.partitionWidth / 2)
+  val touchStart = splitCenter - (PANE_HANDLE_TOUCH_WIDTH / 2)
+
+  Box(
+    contentAlignment = Alignment.Center,
+    modifier = Modifier
+      .align(Alignment.CenterStart)
+      .offset { IntOffset(with(density) { touchStart.roundToPx() }, 0) }
+      .size(PANE_HANDLE_TOUCH_WIDTH, PANE_HANDLE_TOUCH_HEIGHT)
+      .draggable(
+        state = draggableState,
+        orientation = Orientation.Horizontal,
+        interactionSource = interactionSource,
+        onDragStarted = { layout.expansion.onDragStarted() },
+        onDragStopped = { layout.expansion.settle() }
+      )
+      .semantics {
+        contentDescription = dividerDescription
+        onClick(label = moveLabel) {
+          val next = layout.expansion.nextAnchor()
+
+          if (next != null) {
+            layout.expansion.selectAnchor(next)
+            true
+          } else {
+            false
+          }
+        }
+      }
+      .systemGestureExclusion()
+  ) {
+    Box(
+      modifier = Modifier
+        .size(PANE_HANDLE_VISUAL_WIDTH, PANE_HANDLE_VISUAL_HEIGHT)
+        .background(color = Color(0xFF605F5D), RoundedCornerShape(percent = 50))
+    )
+  }
+}
+
+/**
+ * Measures content at no less than [minWidth] while still reporting the slot's actual width. This makes sure that as the user
+ * makes a pane smaller and smaller the UI doesn't distort and look weird.
+ */
+private fun Modifier.minContentWidth(minWidth: Dp): Modifier {
+  return layout { measurable, constraints ->
+    val min = minWidth.roundToPx()
+    val placeable = measurable.measure(
+      constraints.copy(
+        minWidth = min,
+        maxWidth = max(min, constraints.maxWidth)
+      )
+    )
+
+    layout(constraints.maxWidth, placeable.height) {
+      placeable.placeRelative(x = 0, y = 0)
+    }
+  }
+}
+
+/**
+ * Renders [content] wrapped in the list pane chrome, if any has been provided.
+ */
+@Composable
+private fun ListPaneContent(content: @Composable () -> Unit) {
+  val chrome = LocalListPaneChrome.current
+
+  if (chrome != null) {
+    chrome(content)
+  } else {
+    content()
+  }
+}
+
+/**
+ * Single-pane counterpart to [ListDetailScene]. We roll our own because otherwise animations get weird when swapping
+ * between lists.
+ */
+@Immutable
+internal class SinglePaneScene(
+  override val key: Any,
+  override val previousEntries: List<NavEntry<NavKey>>,
+  private val entry: NavEntry<NavKey>,
+  private val isList: Boolean
+) : Scene<NavKey> {
+
+  override val entries: List<NavEntry<NavKey>> = listOf(entry)
+
+  override val content: @Composable () -> Unit = {
+    if (isList) {
+      ListPaneContent { entry.Content() }
+    } else {
+      entry.Content()
+    }
+  }
+
+  override fun equals(other: Any?): Boolean {
+    if (this === other) return true
+    if (other !is SinglePaneScene) return false
+
+    return key == other.key &&
+      entry == other.entry &&
+      isList == other.isList &&
+      previousEntries == other.previousEntries
+  }
+
+  override fun hashCode(): Int {
+    var result = key.hashCode()
+    result = 31 * result + entry.hashCode()
+    result = 31 * result + isList.hashCode()
+    result = 31 * result + previousEntries.hashCode()
+    return result
+  }
+}
+
+/**
+ * Renders the given backstack depending on what kind of screen we're on. Note that we do utilize fixed keys here because we want to make
+ * sure that as we move between screens in either pane we are *only* animating that panes content and not the whole scene.
+ */
+internal class ListDetailSceneStrategy(private val isSplitPane: Boolean) : SceneStrategy<NavKey> {
+
+  override fun SceneStrategyScope<NavKey>.calculateScene(entries: List<NavEntry<NavKey>>): Scene<NavKey>? {
+    val top = entries.lastOrNull() ?: return null
+
+    if (top.isFullScreen) {
+      return SinglePaneScene(
+        key = top.contentKey,
+        previousEntries = entries.dropLast(1),
+        entry = top,
+        isList = false
+      )
+    }
+
+    if (!isSplitPane) {
+      val isList = top.isListPane
+
+      return SinglePaneScene(
+        key = if (isList) LIST_SCENE_KEY else top.contentKey,
+        previousEntries = entries.dropLast(1),
+        entry = top,
+        isList = isList
+      )
+    }
+
+    val listIndex = entries.indexOfLast { it.isListPane }
+    if (listIndex < 0) {
+      return null
+    }
+
+    val detailEntries = entries.subList(listIndex + 1, entries.size)
+
+    return ListDetailScene(
+      key = DETAIL_SCENE_KEY,
+      previousEntries = if (detailEntries.size > 1) entries.dropLast(1) else entries.take(listIndex),
+      listEntry = entries[listIndex],
+      detailEntry = detailEntries.lastOrNull()
+    )
+  }
+}
+
+/** Fixed detail key */
+private const val DETAIL_SCENE_KEY = "signal.listDetail"
+
+/** Fixed list key */
+private const val LIST_SCENE_KEY = "signal.list"

core/ui/src/main/java/org/signal/core/ui/compose/split/PaneAnchor.kt

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.ui.unit.Density
+import androidx.compose.ui.unit.Dp
+
+/**
+ * How a split-pane window divides the list and detail panes.
+ */
+enum class PaneAnchor {
+  /** The detail pane fills the window, with the list pushed off the start edge. */
+  DETAIL_ONLY,
+
+  /** Both panes are visible. */
+  SPLIT,
+
+  /** The list fills the window, with the detail pane pushed off the end edge. */
+  LIST_ONLY
+}
+
+/**
+ * The width the list pane takes at each [PaneAnchor].
+ *
+ * @property detailOnly width left to the list once the detail fills the window. Not necessarily zero: a
+ *   navigation rail living inside the list pane stays on screen.
+ * @property split width of the list pane while both panes are visible.
+ * @property listOnly width the list grows to once it fills the window, which leaves room for whatever
+ *   padding the detail pane's edge needs.
+ */
+data class PaneAnchorWidths(
+  val detailOnly: Dp,
+  val split: Dp,
+  val listOnly: Dp
+) {
+  internal fun toOffsets(density: Density): Map<PaneAnchor, Float> {
+    return with(density) {
+      mapOf(
+        PaneAnchor.DETAIL_ONLY to detailOnly.toPx(),
+        PaneAnchor.SPLIT to split.toPx(),
+        PaneAnchor.LIST_ONLY to listOnly.toPx()
+      )
+    }
+  }
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/PaneAnchorController.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.lifecycle.SavedStateHandle
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.map
+
+/**
+ * Owns which [PaneAnchor] a split-pane window sits at, and the rules for moving between them. Intended
+ * to be held by the view-model that owns the window's navigation state.
+ */
+internal class PaneAnchorController(private val savedStateHandle: SavedStateHandle) {
+
+  companion object {
+    private const val KEY = "pane_anchor"
+  }
+
+  val anchor: StateFlow<PaneAnchor> = savedStateHandle.getStateFlow(KEY, PaneAnchor.SPLIT)
+
+  /**
+   * Whether one pane currently occupies the whole window.
+   */
+  val isFullScreenPane: Flow<Boolean> = anchor.map { it != PaneAnchor.SPLIT }
+
+  /**
+   * The user dragged the pane divider to [anchor].
+   */
+  fun onAnchorSelected(anchor: PaneAnchor) {
+    set(anchor)
+  }
+
+  /**
+   * Opening detail content while the list fills the window has to reveal the detail. A window already
+   * showing both panes is left as the user arranged it.
+   */
+  fun revealDetailPane() {
+    if (anchor.value == PaneAnchor.LIST_ONLY) {
+      set(PaneAnchor.DETAIL_ONLY)
+    }
+  }
+
+  /**
+   * Losing detail content, or explicitly asking for the list, has to reveal the list if the detail
+   * filled the window.
+   */
+  fun revealListPane() {
+    if (anchor.value == PaneAnchor.DETAIL_ONLY) {
+      set(PaneAnchor.LIST_ONLY)
+    }
+  }
+
+  private fun set(value: PaneAnchor) {
+    savedStateHandle[KEY] = value
+  }
+}

core/ui/src/main/java/org/signal/core/ui/compose/split/PaneEntries.kt

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.runtime.Composable
+import androidx.navigation3.runtime.EntryProviderScope
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.NavMetadataKey
+import androidx.navigation3.runtime.contains
+import androidx.navigation3.runtime.metadata
+
+/**
+ * Marks an entry as list-pane content, so that a scene can tell which of the entries it has been handed
+ * belongs in the list pane, since it can't gather that information for itself via a key which is private.
+ */
+internal object ListPane : NavMetadataKey<Boolean>
+
+/**
+ * Marks an entry as detail-pane content, the counterpart to [ListPane].
+ */
+internal object DetailPane : NavMetadataKey<Boolean>
+
+/**
+ * Registers [content] as the list pane for [K]. The counterpart to `entry` for a list location, and the
+ * only way to mark one.
+ *
+ * @param metadata additional metadata for the display, such as a transition spec.
+ */
+inline fun <reified K : ListNavKey> EntryProviderScope<NavKey>.listEntry(
+  metadata: Map<String, Any> = emptyMap(),
+  noinline content: @Composable (K) -> Unit
+) {
+  entry<K>(metadata = metadata + listPaneMetadata(), content = content)
+}
+
+/**
+ * Registers [content] as the detail pane for [K], displayed beside the list in a split-pane window and
+ * over it in a single-pane one.
+ *
+ * @param metadata additional metadata for the display, such as a transition spec.
+ */
+inline fun <reified K : DetailNavKey> EntryProviderScope<NavKey>.detailEntry(
+  metadata: Map<String, Any> = emptyMap(),
+  noinline content: @Composable (K) -> Unit
+) {
+  entry<K>(metadata = metadata + detailPaneMetadata(), content = content)
+}
+
+/**
+ * Metadata marking an entry as list-pane content, applied by [listEntry].
+ */
+@PublishedApi
+internal fun listPaneMetadata(): Map<String, Any> = metadata { put(ListPane, true) }
+
+/**
+ * Metadata marking an entry as detail-pane content, applied by [detailEntry].
+ */
+@PublishedApi
+internal fun detailPaneMetadata(): Map<String, Any> = metadata { put(DetailPane, true) }
+
+/**
+ * Whether this entry belongs in the list pane.
+ */
+internal val NavEntry<*>.isListPane: Boolean
+  get() = ListPane in metadata
+
+/**
+ * Whether this entry belongs in the detail pane.
+ */
+internal val NavEntry<*>.isDetailPane: Boolean
+  get() = DetailPane in metadata
+
+/**
+ * Whether this entry takes the whole window. An entry that claims neither pane gets both, which is what a
+ * plain `entry` registration means.
+ */
+internal val NavEntry<*>.isFullScreen: Boolean
+  get() = !isListPane && !isDetailPane

