Replace main activity xml with AppScaffold.

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

@@ -12,7 +12,15 @@ import android.content.Intent
 import android.os.Bundle
 import android.view.View
 import android.view.ViewTreeObserver
+import androidx.activity.compose.setContent
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Modifier
 import androidx.core.content.ContextCompat
+import androidx.fragment.compose.AndroidFragment
+import androidx.fragment.compose.rememberFragmentState
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import com.google.android.material.dialog.MaterialAlertDialogBuilder
 import org.signal.core.util.concurrent.LifecycleDisposable
 import org.signal.core.util.getSerializableCompat
@@ -29,11 +37,14 @@ import org.thoughtcrime.securesms.conversationlist.RelinkDevicesReminderBottomSh
 import org.thoughtcrime.securesms.conversationlist.RestoreCompleteBottomSheetDialog
 import org.thoughtcrime.securesms.devicetransfer.olddevice.OldDeviceExitActivity
 import org.thoughtcrime.securesms.keyvalue.SignalStore
+import org.thoughtcrime.securesms.main.MainActivityListHostFragment
+import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
 import org.thoughtcrime.securesms.net.DeviceTransferBlockingInterceptor
 import org.thoughtcrime.securesms.notifications.VitalsViewModel
 import org.thoughtcrime.securesms.stories.Stories
 import org.thoughtcrime.securesms.stories.tabs.ConversationListTab
 import org.thoughtcrime.securesms.stories.tabs.ConversationListTabRepository
+import org.thoughtcrime.securesms.stories.tabs.ConversationListTabsFragment
 import org.thoughtcrime.securesms.stories.tabs.ConversationListTabsViewModel
 import org.thoughtcrime.securesms.util.AppStartup
 import org.thoughtcrime.securesms.util.CachedInflater
@@ -42,6 +53,7 @@ import org.thoughtcrime.securesms.util.DynamicNoActionBarTheme
 import org.thoughtcrime.securesms.util.SplashScreenUtil
 import org.thoughtcrime.securesms.util.WindowUtil
 import org.thoughtcrime.securesms.util.viewModel
+import org.thoughtcrime.securesms.window.AppScaffold
 
 class MainActivity : PassphraseRequiredActivity(), VoiceNoteMediaControllerOwner, MainNavigator.NavigatorProvider {
 
@@ -85,7 +97,39 @@ class MainActivity : PassphraseRequiredActivity(), VoiceNoteMediaControllerOwner
     AppStartup.getInstance().onCriticalRenderEventStart()
     super.onCreate(savedInstanceState, ready)
 
-    setContentView(R.layout.main_activity)
+    setContent {
+      val navState = rememberFragmentState()
+      val listHostState = rememberFragmentState()
+      val detailLocation by navigator.viewModel.detailLocation.collectAsStateWithLifecycle()
+
+      LaunchedEffect(detailLocation) {
+        if (detailLocation is MainNavigationDetailLocation.Conversation) {
+          startActivity((detailLocation as MainNavigationDetailLocation.Conversation).intent)
+          overridePendingTransition(R.anim.slide_from_end, R.anim.fade_scale_out)
+        }
+      }
+
+      AppScaffold(
+        bottomNavContent = {
+          AndroidFragment(
+            clazz = ConversationListTabsFragment::class.java,
+            fragmentState = navState
+          )
+        },
+        navRailContent = {
+          AndroidFragment(
+            clazz = ConversationListTabsFragment::class.java,
+            fragmentState = navState
+          )
+        }
+      ) {
+        AndroidFragment(
+          clazz = MainActivityListHostFragment::class.java,
+          fragmentState = listHostState,
+          modifier = Modifier.fillMaxSize()
+        )
+      }
+    }
 
     val content: View = findViewById(android.R.id.content)
     content.viewTreeObserver.addOnPreDrawListener(object : ViewTreeObserver.OnPreDrawListener {

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

@@ -3,15 +3,20 @@ package org.thoughtcrime.securesms;
 import android.app.Activity;
 import android.content.Intent;
 
+import androidx.annotation.MainThread;
 import androidx.annotation.NonNull;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.fragment.app.Fragment;
 import androidx.fragment.app.FragmentManager;
+import androidx.lifecycle.ViewModelProvider;
+import androidx.lifecycle.viewmodel.internal.ViewModelProviders;
 