core/ui/src/main/java/org/signal/core/ui/compose/split/PaneExpansionState.kt

@@ -0,0 +1,130 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.animation.core.animate
+import androidx.compose.foundation.MutatorMutex
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.Stable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableFloatStateOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.unit.Density
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import kotlin.math.abs
+
+/**
+ * Drives how wide the list pane is, in pixels.
+ */
+@Stable
+internal class PaneExpansionState(
+  private val density: Density,
+  initialOffsetPx: Float,
+  private val onAnchorSelected: (PaneAnchor) -> Unit
+) {
+  private var offsetPx by mutableFloatStateOf(initialOffsetPx)
+
+  private var anchorOffsets: Map<PaneAnchor, Float> by mutableStateOf(emptyMap())
+
+  private val animationMutex = MutatorMutex()
+
+  private var isDragging = false
+
+  /** Current width of the list pane. */
+  val listWidth: Dp
+    get() = with(density) { offsetPx.toDp() }
+
+  /** Publishes the widths each anchor corresponds to, from layout. */
+  fun updateAnchors(offsets: Map<PaneAnchor, Float>) {
+    anchorOffsets = offsets
+  }
+
+  /**
+   * Applies a drag delta.
+   */
+  fun dragBy(delta: Float) {
+    val min = anchorOffsets.values.minOrNull() ?: return
+    val max = anchorOffsets.values.maxOrNull() ?: return
+
+    offsetPx = (offsetPx + delta).coerceIn(min, max)
+  }
+
+  fun onDragStarted() {
+    isDragging = true
+  }
+
+  /**
+   * Settles onto whichever anchor the drag ended nearest, and reports it as a selection.
+   */
+  suspend fun settle() {
+    isDragging = false
+
+    val nearest = anchorOffsets.minByOrNull { abs(it.value - offsetPx) }?.key ?: return
+    onAnchorSelected(nearest)
+    animateTo(nearest)
+  }
+
+  /**
+   * Moves the split to [anchor], ignored while a drag is in progress so that a state change mid-gesture
+   * doesn't yank the divider out from under the user's finger.
+   */
+  suspend fun goTo(anchor: PaneAnchor) {
+    if (isDragging) {
+      return
+    }
+
+    animateTo(anchor)
+  }
+
+  /**
+   * The anchor an accessibility action should move to next, cycling through them in declared order. Screen
+   * readers cannot drag, so this is how the divider is operated without a gesture.
+   */
+  fun nextAnchor(): PaneAnchor? {
+    val current = anchorOffsets.minByOrNull { abs(it.value - offsetPx) }?.key ?: return null
+    val anchors = PaneAnchor.entries
+
+    return anchors[(anchors.indexOf(current) + 1) % anchors.size]
+  }
+
+  /**
+   * Reports [anchor] as selected without dragging to it. The resulting state change is what moves the
+   * divider, by the same path a drag takes.
+   */
+  fun selectAnchor(anchor: PaneAnchor) {
+    onAnchorSelected(anchor)
+  }
+
+  private suspend fun animateTo(anchor: PaneAnchor) {
+    val target = anchorOffsets[anchor] ?: return
+
+    animationMutex.mutate {
+      animate(initialValue = offsetPx, targetValue = target) { value, _ -> offsetPx = value }
+    }
+  }
+}
+
+/** [initialOffsetPx] is the width the list pane starts at, read once on the composition that creates this. */
+@Composable
+internal fun rememberPaneExpansionState(initialOffsetPx: Float, onAnchorSelected: (PaneAnchor) -> Unit): PaneExpansionState {
+  val density = LocalDensity.current
+  return remember(density) { PaneExpansionState(density, initialOffsetPx, onAnchorSelected) }
+}
+
+/** Touch width of the pane divider, which is wider than both the visual handle and the gap it sits in. */
+internal val PANE_HANDLE_TOUCH_WIDTH: Dp = 48.dp
+
+/** Width of the visible pill, which tracks the divider rather than the touch target. */
+internal val PANE_HANDLE_VISUAL_WIDTH: Dp = 4.dp
+
+internal val PANE_HANDLE_VISUAL_HEIGHT: Dp = 48.dp
+
+/** Height of the grab area. Bounded rather than full-height so the system gesture exclusion is honoured. */
+internal val PANE_HANDLE_TOUCH_HEIGHT: Dp = 48.dp

core/ui/src/main/java/org/signal/core/ui/navigation/TransitionSpecs.kt

@@ -8,6 +8,7 @@ package org.signal.core.ui.navigation
 import androidx.compose.animation.ContentTransform
 import androidx.compose.animation.EnterTransition
 import androidx.compose.animation.ExitTransition
+import androidx.compose.animation.core.CubicBezierEasing
 import androidx.compose.animation.core.tween
 import androidx.compose.animation.fadeIn
 import androidx.compose.animation.fadeOut
@@ -18,6 +19,14 @@ import androidx.compose.animation.slideInVertically
 import androidx.compose.animation.slideOutHorizontally
 import androidx.compose.animation.slideOutVertically
 import androidx.compose.animation.togetherWith
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.platform.LocalLayoutDirection
+import androidx.compose.ui.unit.Density
+import androidx.compose.ui.unit.IntOffset
+import androidx.compose.ui.unit.LayoutDirection
+import androidx.compose.ui.unit.dp
 import androidx.navigation3.runtime.metadata
 import androidx.navigation3.ui.NavDisplay
 
@@ -26,6 +35,10 @@ import androidx.navigation3.ui.NavDisplay
  */
 object TransitionSpecs {
 
+  private const val PANE_SHIFT_DURATION = 200
+  private val PANE_SHIFT_OFFSET = 48.dp
+  private val PANE_SHIFT_EASING = CubicBezierEasing(0.4f, 0.0f, 0.2f, 1f)
+
   interface Transition {
     companion object {
       val NONE: ContentTransform = EnterTransition.None togetherWith ExitTransition.None
@@ -48,6 +61,54 @@ object TransitionSpecs {
     }
   }
 
+  /**
+   * [paneShift] against the density and layout direction of wherever it is called, which is what a nav
+   * display wants. Every display that uses this reads the same, whether it fills a pane or the window.
+   *
+   * @param pop reverses the direction, for navigating back.
+   */
+  @Composable
+  fun paneShift(pop: Boolean = false): ContentTransform {
+    val density = LocalDensity.current
+    val layoutDirection = LocalLayoutDirection.current
+
+    return remember(density, layoutDirection, pop) { paneShift(density, layoutDirection, pop) }
+  }
+
+  /**
+   * The short horizontal shift a pane makes as it enters or leaves: content moves [PANE_SHIFT_OFFSET]
+   * rather than a whole width, cross-fading as it goes, with no scale.
+   *
+   * A function rather than a [Transition] value because the distance is fixed in dp, so it needs a
+   * [Density]. Values mirror `AppScaffoldAnimationDefaults`, which cannot be referenced from here.
+   *
+   * @param pop reverses the direction, for navigating back.
+   */
+  fun paneShift(density: Density, layoutDirection: LayoutDirection, pop: Boolean = false): ContentTransform {
+    val offset = with(density) { PANE_SHIFT_OFFSET.roundToPx() }
+    val direction = if (layoutDirection == LayoutDirection.Rtl) -1 else 1
+    val sign = if (pop) -1 else 1
+    val slideSpec = tween<IntOffset>(durationMillis = PANE_SHIFT_DURATION, easing = PANE_SHIFT_EASING)
+    val fadeSpec = tween<Float>(durationMillis = PANE_SHIFT_DURATION, easing = PANE_SHIFT_EASING)
+
+    return slideInHorizontally(animationSpec = slideSpec) { offset * sign * direction } + fadeIn(animationSpec = fadeSpec) togetherWith
+      slideOutHorizontally(animationSpec = slideSpec) { -offset * sign * direction } + fadeOut(animationSpec = fadeSpec)
+  }
+
+  /**
+   * Suppresses only the *enter* transition, leaving pops to the display's defaults.
+   *
+   * For destinations that animate their own arrival and would otherwise be animated twice — a conversation
+   * hands off from a bitmap of the list it came from — but which should still animate on the way out. Using
+   * [None] here would suppress both directions, because a pop is resolved against the metadata of the
+   * scene being left.
+   */
+  val suppressEnterMetadata: Map<String, Any> get() = metadata {
+    put(NavDisplay.TransitionKey) {
+      Transition.NONE
+    }
+  }
+
   /**
    * No enter/exit animation.
    */

core/ui/src/main/res/values/strings.xml

@@ -18,6 +18,12 @@
     <!-- Title for dialog shown when a required permission has not been granted -->
     <string name="Permissions_permission_required">Permission required</string>
 
+    <!-- ListDetailPane -->
+    <!-- Content description for the draggable divider between the list and detail panes on large screens -->
+    <string name="ListDetailPane__accessibility_pane_divider">Resize panes</string>
+    <!-- Accessibility action label for moving the divider between the list and detail panes to its next position -->
+    <string name="ListDetailPane__accessibility_move_pane_divider">Move divider</string>
+
     <!-- StorageUtil -->
     <!-- Format string for displaying a storage path as volume/filename -->
     <string name="StorageUtil__s_s">%1$s/%2$s</string>

core/ui/src/test/java/org/signal/core/ui/NavigationTypeTest.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class NavigationTypeTest {
+
+  @Test
+  fun `given a small window, then navigation is a bar`() {
+    assertEquals(NavigationType.BAR, WindowBreakpoint.Small(isWidthExpanded = false, isHeightExpanded = false).navigationType)
+  }
+
+  /**
+   * A small window is a bar however wide it reports itself: a compact height is enough to rule out a rail,
+   * and that is the case a landscape phone lands in.
+   */
+  @Test
+  fun `given a small window that is wide, then navigation is still a bar`() {
+    assertEquals(NavigationType.BAR, WindowBreakpoint.Small(isWidthExpanded = true, isHeightExpanded = false).navigationType)
+  }
+
+  @Test
+  fun `given a medium window that is not wide, then navigation is a bar`() {
+    assertEquals(NavigationType.BAR, WindowBreakpoint.Medium(isWidthExpanded = false, isHeightExpanded = true).navigationType)
+  }
+
+  @Test
+  fun `given a medium window that is wide, then navigation is a rail`() {
+    assertEquals(NavigationType.RAIL, WindowBreakpoint.Medium(isWidthExpanded = true, isHeightExpanded = true).navigationType)
+  }
+
+  @Test
+  fun `given a large window, then navigation is a rail`() {
+    assertEquals(NavigationType.RAIL, WindowBreakpoint.Large(isWidthExpanded = true, isHeightExpanded = true).navigationType)
+  }
+
+  /**
+   * A tablet in portrait is not width-expanded but is still a large window, which is what separates it from
+   * the medium case above.
+   */
+  @Test
+  fun `given a large window that is not wide, then navigation is still a rail`() {
+    assertEquals(NavigationType.RAIL, WindowBreakpoint.Large(isWidthExpanded = false, isHeightExpanded = true).navigationType)
+  }
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/ListDetailBackStackPersistenceTest.kt

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import android.app.Application
+import androidx.lifecycle.SavedStateHandle
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import org.signal.core.ui.backStack
+
+/**
+ * A stack owned by a [SavedStateHandle] is persisted through `kotlinx.serialization` rather than
+ * `Parcelable`, via `NavKeySerializer`, which writes each entry's concrete class name and reflects the
+ * serializer back on restore. That only fails at runtime, and only after process death, so it is worth
+ * covering directly.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class)
+class ListDetailBackStackPersistenceTest {
+
+  private class Host(savedStateHandle: SavedStateHandle) {
+    val backStack: ListDetailBackStack by savedStateHandle.backStack("test_back_stack", TestListKey.ROOT)
+  }
+
+  /** Saves and restores the handle the way a process death would. */
+  private fun SavedStateHandle.roundTrip(): SavedStateHandle {
+    return SavedStateHandle.createHandle(savedStateProvider().saveState(), null)
+  }
+
+  @Test
+  fun `given a new stack, when restored, then it is still at its root`() {
+    val handle = SavedStateHandle()
+    Host(handle).backStack
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(listOf(TestListKey.ROOT), restored.toList())
+  }
+
+  @Test
+  fun `given a stacked detail location, when restored, then the whole stack comes back`() {
+    val handle = SavedStateHandle()
+    Host(handle).backStack.apply {
+      push(TestDetailKey(7))
+      push(TestSubScreenKey(7, "movie night"))
+    }
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(
+      listOf(TestListKey.ROOT, TestDetailKey(7), TestSubScreenKey(7, "movie night")),
+      restored.toList()
+    )
+  }
+
+  /**
+   * A list key is free to be an enum, which `@Parcelize` cannot handle — this is what lets list locations
+   * sit at the root of a stack.
+   */
+  @Test
+  fun `given a list location on the stack, when restored, then the enum entry comes back`() {
+    val handle = SavedStateHandle()
+    Host(handle).backStack.push(TestListKey.PUSHED)
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(TestListKey.PUSHED, restored.last())
+  }
+
+  @Test
+  fun `given a stack mixing list and detail keys, when restored, then order and types are preserved`() {
+    val handle = SavedStateHandle()
+    Host(handle).backStack.apply {
+      push(TestDetailKey(34))
+      push(TestListKey.PUSHED)
+    }
+
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(
+      listOf(TestListKey.ROOT, TestListKey.PUSHED, TestDetailKey(34)),
+      restored.toList()
+    )
+  }
+
+  /**
+   * The stack is a snapshot state list, so a change made after the first save has to be written by the
+   * next one rather than the handle holding on to the state it was given.
+   */
+  @Test
+  fun `given a stack that changed since the last save, when restored, then the change is included`() {
+    val handle = SavedStateHandle()
+    val backStack = Host(handle).backStack
+    handle.roundTrip()
+
+    backStack.push(TestDetailKey(1))
+    val restored = Host(handle.roundTrip()).backStack
+
+    assertEquals(listOf(TestListKey.ROOT, TestDetailKey(1)), restored.toList())
+  }
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/ListDetailBackStackTest.kt

@@ -0,0 +1,261 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.navigation3.runtime.NavBackStack
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class ListDetailBackStackTest {
+
+  private val contentRoot = TestDetailKey(1)
+  private val otherContentRoot = TestDetailKey(2)
+  private val subScreen = TestSubScreenKey(1)
+
+  private fun backStack(root: TestListKey = TestListKey.ROOT): ListDetailBackStack {
+    return NavBackStack(root)
+  }
+
+  @Test
+  fun `given a new stack, then it displays its root list with no detail`() {
+    val backStack = backStack()
+
+    assertEquals(TestListKey.ROOT, backStack.listLocation<TestListKey>())
+    assertFalse(backStack.hasDetail)
+  }
+
+  @Test
+  fun `given a stack with only a list, when pushing detail, then detail is displayed above it`() {
+    val backStack = backStack()
+
+    backStack.push(contentRoot)
+
+    assertTrue(backStack.hasDetail)
+    assertEquals(TestListKey.ROOT, backStack.listLocation<TestListKey>())
+    assertEquals(listOf(TestListKey.ROOT, contentRoot), backStack.toList())
+  }
+
+  @Test
+  fun `given detail is displayed, when pushing another content root, then it replaces the previous detail`() {
+    val backStack = backStack()
+
+    backStack.push(contentRoot)
+    backStack.push(subScreen)
+    backStack.push(otherContentRoot)
+
+    assertEquals(listOf(TestListKey.ROOT, otherContentRoot), backStack.toList())
+  }
+
+  @Test
+  fun `given detail is displayed, when pushing a sub screen, then it stacks on top`() {
+    val backStack = backStack()
+
+    backStack.push(contentRoot)
+    backStack.push(subScreen)
+
+    assertEquals(listOf(TestListKey.ROOT, contentRoot, subScreen), backStack.toList())
+  }
+
+  @Test
+  fun `given an entry on top, when pushing that same entry, then it is not duplicated`() {
+    val backStack = backStack()
+
+    backStack.push(contentRoot)
+    backStack.push(contentRoot)
+
+    assertEquals(listOf(TestListKey.ROOT, contentRoot), backStack.toList())
+  }
+
+  @Test
+  fun `given a list is displayed, when pushing another list, then that list is displayed`() {
+    val backStack = backStack()
+
+    backStack.push(TestListKey.PUSHED)
+
+    assertEquals(TestListKey.PUSHED, backStack.listLocation<TestListKey>())
+    assertFalse(backStack.hasDetail)
+  }
+
+  @Test
+  fun `given a list is already displayed, when pushing it again, then it is not duplicated`() {
+    val backStack = backStack()
+
+    backStack.push(TestListKey.ROOT)
+
+    assertEquals(listOf(TestListKey.ROOT), backStack.toList())
+  }
+
+  /**
+   * Pushing a list while detail is open must not blank the detail pane, so the list is inserted beneath
+   * the detail content rather than on top of it.
+   */
+  @Test
+  fun `given detail is displayed, when pushing a list, then the detail stays displayed`() {
+    val backStack = backStack()
+    backStack.push(contentRoot)
+
+    backStack.push(TestListKey.PUSHED)
+
+    assertEquals(TestListKey.PUSHED, backStack.listLocation<TestListKey>())
+    assertTrue(backStack.hasDetail)
+    assertEquals(
+      listOf(TestListKey.ROOT, TestListKey.PUSHED, contentRoot),
+      backStack.toList()
+    )
+  }
+
+  /**
+   * Because lists sit beneath detail content, back closes the detail before leaving the list it was
+   * opened from.
+   */
+  @Test
+  fun `given detail over a pushed list, when popping, then the detail closes before the list`() {
+    val backStack = backStack()
+    backStack.push(contentRoot)
+    backStack.push(TestListKey.PUSHED)
+
+    backStack.pop()
+
+    assertEquals(TestListKey.PUSHED, backStack.listLocation<TestListKey>())
+    assertFalse(backStack.hasDetail)
+
+    backStack.pop()
+
+    assertEquals(TestListKey.ROOT, backStack.listLocation<TestListKey>())
+  }
+
+  @Test
+  fun `given a pushed list, when popping to the list beneath it, then that list is displayed`() {
+    val backStack = backStack()
+    backStack.push(TestListKey.PUSHED)
+
+    backStack.popToList(TestListKey.ROOT)
+
+    assertEquals(listOf(TestListKey.ROOT), backStack.toList())
+  }
+
+  @Test
+  fun `given detail over a pushed list, when popping to the list beneath it, then the detail stays displayed`() {
+    val backStack = backStack()
+    backStack.push(contentRoot)
+    backStack.push(TestListKey.PUSHED)
+
+    backStack.popToList(TestListKey.ROOT)
+
+    assertEquals(TestListKey.ROOT, backStack.listLocation<TestListKey>())
+    assertTrue(backStack.hasDetail)
+    assertEquals(listOf(TestListKey.ROOT, contentRoot), backStack.toList())
+  }
+
+  @Test
+  fun `given the displayed list is already the target, when popping to it, then the stack is unchanged`() {
+    val backStack = backStack()
+    backStack.push(contentRoot)
+
+    backStack.popToList(TestListKey.ROOT)
+
+    assertEquals(listOf(TestListKey.ROOT, contentRoot), backStack.toList())
+  }
+
+  /**
+   * Every stack keeps its own root list, so a list belonging to another stack is not on it. Stripping
+   * lists in search of one would leave nothing to display.
+   */
+  @Test
+  fun `given a location that is not on the stack, when popping to it, then the stack is unchanged`() {
+    val backStack = backStack()
+    backStack.push(TestListKey.PUSHED)
+    backStack.push(contentRoot)
+
+    backStack.popToList(TestListKey.OTHER)
+
+    assertEquals(
+      listOf(TestListKey.ROOT, TestListKey.PUSHED, contentRoot),
+      backStack.toList()
+    )
+  }
+
+  @Test
+  fun `given a pushed list, when popped, then the previous list is displayed again`() {
+    val backStack = backStack()
+    backStack.push(TestListKey.PUSHED)
+
+    assertTrue(backStack.pop())
+
+    assertEquals(TestListKey.ROOT, backStack.listLocation<TestListKey>())
+  }
+
+  @Test
+  fun `given a stack at its root, when popped, then nothing is removed`() {
+    val backStack = backStack()
+
+    assertFalse(backStack.pop())
+
+    assertEquals(listOf(TestListKey.ROOT), backStack.toList())
+  }
+
+  /**
+   * Handing an empty list to a NavDisplay throws, so popping must never drain the stack no matter how
+   * many times it is called.
+   */
+  @Test
+  fun `given a deep stack, when popped past the root, then the stack is never empty`() {
+    val backStack = backStack()
+    backStack.push(TestListKey.PUSHED)
+    backStack.push(contentRoot)
+    backStack.push(subScreen)
+
+    repeat(10) { backStack.pop() }
+
+    assertTrue(backStack.isNotEmpty())
+    assertEquals(listOf(TestListKey.ROOT), backStack.toList())
+  }
+
+  @Test
+  fun `given stacked detail, when exiting detail, then only the detail is dropped`() {
+    val backStack = backStack()
+    backStack.push(contentRoot)
+    backStack.push(subScreen)
+
+    backStack.exitDetail()
+
+    assertFalse(backStack.hasDetail)
+    assertEquals(listOf(TestListKey.ROOT), backStack.toList())
+  }
+
+  @Test
+  fun `given detail opened from a pushed list, when exiting detail, then the pushed list stays`() {
+    val backStack = backStack()
+    backStack.push(TestListKey.PUSHED)
+    backStack.push(contentRoot)
+
+    backStack.exitDetail()
+
+    assertEquals(TestListKey.PUSHED, backStack.listLocation<TestListKey>())
+    assertFalse(backStack.hasDetail)
+    assertEquals(listOf(TestListKey.ROOT, TestListKey.PUSHED), backStack.toList())
+  }
+
+  @Test
+  fun `given no detail, when exiting detail, then the stack is unchanged`() {
+    val backStack = backStack()
+
+    backStack.exitDetail()
+
+    assertEquals(listOf(TestListKey.ROOT), backStack.toList())
+  }
+
+  @Test(expected = ClassCastException::class)
+  fun `given a list key of another type, when reading the list location, then it throws`() {
+    val backStack: ListDetailBackStack = NavBackStack(OtherListKey)
+
+    backStack.listLocation<TestListKey>()
+  }
+
+  private object OtherListKey : ListNavKey
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/ListDetailNavigatorTest.kt