 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.MainNavigationViewModel;
 import org.thoughtcrime.securesms.recipients.RecipientId;
 
 import io.reactivex.rxjava3.disposables.Disposable;
@@ -20,8 +25,10 @@ public class MainNavigator {
 
   public static final int REQUEST_CONFIG_CHANGES = 901;
 
-  private final AppCompatActivity   activity;
-  private final LifecycleDisposable lifecycleDisposable;
+  private final AppCompatActivity       activity;
+  private final LifecycleDisposable     lifecycleDisposable;
+
+  private MainNavigationViewModel viewModel;
 
   public MainNavigator(@NonNull AppCompatActivity activity) {
     this.activity            = activity;
@@ -30,6 +37,15 @@ public class MainNavigator {
     lifecycleDisposable.bindTo(activity);
   }
 
+  @MainThread
+  public @NonNull MainNavigationViewModel getViewModel() {
+    if (viewModel == null) {
+      viewModel = new ViewModelProvider(activity).get(MainNavigationViewModel.class);
+    }
+    
+    return viewModel;
+  }
+
   public static MainNavigator get(@NonNull Activity activity) {
     if (!(activity instanceof MainActivity)) {
       throw new IllegalArgumentException("Activity must be an instance of MainActivity!");
@@ -57,10 +73,7 @@ public class MainNavigator {
                                                .map(builder -> builder.withDistributionType(distributionType)
                                                                       .withStartingPosition(startingPosition)
                                                                       .build())
-                                               .subscribe(intent -> {
-                                                 activity.startActivity(intent);
-                                                 activity.overridePendingTransition(R.anim.slide_from_end, R.anim.fade_scale_out);
-                                               });
+                                               .subscribe(intent -> viewModel.goTo(new MainNavigationDetailLocation.Conversation(intent)));
 
     lifecycleDisposable.add(disposable);
   }

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

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+import android.content.Intent
+
+/**
+ * Describes which content to display in the detail view.
+ */
+sealed interface MainNavigationDetailLocation {
+  data object Empty : MainNavigationDetailLocation
+  data class Conversation(val intent: Intent) : MainNavigationDetailLocation
+}

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

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.main
+
+import androidx.lifecycle.ViewModel
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.update
+
+class MainNavigationViewModel : ViewModel() {
+  private val detailLocationFlow = MutableStateFlow<MainNavigationDetailLocation>(MainNavigationDetailLocation.Empty)
+
+  val detailLocation: StateFlow<MainNavigationDetailLocation> = detailLocationFlow
+
+  fun goTo(location: MainNavigationDetailLocation) {
+    detailLocationFlow.update { location }
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/stories/tabs/ConversationListTabsFragment.kt

@@ -8,7 +8,10 @@ import androidx.fragment.app.viewModels
 import org.thoughtcrime.securesms.compose.ComposeFragment
 import org.thoughtcrime.securesms.main.MainNavigationBar
 import org.thoughtcrime.securesms.main.MainNavigationDestination
+import org.thoughtcrime.securesms.main.MainNavigationRail
 import org.thoughtcrime.securesms.main.MainNavigationState
+import org.thoughtcrime.securesms.window.Navigation
+import org.thoughtcrime.securesms.window.WindowSizeClass
 
 /**
  * Displays the "Chats" and "Stories" tab to a user.
@@ -37,16 +40,28 @@ class ConversationListTabsFragment : ComposeFragment() {
     }
 
     if (state.visibilityState.isVisible()) {
-      MainNavigationBar(
-        state = navState,
-        onDestinationSelected = {
+      val windowSizeClass = WindowSizeClass.rememberWindowSizeClass()
+      val onDestinationSelected: (MainNavigationDestination) -> Unit = remember {
+        {
           when (it) {
             MainNavigationDestination.CHATS -> viewModel.onChatsSelected()
             MainNavigationDestination.CALLS -> viewModel.onCallsSelected()
             MainNavigationDestination.STORIES -> viewModel.onStoriesSelected()
           }
         }
-      )
+      }
+
+      if (windowSizeClass.navigation == Navigation.BAR) {
+        MainNavigationBar(
+          state = navState,
+          onDestinationSelected = onDestinationSelected
+        )
+      } else {
+        MainNavigationRail(
+          state = navState,
+          onDestinationSelected = onDestinationSelected
+        )
+      }
     }
   }
 

app/src/main/java/org/thoughtcrime/securesms/util/RemoteConfig.kt

@@ -1121,5 +1121,12 @@ object RemoteConfig {
     hotSwappable = true
   )
 