@@ -0,0 +1,235 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import android.app.Application
+import androidx.compose.runtime.snapshots.Snapshot
+import androidx.lifecycle.SavedStateHandle
+import kotlinx.coroutines.test.TestScope
+import kotlinx.coroutines.test.runCurrent
+import kotlinx.coroutines.test.runTest
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+/**
+ * Robolectric because the stacks are owned by a [SavedStateHandle], which stores them in a `Bundle`.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class)
+class ListDetailNavigatorTest {
+
+  private val detail = TestDetailKey(1)
+  private val otherDetail = TestDetailKey(2)
+  private val subScreen = TestSubScreenKey(1)
+
+  private fun TestScope.navigator(initialRoot: TestListKey = TestListKey.ROOT): ListDetailNavigator<TestListKey, DetailNavKey> {
+    return ListDetailNavigator(
+      savedStateHandle = SavedStateHandle(),
+      scope = backgroundScope,
+      stackKeys = mapOf(
+        TestListKey.ROOT to "root_stack",
+        TestListKey.OTHER to "other_stack"
+      ),
+      initialRoot = initialRoot
+    )
+  }
+
+  @Test
+  fun `given a new navigator, then each root has its own stack`() = runTest {
+    val navigator = navigator()
+
+    assertEquals(listOf(TestListKey.ROOT), navigator[TestListKey.ROOT])
+    assertEquals(listOf(TestListKey.OTHER), navigator[TestListKey.OTHER])
+    assertEquals(TestListKey.ROOT, navigator.currentRoot.value)
+  }
+
+  @Test
+  fun `when pushing detail, then it lands on that root's stack and the detail pane is revealed`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.LIST_ONLY))
+
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+
+    assertEquals(listOf(TestListKey.ROOT, detail), navigator[TestListKey.ROOT])
+    assertEquals(PaneAnchor.DETAIL_ONLY, navigator.paneAnchor.value)
+  }
+
+  /**
+   * A root that is not displayed can still be pushed to — a deep link arriving for another root leaves it
+   * waiting there rather than switching the window to it.
+   */
+  @Test
+  fun `when pushing detail onto another root, then the displayed stack is untouched`() = runTest {
+    val navigator = navigator()
+
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.OTHER))
+
+    assertEquals(listOf(TestListKey.ROOT), navigator[TestListKey.ROOT])
+    assertEquals(listOf(TestListKey.OTHER, detail), navigator[TestListKey.OTHER])
+    assertEquals(TestListKey.ROOT, navigator.currentRoot.value)
+  }
+
+  @Test
+  fun `when exiting detail, then the current stack drops it and the list pane is revealed`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.DETAIL_ONLY))
+
+    navigator.processEvent(ListDetailEvents.ExitDetail)
+
+    assertEquals(listOf(TestListKey.ROOT), navigator[TestListKey.ROOT])
+    assertEquals(PaneAnchor.LIST_ONLY, navigator.paneAnchor.value)
+  }
+
+  /**
+   * Popping one of several detail entries leaves detail on screen, so the pane it is displayed in has to
+   * stay as the user left it.
+   */
+  @Test
+  fun `given stacked detail, when popping, then the list pane is not revealed`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+    navigator.processEvent(ListDetailEvents.Push(subScreen, TestListKey.ROOT))
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.DETAIL_ONLY))
+
+    navigator.processEvent(ListDetailEvents.Back)
+
+    assertEquals(listOf(TestListKey.ROOT, detail), navigator[TestListKey.ROOT])
+    assertEquals(PaneAnchor.DETAIL_ONLY, navigator.paneAnchor.value)
+  }
+
+  @Test
+  fun `given one detail, when popping, then the list pane is revealed`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.DETAIL_ONLY))
+
+    navigator.processEvent(ListDetailEvents.Back)
+
+    assertEquals(PaneAnchor.LIST_ONLY, navigator.paneAnchor.value)
+  }
+
+  @Test
+  fun `when going to another root, then it is displayed and the list pane is revealed`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.DETAIL_ONLY))
+
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.OTHER))
+
+    assertEquals(TestListKey.OTHER, navigator.currentRoot.value)
+    assertEquals(PaneAnchor.LIST_ONLY, navigator.paneAnchor.value)
+  }
+
+  /**
+   * Each root keeps its own stack, so a root that had detail open comes back to it rather than to its
+   * list.
+   */
+  @Test
+  fun `given detail open on a root, when leaving and returning to it, then the detail is still there`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.OTHER))
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.ROOT))
+
+    assertEquals(listOf(TestListKey.ROOT, detail), navigator[TestListKey.ROOT])
+  }
+
+  @Test
+  fun `when pushing a list onto a root, then it is displayed above that root`() = runTest {
+    val navigator = navigator()
+
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.PUSHED, root = TestListKey.ROOT, push = true))
+
+    assertEquals(listOf(TestListKey.ROOT, TestListKey.PUSHED), navigator[TestListKey.ROOT])
+    assertEquals(TestListKey.ROOT, navigator.currentRoot.value)
+  }
+
+  @Test
+  fun `given a pushed list, when going back to its root, then the pushed list is dropped`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.PUSHED, root = TestListKey.ROOT, push = true))
+
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.ROOT))
+
+    assertEquals(listOf(TestListKey.ROOT), navigator[TestListKey.ROOT])
+  }
+
+  @Test
+  fun `when revealing the list, then the pane moves without the stack changing`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.DETAIL_ONLY))
+
+    navigator.processEvent(ListDetailEvents.RevealList)
+
+    assertEquals(PaneAnchor.LIST_ONLY, navigator.paneAnchor.value)
+    assertEquals(listOf(TestListKey.ROOT, detail), navigator[TestListKey.ROOT])
+  }
+
+  @Test
+  fun `given a split window, then no pane is full screen`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.SPLIT))
+    runCurrent()
+
+    assertFalse(navigator.isFullScreenPane.value)
+
+    navigator.processEvent(ListDetailEvents.AnchorSelected(PaneAnchor.DETAIL_ONLY))
+    runCurrent()
+
+    assertTrue(navigator.isFullScreenPane.value)
+  }
+
+  @Test
+  fun `when the displayed stack changes, then the displayed list and detail follow it`() = runTest {
+    val navigator = navigator()
+    runCurrent()
+
+    assertEquals(TestListKey.ROOT, navigator.displayedList.value)
+    assertNull(navigator.detail.value)
+    assertFalse(navigator.hasDetail.value)
+
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+    settle()
+
+    assertEquals(detail, navigator.detail.value)
+    assertTrue(navigator.hasDetail.value)
+
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.PUSHED, root = TestListKey.ROOT, push = true))
+    settle()
+
+    assertEquals(TestListKey.PUSHED, navigator.displayedList.value)
+  }
+
+  @Test
+  fun `when moving to another root, then the displayed detail is that root's`() = runTest {
+    val navigator = navigator()
+    navigator.processEvent(ListDetailEvents.Push(detail, TestListKey.ROOT))
+    navigator.processEvent(ListDetailEvents.Push(otherDetail, TestListKey.OTHER))
+    settle()
+
+    assertEquals(detail, navigator.detail.value)
+
+    navigator.processEvent(ListDetailEvents.GoToList(TestListKey.OTHER))
+    settle()
+
+    assertEquals(otherDetail, navigator.detail.value)
+    assertEquals(TestListKey.OTHER, navigator.displayedList.value)
+  }
+
+  /** Publishes snapshot writes to `snapshotFlow`, then lets the flows collecting them run. */
+  private fun TestScope.settle() {
+    Snapshot.sendApplyNotifications()
+    runCurrent()
+  }
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/ListDetailSceneStrategyTest.kt

@@ -0,0 +1,210 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.scene.Scene
+import androidx.navigation3.scene.SceneStrategyScope
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * Covers what a scene reports as its previous entries, because that is the whole of the display's back
+ * behaviour: `NavDisplay` enables its back handler when a scene has previous entries, and pops the
+ * difference between the stack and them. Both are reproduced here by [isBackEnabled] and [popsOnBack].
+ */
+class ListDetailSceneStrategyTest {
+
+  private val rootEntry = listEntry(TestListKey.ROOT)
+
+  private val pushedEntry = listEntry(TestListKey.PUSHED)
+
+  private val otherRootEntry = listEntry(TestListKey.OTHER)
+
+  private val detailEntry = detailEntry(TestDetailKey(1))
+  private val subScreenEntry = detailEntry(TestSubScreenKey(1))
+
+  private val fullScreenEntry = plainEntry(TestFullScreenKey)
+
+  @Test
+  fun `given split pane showing a list on its own, then back is left to the rest of the app`() {
+    val entries = listOf(rootEntry)
+
+    assertFalse(splitPane(entries).isBackEnabled)
+  }
+
+  /**
+   * The empty detail pane is not a destination: closing the last piece of detail content would leave the
+   * user on the same list looking at a placeholder, so back is left to the handlers outside the display.
+   */
+  @Test
+  fun `given split pane showing a list and one detail, then back is left to the rest of the app`() {
+    val entries = listOf(rootEntry, detailEntry)
+
+    assertFalse(splitPane(entries).isBackEnabled)
+  }
+
+  @Test
+  fun `given split pane showing stacked detail, when going back, then only the top detail is popped`() {
+    val entries = listOf(rootEntry, detailEntry, subScreenEntry)
+    val scene = splitPane(entries)
+
+    assertTrue(scene.isBackEnabled)
+    assertEquals(1, scene.popsOnBack(entries))
+    assertEquals(listOf(rootEntry, detailEntry), scene.previousEntries)
+  }
+
+  @Test
+  fun `given split pane showing a pushed list, when going back, then that list is popped`() {
+    val entries = listOf(rootEntry, pushedEntry)
+    val scene = splitPane(entries)
+
+    assertTrue(scene.isBackEnabled)
+    assertEquals(1, scene.popsOnBack(entries))
+    assertEquals(listOf(rootEntry), scene.previousEntries)
+  }
+
+  /**
+   * Backing out of a pushed list with detail open takes the detail with it, rather than stopping on the
+   * pushed list with an empty detail pane.
+   */
+  @Test
+  fun `given split pane showing detail over a pushed list, when going back, then both are popped`() {
+    val entries = listOf(rootEntry, pushedEntry, detailEntry)
+    val scene = splitPane(entries)
+
+    assertTrue(scene.isBackEnabled)
+    assertEquals(2, scene.popsOnBack(entries))
+    assertEquals(listOf(rootEntry), scene.previousEntries)
+  }
+
+  @Test
+  fun `given split pane, then it displays the current list beside the topmost detail`() {
+    val scene = splitPane(listOf(rootEntry, pushedEntry, detailEntry, subScreenEntry))
+
+    assertEquals(listOf(pushedEntry, subScreenEntry), scene.entries)
+  }
+
+  /**
+   * One scene identity for every list, so that swapping between them is not a scene change and the
+   * display does not cross-fade the whole window.
+   */
+  @Test
+  fun `given split pane, then every list shares one scene identity`() {
+    val root = splitPane(listOf(rootEntry))
+    val other = splitPane(listOf(otherRootEntry))
+
+    assertEquals(root.key, other.key)
+  }
+
+  @Test
+  fun `given a single pane showing a list on its own, then back is left to the rest of the app`() {
+    assertFalse(singlePane(listOf(rootEntry)).isBackEnabled)
+  }
+
+  /**
+   * The single-pane counterpart of the split-pane case above: here the detail covers the list rather than
+   * sitting beside it, so closing it is a real destination.
+   */
+  @Test
+  fun `given a single pane showing detail, when going back, then the detail is popped`() {
+    val entries = listOf(rootEntry, detailEntry)
+    val scene = singlePane(entries)
+
+    assertTrue(scene.isBackEnabled)
+    assertEquals(1, scene.popsOnBack(entries))
+  }
+
+  @Test
+  fun `given a single pane showing detail over a pushed list, when going back, then the list stays`() {
+    val entries = listOf(rootEntry, pushedEntry, detailEntry)
+    val scene = singlePane(entries)
+
+    assertEquals(1, scene.popsOnBack(entries))
+    assertEquals(listOf(rootEntry, pushedEntry), scene.previousEntries)
+  }
+
+  @Test
+  fun `given a single pane, then it displays only the top entry`() {
+    val scene = singlePane(listOf(rootEntry, detailEntry))
+
+    assertEquals(listOf(detailEntry), scene.entries)
+  }
+
+  @Test
+  fun `given a single pane, then lists share an identity that detail content does not`() {
+    val root = singlePane(listOf(rootEntry))
+    val other = singlePane(listOf(otherRootEntry))
+    val detail = singlePane(listOf(rootEntry, detailEntry))
+
+    assertEquals(root.key, other.key)
+    assertNotEquals(root.key, detail.key)
+  }
+
+  @Test
+  fun `given split pane with a full screen entry on top, then it is displayed instead of the panes`() {
+    val scene = splitPane(listOf(rootEntry, detailEntry, fullScreenEntry))
+
+    assertEquals(listOf(fullScreenEntry), scene.entries)
+  }
+
+  @Test
+  fun `given a full screen entry on top, when going back, then only it is popped`() {
+    val entries = listOf(rootEntry, detailEntry, fullScreenEntry)
+
+    listOf(splitPane(entries), singlePane(entries)).forEach { scene ->
+      assertTrue(scene.isBackEnabled)
+      assertEquals(1, scene.popsOnBack(entries))
+      assertEquals(listOf(rootEntry, detailEntry), scene.previousEntries)
+    }
+  }
+
+  /**
+   * The list keeps one scene identity so that swapping lists does not cross-fade the window; a full screen
+   * entry is a window of its own, and moving to it should be a scene change.
+   */
+  @Test
+  fun `given a full screen entry, then it does not share the list scene identity`() {
+    val list = splitPane(listOf(rootEntry))
+    val fullScreen = splitPane(listOf(rootEntry, fullScreenEntry))
+
+    assertNotEquals(list.key, fullScreen.key)
+  }
+
+  private fun splitPane(entries: List<NavEntry<NavKey>>): Scene<NavKey> = scene(entries, isSplitPane = true)
+
+  private fun singlePane(entries: List<NavEntry<NavKey>>): Scene<NavKey> = scene(entries, isSplitPane = false)
+
+  private fun scene(entries: List<NavEntry<NavKey>>, isSplitPane: Boolean): Scene<NavKey> {
+    return with(ListDetailSceneStrategy(isSplitPane)) {
+      SceneStrategyScope<NavKey>().calculateScene(entries)
+    }!!
+  }
+
+  /** What `NavDisplay` uses to decide whether to register a back handler at all. */
+  private val Scene<NavKey>.isBackEnabled: Boolean
+    get() = previousEntries.isNotEmpty()
+
+  /** What `NavDisplay` pops when its back handler completes. */
+  private fun Scene<NavKey>.popsOnBack(entries: List<NavEntry<NavKey>>): Int = entries.size - previousEntries.size
+
+  private fun listEntry(location: ListNavKey): NavEntry<NavKey> {
+    return NavEntry(key = location, metadata = listPaneMetadata()) {}
+  }
+
+  private fun detailEntry(location: DetailNavKey): NavEntry<NavKey> {
+    return NavEntry(key = location, metadata = detailPaneMetadata()) {}
+  }
+
+  /** An entry registered with a plain `entry`, claiming neither pane. */
+  private fun plainEntry(location: NavKey): NavEntry<NavKey> {
+    return NavEntry(key = location) {}
+  }
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/PaneAnchorControllerTest.kt

@@ -0,0 +1,120 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.lifecycle.SavedStateHandle
+import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.test.runTest
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class PaneAnchorControllerTest {
+
+  private fun controller(initial: PaneAnchor? = null): PaneAnchorController {
+    val controller = PaneAnchorController(SavedStateHandle())
+    if (initial != null) {
+      controller.onAnchorSelected(initial)
+    }
+    return controller
+  }
+
+  @Test
+  fun `given a fresh controller, then both panes are shown`() {
+    assertEquals(PaneAnchor.SPLIT, controller().anchor.value)
+  }
+
+  @Test
+  fun `given a drag settles on an anchor, then that anchor is selected`() {
+    val controller = controller()
+
+    controller.onAnchorSelected(PaneAnchor.DETAIL_ONLY)
+
+    assertEquals(PaneAnchor.DETAIL_ONLY, controller.anchor.value)
+  }
+
+  @Test
+  fun `given the list fills the window, when detail content opens, then the detail is revealed`() {
+    val controller = controller(PaneAnchor.LIST_ONLY)
+
+    controller.revealDetailPane()
+
+    assertEquals(PaneAnchor.DETAIL_ONLY, controller.anchor.value)
+  }
+
+  /**
+   * A window already showing both panes is an arrangement the user chose; opening a conversation in the
+   * detail pane must not collapse the list out from under them.
+   */
+  @Test
+  fun `given both panes are shown, when detail content opens, then the arrangement is left alone`() {
+    val controller = controller(PaneAnchor.SPLIT)
+
+    controller.revealDetailPane()
+
+    assertEquals(PaneAnchor.SPLIT, controller.anchor.value)
+  }
+
+  @Test
+  fun `given the detail already fills the window, when detail content opens, then it stays filled`() {
+    val controller = controller(PaneAnchor.DETAIL_ONLY)
+
+    controller.revealDetailPane()
+
+    assertEquals(PaneAnchor.DETAIL_ONLY, controller.anchor.value)
+  }
+
+  @Test
+  fun `given the detail fills the window, when the list is revealed, then the list fills the window`() {
+    val controller = controller(PaneAnchor.DETAIL_ONLY)
+
+    controller.revealListPane()
+
+    assertEquals(PaneAnchor.LIST_ONLY, controller.anchor.value)
+  }
+
+  @Test
+  fun `given both panes are shown, when the list is revealed, then the arrangement is left alone`() {
+    val controller = controller(PaneAnchor.SPLIT)
+
+    controller.revealListPane()
+
+    assertEquals(PaneAnchor.SPLIT, controller.anchor.value)
+  }
+
+  @Test
+  fun `given the list already fills the window, when the list is revealed, then it stays filled`() {
+    val controller = controller(PaneAnchor.LIST_ONLY)
+
+    controller.revealListPane()
+
+    assertEquals(PaneAnchor.LIST_ONLY, controller.anchor.value)
+  }
+
+  @Test
+  fun `given both panes are shown, then neither pane is full screen`() = runTest {
+    assertFalse(controller(PaneAnchor.SPLIT).isFullScreenPane.first())
+  }
+
+  @Test
+  fun `given one pane fills the window, then it is reported as full screen`() = runTest {
+    assertTrue(controller(PaneAnchor.DETAIL_ONLY).isFullScreenPane.first())
+    assertTrue(controller(PaneAnchor.LIST_ONLY).isFullScreenPane.first())
+  }
+
+  /**
+   * The anchor is persisted so that it survives a configuration change without the layout having to
+   * re-derive it.
+   */
+  @Test
+  fun `given a selected anchor, when rebuilt from the same saved state, then the anchor is restored`() {
+    val savedStateHandle = SavedStateHandle()
+    PaneAnchorController(savedStateHandle).onAnchorSelected(PaneAnchor.LIST_ONLY)
+
+    assertEquals(PaneAnchor.LIST_ONLY, PaneAnchorController(savedStateHandle).anchor.value)
+  }
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/PaneExpansionStateTest.kt

@@ -0,0 +1,153 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.compose.runtime.MonotonicFrameClock
+import androidx.compose.ui.unit.Density
+import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.test.runTest
+import kotlinx.coroutines.withContext
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class PaneExpansionStateTest {
+
+  private val anchorOffsets = mapOf(
+    PaneAnchor.DETAIL_ONLY to 100f,
+    PaneAnchor.SPLIT to 500f,
+    PaneAnchor.LIST_ONLY to 900f
+  )
+
+  private val selected = mutableListOf<PaneAnchor>()
+
+  private fun expansion(initialOffsetPx: Float = 500f, withAnchors: Boolean = true): PaneExpansionState {
+    return PaneExpansionState(Density(2f), initialOffsetPx) { selected += it }.apply {
+      if (withAnchors) {
+        updateAnchors(anchorOffsets)
+      }
+    }
+  }
+
+  @Test
+  fun `given an initial offset, then the list is that wide at this density`() {
+    assertEquals(250.dp, expansion(initialOffsetPx = 500f).listWidth)
+  }
+
+  /**
+   * The layout publishes the anchors from measurement, which happens after construction. A drag arriving
+   * first has nothing to clamp against, so it is dropped rather than moving the divider anywhere.
+   */
+  @Test
+  fun `given anchors have not been published, when dragging, then the divider does not move`() {
+    val expansion = expansion(withAnchors = false)
+
+    expansion.dragBy(100f)
+
+    assertEquals(250.dp, expansion.listWidth)
+  }
+
+  @Test
+  fun `given a drag within the anchors, then the divider follows it`() {
+    val expansion = expansion()
+
+    expansion.dragBy(-100f)
+
+    assertEquals(200.dp, expansion.listWidth)
+  }
+
+  @Test
+  fun `given a drag past the widest anchor, then the divider stops there`() {
+    val expansion = expansion()
+
+    expansion.dragBy(1000f)
+
+    assertEquals(450.dp, expansion.listWidth)
+  }
+
+  @Test
+  fun `given a drag past the narrowest anchor, then the divider stops there`() {
+    val expansion = expansion()
+
+    expansion.dragBy(-1000f)
+
+    assertEquals(50.dp, expansion.listWidth)
+  }
+
+  @Test
+  fun `given a drag that ended nearer another anchor, when settling, then that anchor is selected and moved to`() = runTest {
+    val expansion = expansion()
+    expansion.onDragStarted()
+    expansion.dragBy(300f)
+
+    withContext(TestFrameClock()) { expansion.settle() }
+
+    assertEquals(listOf(PaneAnchor.LIST_ONLY), selected)
+    assertEquals(450.dp, expansion.listWidth)
+  }
+
+  /**
+   * A state change arriving mid-gesture must not yank the divider out from under the user's finger.
+   */
+  @Test
+  fun `given a drag in progress, when told to go to an anchor, then it is ignored`() = runTest {
+    val expansion = expansion()
+    expansion.onDragStarted()
+
+    withContext(TestFrameClock()) { expansion.goTo(PaneAnchor.DETAIL_ONLY) }
+
+    assertEquals(250.dp, expansion.listWidth)
+  }
+
+  @Test
+  fun `given no drag in progress, when told to go to an anchor, then the divider moves there`() = runTest {
+    val expansion = expansion()
+
+    withContext(TestFrameClock()) { expansion.goTo(PaneAnchor.DETAIL_ONLY) }
+
+    assertEquals(50.dp, expansion.listWidth)
+  }
+
+  /**
+   * How the divider is operated without a gesture: screen readers cannot drag, so the accessibility action
+   * steps through the anchors in declared order and wraps around.
+   */
+  @Test
+  fun `when asked for the next anchor, then it cycles through them in declared order`() {
+    assertEquals(PaneAnchor.SPLIT, expansion(initialOffsetPx = 100f).nextAnchor())
+    assertEquals(PaneAnchor.LIST_ONLY, expansion(initialOffsetPx = 500f).nextAnchor())
+    assertEquals(PaneAnchor.DETAIL_ONLY, expansion(initialOffsetPx = 900f).nextAnchor())
+  }
+
+  @Test
+  fun `given anchors have not been published, when asked for the next anchor, then there is none`() {
+    assertNull(expansion(withAnchors = false).nextAnchor())
+  }
+
+  /**
+   * Selecting reports the anchor and nothing else — the state change that comes back is what moves the
+   * divider, by the same path a drag takes.
+   */
+  @Test
+  fun `when selecting an anchor, then it is reported without moving the divider`() {
+    val expansion = expansion()
+
+    expansion.selectAnchor(PaneAnchor.DETAIL_ONLY)
+
+    assertEquals(listOf(PaneAnchor.DETAIL_ONLY), selected)
+    assertEquals(250.dp, expansion.listWidth)
+  }
+
+  /** Runs animations to completion a frame at a time, without waiting on a real one. */
+  private class TestFrameClock : MonotonicFrameClock {
+    private var frameTimeNanos = 0L
+
+    override suspend fun <R> withFrameNanos(onFrame: (Long) -> R): R {
+      frameTimeNanos += 16_000_000
+      return onFrame(frameTimeNanos)
+    }
+  }
+}

core/ui/src/test/java/org/signal/core/ui/compose/split/TestNavKeys.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.ui.compose.split
+
+import androidx.navigation3.runtime.NavKey
+import kotlinx.serialization.Serializable
+
+/**
+ * Keys the split-pane tests navigate with. Serializable because the back stack is persisted through
+ * `kotlinx.serialization`, which resolves each entry's serializer from its concrete class.
+ */
+
+/** The list a stack is rooted at, a list pushed on top of it, and the root of a different stack. */
+@Serializable
+enum class TestListKey : ListNavKey {
+  ROOT,
+  PUSHED,
+  OTHER
+}
+
+/** Detail content root, so pushing one replaces the detail content already displayed. */
+@Serializable
+data class TestDetailKey(val id: Int) : DetailNavKey {
+  override val isContentRoot: Boolean get() = true
+}
+
+/** Not a content root, so pushing one stacks on top of the detail content already displayed. */
+@Serializable
+data class TestSubScreenKey(val id: Int, val label: String = "") : DetailNavKey {
+  override val isContentRoot: Boolean get() = false
+}
+
+/** Neither a list nor detail content: a screen that takes the whole window. */
+@Serializable
+data object TestFullScreenKey : NavKey