+  /** Whether to allow different WindowSizeClasses to be used to determine screen layout */
+  val largeScreenUi: Boolean by remoteBoolean(
+    key = "android.largeScreenUI",
+    defaultValue = false,
+    hotSwappable = false
+  )
+
   // endregion
 }

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

@@ -21,6 +21,7 @@ import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.platform.LocalConfiguration
+import androidx.compose.ui.platform.LocalInspectionMode
 import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
@@ -30,6 +31,7 @@ import org.signal.core.ui.compose.Previews
 import org.thoughtcrime.securesms.main.MainNavigationBar
 import org.thoughtcrime.securesms.main.MainNavigationRail
 import org.thoughtcrime.securesms.main.MainNavigationState
+import org.thoughtcrime.securesms.util.RemoteConfig
 
 enum class Navigation {
   RAIL,
@@ -39,6 +41,10 @@ enum class Navigation {
 /**
  * Describes the size of screen we are displaying, and what components should be displayed.
  *
+ * Screens should utilize this class by convention instead of calling [currentWindowAdaptiveInfo]
+ * themselves, as this class includes checks with [RemoteConfig] to ensure we're allowed to display
+ * content in different screen sizes.
+ *
  * https://developer.android.com/develop/ui/compose/layouts/adaptive/use-window-size-classes
  */
 enum class WindowSizeClass(
@@ -54,9 +60,20 @@ enum class WindowSizeClass(
   companion object {
     @Composable
     fun rememberWindowSizeClass(): WindowSizeClass {
-      val wsc = currentWindowAdaptiveInfo().windowSizeClass
       val orientation = LocalConfiguration.current.orientation
 
+      if (!LocalInspectionMode.current && !RemoteConfig.largeScreenUi) {
+        return when (orientation) {
+          Configuration.ORIENTATION_PORTRAIT, Configuration.ORIENTATION_UNDEFINED, Configuration.ORIENTATION_SQUARE -> {
+            COMPACT_PORTRAIT
+          }
+          Configuration.ORIENTATION_LANDSCAPE -> COMPACT_LANDSCAPE
+          else -> error("Unexpected orientation: $orientation")
+        }
+      }
+
+      val wsc = currentWindowAdaptiveInfo().windowSizeClass
+
       return remember(orientation, wsc) {
         when (orientation) {
           Configuration.ORIENTATION_PORTRAIT, Configuration.ORIENTATION_UNDEFINED, Configuration.ORIENTATION_SQUARE -> {
@@ -88,10 +105,10 @@ enum class WindowSizeClass(
  */
 @Composable
 fun AppScaffold(
-  listContent: @Composable () -> Unit,
-  detailContent: @Composable () -> Unit,
-  navRailContent: @Composable () -> Unit,
-  bottomNavContent: @Composable () -> Unit
+  detailContent: @Composable () -> Unit = {},
+  navRailContent: @Composable () -> Unit = {},
+  bottomNavContent: @Composable () -> Unit = {},
+  listContent: @Composable () -> Unit
 ) {
   val windowSizeClass = WindowSizeClass.rememberWindowSizeClass()
 

app/src/main/res/layout/main_activity.xml

@@ -1,23 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
-    xmlns:tools="http://schemas.android.com/tools"
-    tools:viewBindingIgnore="true"
-    android:layout_width="match_parent"
-    android:layout_height="match_parent"
-    android:orientation="vertical">
-
-    <androidx.fragment.app.FragmentContainerView
-        android:name="org.thoughtcrime.securesms.main.MainActivityListHostFragment"
-        android:id="@+id/fragment_container"
-        android:layout_width="match_parent"
-        android:layout_height="0dp"
-        android:layout_weight="1"
-        tools:context=".MainActivity" />
-
-    <androidx.fragment.app.FragmentContainerView
-        android:id="@+id/conversation_list_tabs"
-        android:name="org.thoughtcrime.securesms.stories.tabs.ConversationListTabsFragment"
-        android:layout_width="match_parent"
-        android:layout_height="wrap_content" />
-
-</LinearLayout>