core/ui/src/testFixtures/java/org/signal/core/ui/CoreUiDependenciesRule.kt

@@ -26,6 +26,5 @@ class CoreUiDependenciesRule(
     override fun providePackageId(): String = "org.thoughtcrime.securesms"
     override fun provideIsIncognitoKeyboardEnabled(): Boolean = isIncognitoKeyboardEnabled
     override fun provideIsScreenSecurityEnabled(): Boolean = false
-    override fun provideForceSplitPane(): Boolean = false
   }
 }

demo/list-detail/.gitignore

@@ -0,0 +1 @@
+/build

demo/list-detail/build.gradle.kts

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+plugins {
+  id("signal-sample-app")
+  alias(libs.plugins.compose.compiler)
+  alias(libs.plugins.kotlinx.serialization)
+}
+
+android {
+  namespace = "org.signal.listdetail.demo"
+
+  defaultConfig {
+    applicationId = "org.signal.listdetail.demo"
+  }
+}
+
+dependencies {
+  implementation(project(":core:ui"))
+
+  implementation(libs.androidx.navigation3.runtime)
+  implementation(libs.androidx.navigation3.ui)
+
+  implementation(libs.androidx.lifecycle.runtime.compose)
+
+  implementation(libs.androidx.compose.material.icons.extended)
+
+  implementation(libs.kotlinx.serialization.json)
+}

demo/list-detail/src/main/AndroidManifest.xml

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ Copyright 2026 Signal Messenger, LLC
+  ~ SPDX-License-Identifier: AGPL-3.0-only
+  -->
+
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+
+    <application
+        android:allowBackup="true"
+        android:icon="@mipmap/ic_launcher"
+        android:label="@string/app_name"
+        android:roundIcon="@mipmap/ic_launcher_round"
+        android:supportsRtl="true"
+        android:theme="@style/Theme.Signal">
+        <!-- Mirrors MainActivity: no screenSize/screenLayout here, so folding recreates the activity. -->
+        <activity
+            android:name=".MainActivity"
+            android:configChanges="touchscreen|keyboard|keyboardHidden"
+            android:enableOnBackInvokedCallback="true"
+            android:exported="true"
+            android:label="@string/app_name"
+            android:resizeableActivity="true"
+            android:theme="@style/Theme.Signal">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+
+</manifest>

demo/list-detail/src/main/java/org/signal/listdetail/demo/DemoData.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+/** A row in a list, and the thing the detail pane displays. */
+data class DemoItem(val id: Int, val title: String, val subtitle: String)
+
+/**
+ * The contents of each list. All static: the demo is about navigation, not about data.
+ */
+object DemoData {
+
+  val inbox: List<DemoItem> = listOf(
+    DemoItem(1, "Nadia", "Sent a photo"),
+    DemoItem(2, "Weekend Plans", "Miguel: I can drive"),
+    DemoItem(3, "Priya", "Thanks!"),
+    DemoItem(4, "Book Club", "Ana: Chapter four tonight"),
+    DemoItem(5, "Sam", "Are you around later?"),
+    DemoItem(6, "Deniz", "Sent a voice message"),
+    DemoItem(7, "Roommates", "Jo: Trash goes out tomorrow"),
+    DemoItem(8, "Ines", "See you then")
+  )
+
+  val archive: List<DemoItem> = listOf(
+    DemoItem(101, "Old Group", "Archived last spring"),
+    DemoItem(102, "Yusuf", "Archived in March"),
+    DemoItem(103, "Delivery Updates", "Archived in January")
+  )
+
+  val contacts: List<DemoItem> = listOf(
+    DemoItem(201, "Ana", "+1 555 0100"),
+    DemoItem(202, "Deniz", "+1 555 0101"),
+    DemoItem(203, "Ines", "+1 555 0102"),
+    DemoItem(204, "Miguel", "+1 555 0103"),
+    DemoItem(205, "Nadia", "+1 555 0104"),
+    DemoItem(206, "Priya", "+1 555 0105"),
+    DemoItem(207, "Sam", "+1 555 0106"),
+    DemoItem(208, "Yusuf", "+1 555 0107")
+  )
+
+  private val byId: Map<Int, DemoItem> = (inbox + archive + contacts).associateBy { it.id }
+
+  fun itemsFor(route: DemoListRoute): List<DemoItem> = when (route) {
+    DemoListRoute.INBOX -> inbox
+    DemoListRoute.ARCHIVE -> archive
+    DemoListRoute.CONTACTS -> contacts
+  }
+
+  operator fun get(id: Int): DemoItem = byId.getValue(id)
+}

demo/list-detail/src/main/java/org/signal/listdetail/demo/DemoEvents.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+import org.signal.core.ui.compose.split.ListDetailEvents
+
+/**
+ * Everything the user can do in this demo.
+ */
+sealed interface DemoEvents {
+
+  /** Navigating to the archive screen, which could have special processing. */
+  data object ArchiveSelected : DemoEvents
+
+  /** Something that is navigation and nothing else, so the navigator can answer it unaided. */
+  data class ListDetailEvent(val event: ListDetailEvents) : DemoEvents
+}

demo/list-detail/src/main/java/org/signal/listdetail/demo/DemoRoutes.kt

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+import androidx.navigation3.runtime.NavKey
+import kotlinx.serialization.Serializable
+import org.signal.core.ui.compose.split.DetailNavKey
+import org.signal.core.ui.compose.split.ListNavKey
+
+/**
+ * The lists this demo can display. Implementing [ListNavKey] is what puts them in the list pane.
+ *
+ * [INBOX] and [CONTACTS] are tabs, each with a stack of its own. [ARCHIVE] is not: it is pushed onto the
+ * inbox's stack, so opening it keeps whatever item was already open beside it.
+ */
+@Serializable
+enum class DemoListRoute(val label: String) : ListNavKey {
+  INBOX("Inbox"),
+  ARCHIVE("Archive"),
+  CONTACTS("Contacts");
+
+  /** The tab this list is displayed under, which is the root of the stack it lives on. */
+  val tab: DemoListRoute
+    get() = if (this == ARCHIVE) INBOX else this
+}
+
+/**
+ * The detail content this demo can display above a list. Implementing [DetailNavKey] is what puts them in
+ * the detail pane.
+ */
+@Serializable
+sealed interface DemoDetailRoute : DetailNavKey {
+
+  val itemId: Int
+
+  /** An item opened from a list. A content root, so opening another item replaces this one. */
+  @Serializable
+  data class Item(override val itemId: Int) : DemoDetailRoute {
+    override val isContentRoot: Boolean = true
+  }
+
+  /** Opened from an [Item]. Not a content root, so it stacks on top of the item instead of replacing it. */
+  @Serializable
+  data class Notes(override val itemId: Int) : DemoDetailRoute {
+    override val isContentRoot: Boolean = false
+  }
+}
+
+/**
+ * Settings, which are neither a list nor detail content: they take the whole window and navigate for
+ * themselves once open. A plain [NavKey], since nothing about the panes applies to them, and it stacks
+ * above whatever was open so that popping it puts the panes back the way they were.
+ */
+@Serializable
+data object DemoSettingsRoute : NavKey

demo/list-detail/src/main/java/org/signal/listdetail/demo/DemoScreens.kt

@@ -0,0 +1,262 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material.icons.automirrored.filled.KeyboardArrowRight
+import androidx.compose.material.icons.filled.Archive
+import androidx.compose.material3.Button
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.TopAppBarDefaults
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.vector.ImageVector
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import org.signal.core.ui.compose.DayNightPreviews
+import org.signal.core.ui.compose.Previews
+import org.signal.core.ui.compose.split.ListDetailEvents
+
+/**
+ * List-pane content. Every list is the same nav entry, so moving between the inbox, the archive and the
+ * contacts animates this pane alone and leaves the chrome around it untouched.
+ *
+ * @param selectedItemId the item open in the detail pane, highlighted so that a split-pane window shows
+ *   which row the detail beside it belongs to.
+ */
+@Composable
+fun ItemListPane(
+  route: DemoListRoute,
+  selectedItemId: Int?,
+  onEvent: (DemoEvents) -> Unit,
+  modifier: Modifier = Modifier
+) {
+  LazyColumn(modifier = modifier.fillMaxSize()) {
+    if (route == DemoListRoute.INBOX) {
+      item {
+        ItemRow(
+          title = DemoListRoute.ARCHIVE.label,
+          subtitle = "${DemoData.archive.size} conversations",
+          onClick = { onEvent(DemoEvents.ArchiveSelected) },
+          icon = Icons.Filled.Archive,
+          trailingIcon = Icons.AutoMirrored.Filled.KeyboardArrowRight
+        )
+      }
+    }
+
+    items(items = DemoData.itemsFor(route), key = { it.id }) { item ->
+      ItemRow(
+        title = item.title,
+        subtitle = item.subtitle,
+        onClick = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Push(DemoDetailRoute.Item(item.id)))) },
+        isSelected = item.id == selectedItemId
+      )
+    }
+  }
+}
+
+/** Detail-pane content for an item. */
+@Composable
+fun ItemDetailPane(
+  item: DemoItem,
+  onEvent: (DemoEvents) -> Unit,
+  modifier: Modifier = Modifier
+) {
+  DetailScaffold(title = item.title, onBack = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Back)) }, modifier = modifier) {
+    Text(
+      text = item.subtitle,
+      style = MaterialTheme.typography.bodyLarge,
+      color = MaterialTheme.colorScheme.onSurfaceVariant
+    )
+
+    Button(onClick = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Push(DemoDetailRoute.Notes(item.id)))) }) {
+      Text(text = "Open notes")
+    }
+  }
+}
+
+/** Detail-pane content stacked on top of [ItemDetailPane], rather than replacing it. */
+@Composable
+fun ItemNotesPane(
+  item: DemoItem,
+  onEvent: (DemoEvents) -> Unit,
+  modifier: Modifier = Modifier
+) {
+  DetailScaffold(title = "Notes", onBack = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Back)) }, modifier = modifier) {
+    Text(
+      text = "Notes about ${item.title}. Back returns to the item, not to the list.",
+      style = MaterialTheme.typography.bodyLarge,
+      textAlign = TextAlign.Center
+    )
+  }
+}
+
+/**
+ * Fills the detail pane before anything has been opened. Only ever seen in a split-pane window; a single
+ * pane displays the list instead.
+ */
+@Composable
+fun EmptyDetailPane(modifier: Modifier = Modifier) {
+  Column(
+    horizontalAlignment = Alignment.CenterHorizontally,
+    verticalArrangement = Arrangement.Center,
+    modifier = modifier.fillMaxSize()
+  ) {
+    Text(
+      text = "Select something from the list",
+      style = MaterialTheme.typography.bodyLarge,
+      color = MaterialTheme.colorScheme.onSurfaceVariant
+    )
+  }
+}
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+private fun DetailScaffold(
+  title: String,
+  onBack: () -> Unit,
+  modifier: Modifier = Modifier,
+  content: @Composable () -> Unit
+) {
+  Scaffold(
+    containerColor = Color.Transparent,
+    topBar = {
+      TopAppBar(
+        title = { Text(text = title) },
+        navigationIcon = {
+          IconButton(onClick = onBack) {
+            Icon(imageVector = Icons.AutoMirrored.Filled.ArrowBack, contentDescription = "Back")
+          }
+        },
+        colors = TopAppBarDefaults.topAppBarColors(containerColor = Color.Transparent)
+      )
+    },
+    modifier = modifier
+  ) { paddingValues ->
+    Column(
+      horizontalAlignment = Alignment.CenterHorizontally,
+      verticalArrangement = Arrangement.spacedBy(24.dp, Alignment.CenterVertically),
+      modifier = Modifier
+        .fillMaxSize()
+        .padding(paddingValues)
+        .padding(horizontal = 32.dp)
+    ) {
+      content()
+    }
+  }
+}
+
+@Composable
+private fun ItemRow(
+  title: String,
+  subtitle: String,
+  onClick: () -> Unit,
+  modifier: Modifier = Modifier,
+  icon: ImageVector? = null,
+  trailingIcon: ImageVector? = null,
+  isSelected: Boolean = false
+) {
+  Row(
+    verticalAlignment = Alignment.CenterVertically,
+    modifier = modifier
+      .fillMaxWidth()
+      .padding(horizontal = 8.dp, vertical = 2.dp)
+      .clip(MaterialTheme.shapes.large)
+      .background(if (isSelected) MaterialTheme.colorScheme.secondaryContainer else Color.Transparent)
+      .clickable(onClick = onClick)
+      .padding(horizontal = 16.dp, vertical = 12.dp)
+  ) {
+    Row(
+      horizontalArrangement = Arrangement.Center,
+      verticalAlignment = Alignment.CenterVertically,
+      modifier = Modifier
+        .size(40.dp)
+        .clip(CircleShape)
+        .background(MaterialTheme.colorScheme.surfaceVariant)
+    ) {
+      if (icon != null) {
+        Icon(imageVector = icon, contentDescription = null)
+      } else {
+        Text(text = title.take(1), style = MaterialTheme.typography.titleMedium)
+      }
+    }
+
+    Column(
+      modifier = Modifier
+        .weight(1f)
+        .padding(start = 16.dp)
+    ) {
+      Text(
+        text = title,
+        style = MaterialTheme.typography.bodyLarge,
+        maxLines = 1,
+        overflow = TextOverflow.Ellipsis
+      )
+
+      Text(
+        text = subtitle,
+        style = MaterialTheme.typography.bodyMedium,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+        maxLines = 1,
+        overflow = TextOverflow.Ellipsis
+      )
+    }
+
+    if (trailingIcon != null) {
+      Icon(
+        imageVector = trailingIcon,
+        contentDescription = null,
+        tint = MaterialTheme.colorScheme.onSurfaceVariant
+      )
+    }
+  }
+}
+
+@DayNightPreviews
+@Composable
+private fun ItemListPanePreview() {
+  Previews.Preview {
+    ItemListPane(
+      route = DemoListRoute.INBOX,
+      selectedItemId = 2,
+      onEvent = {}
+    )
+  }
+}
+
+@DayNightPreviews
+@Composable
+private fun ItemDetailPanePreview() {
+  Previews.Preview {
+    ItemDetailPane(
+      item = DemoData.inbox.first(),
+      onEvent = {}
+    )
+  }
+}

demo/list-detail/src/main/java/org/signal/listdetail/demo/DemoSettings.kt

@@ -0,0 +1,145 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material.icons.automirrored.filled.KeyboardArrowRight
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.entryProvider
+import androidx.navigation3.runtime.rememberNavBackStack
+import androidx.navigation3.ui.NavDisplay
+import kotlinx.serialization.Serializable
+import org.signal.core.ui.compose.split.ListDetailEvents
+import org.signal.core.ui.navigation.TransitionSpecs
+
+/**
+ * Settings' own nav keys. The stack outside knows about settings as a single entry; these are the screens
+ * within it, and nothing outside needs to know they exist.
+ */
+@Serializable
+private sealed interface SettingsRoute : NavKey {
+
+  @Serializable
+  data object Root : SettingsRoute
+
+  @Serializable
+  data class Section(val title: String) : SettingsRoute
+}
+
+private val SECTIONS = listOf("Notifications", "Privacy", "Storage")
+
+/**
+ * A section that takes the whole window and navigates for itself, which is what a plain `entry` gets you:
+ * no list pane, no divider, no empty detail — just this, over the panes it was pushed on top of.
+ *
+ * Its back stack is a [rememberNavBackStack], so the entry decorators hold it while the user is off in
+ * another tab and hand it back with settings still open on whatever screen they left it.
+ *
+ * @param onEvent reports back out to the demo, which is how settings gets popped off the stack outside
+ *   once this display is at its own root.
+ */
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+fun SettingsScreen(onEvent: (DemoEvents) -> Unit, modifier: Modifier = Modifier) {
+  val backStack = rememberNavBackStack(SettingsRoute.Root)
+
+  val paneShift = TransitionSpecs.paneShift()
+  val paneShiftPop = TransitionSpecs.paneShift(pop = true)
+
+  NavDisplay(
+    backStack = backStack,
+    onBack = { backStack.removeLastOrNull() },
+    transitionSpec = { paneShift },
+    popTransitionSpec = { paneShiftPop },
+    predictivePopTransitionSpec = { paneShiftPop },
+    modifier = modifier.fillMaxSize(),
+    entryProvider = entryProvider {
+      entry<SettingsRoute.Root> {
+        SettingsScaffold(title = "Settings", onBack = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Back)) }) {
+          SECTIONS.forEach { section ->
+            SettingsRow(title = section, onClick = { backStack.add(SettingsRoute.Section(section)) })
+          }
+        }
+      }
+
+      entry<SettingsRoute.Section> { route ->
+        SettingsScaffold(title = route.title, onBack = { backStack.removeLastOrNull() }) {
+          Text(
+            text = "Back here returns to settings, not to the list underneath.",
+            style = MaterialTheme.typography.bodyLarge,
+            modifier = Modifier.padding(horizontal = 24.dp, vertical = 16.dp)
+          )
+        }
+      }
+    }
+  )
+}
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+private fun SettingsScaffold(
+  title: String,
+  onBack: () -> Unit,
+  content: @Composable () -> Unit
+) {
+  Scaffold(
+    topBar = {
+      TopAppBar(
+        title = { Text(text = title) },
+        navigationIcon = {
+          IconButton(onClick = onBack) {
+            Icon(imageVector = Icons.AutoMirrored.Filled.ArrowBack, contentDescription = "Back")
+          }
+        }
+      )
+    }
+  ) { paddingValues ->
+    Column(
+      modifier = Modifier
+        .fillMaxSize()
+        .padding(paddingValues)
+    ) {
+      content()
+    }
+  }
+}
+
+@Composable
+private fun SettingsRow(title: String, onClick: () -> Unit) {
+  Row(
+    verticalAlignment = Alignment.CenterVertically,
+    modifier = Modifier
+      .fillMaxWidth()
+      .clickable(onClick = onClick)
+      .padding(horizontal = 24.dp, vertical = 20.dp)
+  ) {
+    Text(text = title, style = MaterialTheme.typography.bodyLarge, modifier = Modifier.weight(1f))
+
+    Icon(
+      imageVector = Icons.AutoMirrored.Filled.KeyboardArrowRight,
+      contentDescription = null,
+      tint = MaterialTheme.colorScheme.onSurfaceVariant
+    )
+  }
+}

demo/list-detail/src/main/java/org/signal/listdetail/demo/DemoViewModel.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+import androidx.lifecycle.SavedStateHandle
+import androidx.lifecycle.viewModelScope
+import kotlinx.coroutines.flow.StateFlow
+import org.signal.core.ui.compose.EventDrivenViewModel
+import org.signal.core.ui.compose.split.ListDetailEvents
+import org.signal.core.ui.compose.split.ListDetailNavigator
+import org.signal.core.ui.compose.split.PaneAnchor
+import org.signal.core.util.logging.Log
+
+/**
+ * Owns the navigation. Every event here is answered by a one-liner into [ListDetailNavigator], which is
+ * the point: you keep your screens' rules, and it keeps the stacks, the pane anchor, and the saved state.
+ */
+class DemoViewModel(savedStateHandle: SavedStateHandle) : EventDrivenViewModel<DemoEvents>(TAG) {
+
+  companion object {
+    private val TAG = Log.tag(DemoViewModel::class)
+  }
+
+  val navigator = ListDetailNavigator<DemoListRoute, DemoDetailRoute>(
+    savedStateHandle = savedStateHandle,
+    scope = viewModelScope,
+    stackKeys = mapOf(
+      DemoListRoute.INBOX to "inbox_stack",
+      DemoListRoute.CONTACTS to "contacts_stack"
+    ),
+    initialRoot = DemoListRoute.INBOX
+  )
+
+  /** The selected tab, which is the stack being displayed. */
+  val currentTab: StateFlow<DemoListRoute> = navigator.currentRoot
+
+  /** The list on top of that stack — the archive, whenever it has been pushed. */
+  val displayedList: StateFlow<DemoListRoute> = navigator.displayedList
+
+  /** The detail content above the displayed list, or null when the list is showing on its own. */
+  val detail: StateFlow<DemoDetailRoute?> = navigator.detail
+
+  /** Where the divider sits in a split-pane window. */
+  val paneAnchor: StateFlow<PaneAnchor> = navigator.paneAnchor
+
+  override suspend fun processEvent(event: DemoEvents) {
+    when (event) {
+      DemoEvents.ArchiveSelected -> navigator.processEvent(ListDetailEvents.GoToList(DemoListRoute.ARCHIVE, root = DemoListRoute.INBOX, push = true))
+      is DemoEvents.ListDetailEvent -> navigator.processEvent(event.event)
+    }
+  }
+}

demo/list-detail/src/main/java/org/signal/listdetail/demo/MainActivity.kt

@@ -0,0 +1,261 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.listdetail.demo
+
+import android.os.Bundle
+import androidx.activity.ComponentActivity
+import androidx.activity.compose.setContent
+import androidx.activity.enableEdgeToEdge
+import androidx.activity.viewModels
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxWithConstraints
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.systemBarsPadding
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material.icons.filled.Inbox
+import androidx.compose.material.icons.filled.Person
+import androidx.compose.material.icons.filled.Settings
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.NavigationBar
+import androidx.compose.material3.NavigationBarItem
+import androidx.compose.material3.NavigationRail
+import androidx.compose.material3.NavigationRailItem
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.TopAppBarDefaults
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.vector.ImageVector
+import androidx.compose.ui.platform.LocalResources
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.entryProvider
+import org.signal.core.ui.NavigationType
+import org.signal.core.ui.compose.split.ListDetailEvents
+import org.signal.core.ui.compose.split.ListDetailNavDisplay
+import org.signal.core.ui.compose.split.ListPaneChrome
+import org.signal.core.ui.compose.split.detailEntry
+import org.signal.core.ui.compose.split.listEntry
+import org.signal.core.ui.compose.split.rememberCurrentDecoratedNavEntries
+import org.signal.core.ui.compose.split.rememberListDetailPaneLayout
+import org.signal.core.ui.compose.split.rememberListDetailPaneMetrics
+import org.signal.core.ui.compose.theme.SignalTheme
+import org.signal.core.ui.rememberIsSplitPane
+import org.signal.core.util.logging.AndroidLogger
+import org.signal.core.util.logging.Log
+
+/**
+ * A small list/detail app built on `org.signal.core.ui.compose.split`.
+ *
+ * [DemoScreen] is where it comes together, and every argument [ListDetailNavDisplay] takes is built in
+ * this file. The nav keys are in `DemoRoutes.kt`, the stacks behind them in `DemoViewModel.kt`, and the
+ * panes' content in `DemoScreens.kt`.
+ */
+class MainActivity : ComponentActivity() {
+
+  private val viewModel: DemoViewModel by viewModels()
+
+  override fun onCreate(savedInstanceState: Bundle?) {
+    super.onCreate(savedInstanceState)
+
+    // So that the event each screen reports shows up in logcat, which is half the point of the pattern.
+    Log.initialize(AndroidLogger)
+
+    enableEdgeToEdge()
+
+    setContent {
+      SignalTheme(incognitoKeyboardEnabled = false) {
+        DemoScreen(viewModel)
+      }
+    }
+  }
+}
+
+@Composable
+private fun DemoScreen(viewModel: DemoViewModel) {
+  val onEvent: (DemoEvents) -> Unit = viewModel::onEvent
+
+  val isSplitPane = LocalResources.current.rememberIsSplitPane()
+  val paneAnchor by viewModel.paneAnchor.collectAsStateWithLifecycle()
+  val displayedList by viewModel.displayedList.collectAsStateWithLifecycle()
+  val hasRail = NavigationType.rememberNavigationType() == NavigationType.RAIL
+
+  val entries = rememberDemoEntries(viewModel)
+  val listPaneChrome: ListPaneChrome = remember(displayedList, onEvent) { { content -> DemoListPaneChrome(displayedList, onEvent, content) } }
+  val emptyDetailContent: @Composable () -> Unit = remember { { EmptyDetailPane() } }
+
+  SignalTheme(incognitoKeyboardEnabled = false) {
+    Surface {
+      BoxWithConstraints(
+        modifier = Modifier
+          .fillMaxSize()
+          .background(if (isSplitPane) SignalTheme.colors.colorSurface1 else MaterialTheme.colorScheme.surface)
+          .systemBarsPadding()
+      ) {
+        ListDetailNavDisplay(
+          entries = entries,
+          isSplitPane = isSplitPane,
+          paneAnchor = paneAnchor,
+          onBack = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Back)) },
+          onExitDetail = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.ExitDetail)) },
+          layout = rememberListDetailPaneLayout(
+            paneAnchor = paneAnchor,
+            maxWidth = maxWidth,
+            onAnchorSelected = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.AnchorSelected(it))) },
+            collapsedListWidth = if (hasRail) RAIL_WIDTH else 0.dp
+          ),
+          listPaneChrome = listPaneChrome,
+          emptyDetailContent = emptyDetailContent
+        )
+      }
+    }
+  }
+}
+
+/**
+ * The entry provider, and the decorated entries of the displayed tab's stack. Lists go through
+ * [listEntry], detail content through [detailEntry], and a screen that takes the whole window through a
+ * plain `entry`.
+ */
+@Composable
+private fun rememberDemoEntries(viewModel: DemoViewModel): List<NavEntry<NavKey>> {
+  val entryProvider = remember(viewModel) {
+    entryProvider {
+      listEntry<DemoListRoute> { route ->
+        val detail by viewModel.detail.collectAsStateWithLifecycle()
+
+        ItemListPane(
+          route = route,
+          selectedItemId = detail?.itemId,
+          onEvent = viewModel::onEvent
+        )
+      }
+
+      detailEntry<DemoDetailRoute.Item> { route ->
+        ItemDetailPane(
+          item = DemoData[route.itemId],
+          onEvent = viewModel::onEvent
+        )
+      }
+
+      detailEntry<DemoDetailRoute.Notes> { route ->
+        ItemNotesPane(
+          item = DemoData[route.itemId],
+          onEvent = viewModel::onEvent
+        )
+      }
+
+      // A plain entry: neither pane claims it, so it is displayed over both of them.
+      entry<DemoSettingsRoute> {
+        SettingsScreen(onEvent = viewModel::onEvent)
+      }
+    }
+  }
+
+  return rememberCurrentDecoratedNavEntries(viewModel.navigator, entryProvider)
+}
+
+/**
+ * List pane chrome that won't animate when the list changes (other than your own animations ofc.) for stuff like nav rails, megaphones, etc.
+ */
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+private fun DemoListPaneChrome(
+  displayedList: DemoListRoute,
+  onEvent: (DemoEvents) -> Unit,
+  content: @Composable () -> Unit
+) {
+  val navigationType = NavigationType.rememberNavigationType()
+  val metrics = rememberListDetailPaneMetrics()
+
+  Row(modifier = Modifier.fillMaxSize()) {
+    if (navigationType == NavigationType.RAIL) {
+      NavigationRail(containerColor = Color.Transparent) {
+        TABS.forEach { (route, icon) ->
+          NavigationRailItem(
+            selected = displayedList.tab == route,
+            onClick = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.GoToList(route))) },
+            icon = { TabIcon(icon, route) },
+            label = { Text(text = route.label) }
+          )
+        }
+      }
+    }
+
+    Column(
+      modifier = Modifier
+        .weight(1f)
+        .fillMaxSize()
+        .background(MaterialTheme.colorScheme.surface, metrics.shape)
+        .clip(metrics.shape)
+    ) {
+      TopAppBar(
+        title = { Text(text = displayedList.label) },
+        navigationIcon = {
+          if (displayedList == DemoListRoute.ARCHIVE) {
+            IconButton(onClick = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Back)) }) {
+              Icon(imageVector = Icons.AutoMirrored.Filled.ArrowBack, contentDescription = "Back")
+            }
+          }
+        },
+        actions = {
+          IconButton(onClick = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.Push(DemoSettingsRoute))) }) {
+            Icon(imageVector = Icons.Filled.Settings, contentDescription = "Settings")
+          }
+        },
+        colors = TopAppBarDefaults.topAppBarColors(containerColor = Color.Transparent)
+      )
+
+      Box(modifier = Modifier.weight(1f)) {
+        content()
+      }
+
+      if (navigationType == NavigationType.BAR) {
+        NavigationBar(
+          containerColor = Color.Transparent,
+          modifier = Modifier.clip(metrics.navigationBarShape)
+        ) {
+          TABS.forEach { (route, icon) ->
+            NavigationBarItem(
+              selected = displayedList.tab == route,
+              onClick = { onEvent(DemoEvents.ListDetailEvent(ListDetailEvents.GoToList(route))) },
+              icon = { TabIcon(icon, route) },
+              label = { Text(text = route.label) }
+            )
+          }
+        }
+      }
+    }
+  }
+}
+
+@Composable
+private fun TabIcon(icon: ImageVector, route: DemoListRoute) {
+  Icon(imageVector = icon, contentDescription = route.label)
+}
+
+/** The tabs the chrome offers. The archive is reached from inside the inbox, so it is not one of them. */
+private val TABS = listOf(
+  DemoListRoute.INBOX to Icons.Filled.Inbox,
+  DemoListRoute.CONTACTS to Icons.Filled.Person
+)
+
+/** What is left of the list pane once the detail fills the window: the navigation rail, when there is one. */
+private val RAIL_WIDTH = 80.dp

demo/list-detail/src/main/res/drawable/ic_launcher_background.xml

@@ -0,0 +1,171 @@
+<?xml version="1.0" encoding="utf-8"?>
+<vector
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="108dp"
+    android:height="108dp"
+    android:viewportWidth="108"
+    android:viewportHeight="108">
+  <path
+      android:fillColor="#3DDC84"
+      android:pathData="M0,0h108v108h-108z" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M9,0L9,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,0L19,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M29,0L29,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M39,0L39,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M49,0L49,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M59,0L59,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M69,0L69,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M79,0L79,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M89,0L89,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M99,0L99,108"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,9L108,9"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,19L108,19"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,29L108,29"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,39L108,39"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,49L108,49"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,59L108,59"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,69L108,69"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,79L108,79"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,89L108,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M0,99L108,99"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,29L89,29"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,39L89,39"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,49L89,49"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,59L89,59"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,69L89,69"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M19,79L89,79"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M29,19L29,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M39,19L39,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M49,19L49,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M59,19L59,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M69,19L69,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+  <path
+      android:fillColor="#00000000"
+      android:pathData="M79,19L79,89"
+      android:strokeWidth="0.8"
+      android:strokeColor="#33FFFFFF" />
+</vector>

demo/list-detail/src/main/res/drawable/ic_launcher_foreground.xml

@@ -0,0 +1,31 @@
+<vector
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:aapt="http://schemas.android.com/aapt"
+    android:width="108dp"
+    android:height="108dp"
+    android:viewportWidth="108"
+    android:viewportHeight="108">
+  <path android:pathData="M31,63.928c0,0 6.4,-11 12.1,-13.1c7.2,-2.6 26,-1.4 26,-1.4l38.1,38.1L107,108.928l-32,-1L31,63.928z">
+    <aapt:attr name="android:fillColor">
+      <gradient
+          android:endX="85.84757"
+          android:endY="92.4963"
+          android:startX="42.9492"
+          android:startY="49.59793"
+          android:type="linear">
+        <item
+            android:color="#44000000"
+            android:offset="0.0" />
+        <item
+            android:color="#00000000"
+            android:offset="1.0" />
+      </gradient>
+    </aapt:attr>
+  </path>
+  <path
+      android:fillColor="#FFFFFF"
+      android:fillType="nonZero"
+      android:pathData="M65.3,45.828l3.8,-6.6c0.2,-0.4 0.1,-0.9 -0.3,-1.1c-0.4,-0.2 -0.9,-0.1 -1.1,0.3l-3.9,6.7c-6.3,-2.8 -13.4,-2.8 -19.7,0l-3.9,-6.7c-0.2,-0.4 -0.7,-0.5 -1.1,-0.3C38.8,38.328 38.7,38.828 38.9,39.228l3.8,6.6C36.2,49.428 31.7,56.028 31,63.928h46C76.3,56.028 71.8,49.428 65.3,45.828zM43.4,57.328c-0.8,0 -1.5,-0.5 -1.8,-1.2c-0.3,-0.7 -0.1,-1.5 0.4,-2.1c0.5,-0.5 1.4,-0.7 2.1,-0.4c0.7,0.3 1.2,1 1.2,1.8C45.3,56.528 44.5,57.328 43.4,57.328L43.4,57.328zM64.6,57.328c-0.8,0 -1.5,-0.5 -1.8,-1.2s-0.1,-1.5 0.4,-2.1c0.5,-0.5 1.4,-0.7 2.1,-0.4c0.7,0.3 1.2,1 1.2,1.8C66.5,56.528 65.6,57.328 64.6,57.328L64.6,57.328z"
+      android:strokeWidth="1"
+      android:strokeColor="#00000000" />
+</vector>

demo/list-detail/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon
+    xmlns:android="http://schemas.android.com/apk/res/android">
+  <background android:drawable="@drawable/ic_launcher_background" />
+  <foreground android:drawable="@drawable/ic_launcher_foreground" />
+  <monochrome android:drawable="@drawable/ic_launcher_foreground" />
+</adaptive-icon>

demo/list-detail/src/main/res/mipmap-anydpi-v26/ic_launcher_round.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon
+    xmlns:android="http://schemas.android.com/apk/res/android">
+  <background android:drawable="@drawable/ic_launcher_background" />
+  <foreground android:drawable="@drawable/ic_launcher_foreground" />
+  <monochrome android:drawable="@drawable/ic_launcher_foreground" />
+</adaptive-icon>

demo/list-detail/src/main/res/values/strings.xml

@@ -0,0 +1,8 @@
+<!--
+  ~ Copyright 2026 Signal Messenger, LLC
+  ~ SPDX-License-Identifier: AGPL-3.0-only
+  -->
+
+<resources>
+    <string name="app_name">List Detail Demo</string>
+</resources>

demo/list-detail/src/main/res/values/themes.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ Copyright 2026 Signal Messenger, LLC
+  ~ SPDX-License-Identifier: AGPL-3.0-only
+  -->
+
+<resources>
+
+    <style name="Theme.Signal" parent="Theme.AppCompat.DayNight.NoActionBar" />
+</resources>

demo/registration/src/main/java/org/signal/registration/sample/RegistrationApplication.kt

@@ -99,7 +99,6 @@ class RegistrationApplication : Application() {
         override fun providePackageId(): String = BuildConfig.APPLICATION_ID
         override fun provideIsIncognitoKeyboardEnabled(): Boolean = false
         override fun provideIsScreenSecurityEnabled(): Boolean = false
-        override fun provideForceSplitPane(): Boolean = false
       }
     )
   }

settings.gradle.kts

@@ -147,6 +147,7 @@ include(":demo:debuglogs-viewer")
 include(":demo:registration")
 include(":demo:camera")
 include(":demo:apng")
+include(":demo:list-detail")
 
 // Testing/Lint modules
 include(":lintchecks")