Add MainContentLayoutData object and proper scaffolding directive.

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

@@ -28,10 +28,12 @@ import androidx.compose.foundation.layout.BoxWithConstraintsScope
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.displayCutoutPadding
 import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.layout.padding
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.adaptive.ExperimentalMaterial3AdaptiveApi
+import androidx.compose.material3.adaptive.currentWindowAdaptiveInfo
 import androidx.compose.material3.adaptive.layout.ThreePaneScaffoldRole
+import androidx.compose.material3.adaptive.layout.calculatePaneScaffoldDirectiveWithTwoPanesOnMediumWidth
 import androidx.compose.material3.adaptive.navigation.rememberListDetailPaneScaffoldNavigator
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
@@ -41,9 +43,6 @@ 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.RectangleShape
-import androidx.compose.ui.graphics.Shape
-import androidx.compose.ui.unit.dp
 import androidx.fragment.app.DialogFragment
 import androidx.fragment.compose.AndroidFragment
 import androidx.fragment.compose.rememberFragmentState
@@ -83,6 +82,7 @@ import org.thoughtcrime.securesms.main.MainActivityListHostFragment
 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.MainMegaphoneState
 import org.thoughtcrime.securesms.main.MainNavigationBar
 import org.thoughtcrime.securesms.main.MainNavigationDetailLocation
@@ -231,28 +231,29 @@ class MainActivity : PassphraseRequiredActivity(), VoiceNoteMediaControllerOwner
         )
       }
 
-      val scaffoldNavigator = rememberListDetailPaneScaffoldNavigator<Any>()
       val windowSizeClass = WindowSizeClass.rememberWindowSizeClass()
-
-      val contentClip: Shape = remember(windowSizeClass) {
-        if (windowSizeClass.isExtended()) {
-          RoundedCornerShape(18.dp)
-        } else {
-          RectangleShape
-        }
-      }
-
-      LaunchedEffect(detailLocation) {
-        if (detailLocation is MainNavigationDetailLocation.Conversation) {
-          if (SignalStore.internal.largeScreenUi) {
-            scaffoldNavigator.navigateTo(ThreePaneScaffoldRole.Primary, detailLocation)
-          } else {
-            startActivity((detailLocation as MainNavigationDetailLocation.Conversation).intent)
-          }
-        }
-      }
+      val contentLayoutData = MainContentLayoutData.rememberContentLayoutData()
 
       MainContainer {
+        val scaffoldNavigator = rememberListDetailPaneScaffoldNavigator<Any>(
+          scaffoldDirective = calculatePaneScaffoldDirectiveWithTwoPanesOnMediumWidth(
+            currentWindowAdaptiveInfo()
+          ).copy(
+            horizontalPartitionSpacerSize = contentLayoutData.partitionWidth,
+            defaultPanePreferredWidth = contentLayoutData.rememberDefaultPanePreferredWidth(maxWidth)
+          )
+        )
+
+        LaunchedEffect(detailLocation) {
+          if (detailLocation is MainNavigationDetailLocation.Conversation) {
+            if (SignalStore.internal.largeScreenUi) {
+              scaffoldNavigator.navigateTo(ThreePaneScaffoldRole.Primary, detailLocation)
+            } else {
+              startActivity((detailLocation as MainNavigationDetailLocation.Conversation).intent)
+            }
+          }
+        }
+
         AppScaffold(
           navigator = scaffoldNavigator,
           bottomNavContent = {
@@ -280,9 +281,10 @@ class MainActivity : PassphraseRequiredActivity(), VoiceNoteMediaControllerOwner
 
             Column(
               modifier = Modifier
+                .padding(start = contentLayoutData.listPaddingStart)
                 .fillMaxSize()
                 .background(listContainerColor)
-                .clip(contentClip)
+                .clip(contentLayoutData.shape)
             ) {
               MainToolbar(
                 state = mainToolbarState,
@@ -316,13 +318,17 @@ class MainActivity : PassphraseRequiredActivity(), VoiceNoteMediaControllerOwner
                   fragmentState = fragmentState,
                   arguments = requireNotNull(destination.intent.extras) { "Handed null Conversation intent arguments." },
                   modifier = Modifier
+                    .padding(end = contentLayoutData.detailPaddingEnd)
+                    .clip(contentLayoutData.shape)
                     .background(color = MaterialTheme.colorScheme.surface)
                     .fillMaxSize()
-                    .clip(contentClip)
                 )
               }
             }
-          }
+          },
+          paneExpansionDragHandle = if (contentLayoutData.hasDragHandle()) {
+            { }
+          } else null
         )
       }
     }

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

@@ -0,0 +1,93 @@
+/*
+ * 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.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.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.thoughtcrime.securesms.window.WindowSizeClass
+
+/**
+ * Describes metrics for the content layout (list and detail) of the main screen.
+ *
+ * @param shape The shape of each of the list and detail fragments
+ * @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 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 WindowSizeClass.rememberWindowSizeClass().isExtended()
+  }
+
+  /**
+   * Calculates the default preferred width
+   */
+  @Composable
+  fun rememberDefaultPanePreferredWidth(maxWidth: Dp): Dp {
+    val windowSizeClass = WindowSizeClass.rememberWindowSizeClass()
+
+    return remember(maxWidth, windowSizeClass) {
+      when {
+        windowSizeClass.isCompact() -> maxWidth
+        windowSizeClass.isMedium() -> (maxWidth - extraPadding) / 2f
+        else -> 416.dp
+      }
+    }
+  }
+
+  companion object {
+    /**
+     * Uses the WindowSizeClass to build out a MainContentLayoutData.
+     */
+    @Composable
+    fun rememberContentLayoutData(): MainContentLayoutData {
+      val windowSizeClass = WindowSizeClass.rememberWindowSizeClass()
+
+      return remember(windowSizeClass) {
+        MainContentLayoutData(
+          shape = when {
+            windowSizeClass.isCompact() -> RectangleShape
+            windowSizeClass.isMedium() -> RoundedCornerShape(18.dp)
+            else -> RoundedCornerShape(14.dp)
+          },
+          partitionWidth = when {
+            windowSizeClass.isCompact() -> 0.dp
+            windowSizeClass.isMedium() -> 13.dp
+            else -> 16.dp
+          },
+          listPaddingStart = when {
+            windowSizeClass.isCompact() -> 0.dp
+            windowSizeClass.isMedium() -> 12.dp
+            else -> 16.dp
+          },
+          detailPaddingEnd = when {
+            windowSizeClass.isCompact() -> 0.dp
+            windowSizeClass.isMedium() -> 12.dp
+            else -> 24.dp
+          }
+        )
+      }
+    }
+  }
+}

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

@@ -16,6 +16,10 @@ import androidx.compose.material3.Text
 import androidx.compose.material3.adaptive.ExperimentalMaterial3AdaptiveApi
 import androidx.compose.material3.adaptive.currentWindowAdaptiveInfo
 import androidx.compose.material3.adaptive.layout.AnimatedPane
+import androidx.compose.material3.adaptive.layout.PaneExpansionState
+import androidx.compose.material3.adaptive.layout.ThreePaneScaffoldScope
+import androidx.compose.material3.adaptive.layout.calculatePaneScaffoldDirectiveWithTwoPanesOnMediumWidth
+import androidx.compose.material3.adaptive.layout.rememberPaneExpansionState
 import androidx.compose.material3.adaptive.navigation.NavigableListDetailPaneScaffold
 import androidx.compose.material3.adaptive.navigation.ThreePaneScaffoldNavigator
 import androidx.compose.material3.adaptive.navigation.rememberListDetailPaneScaffoldNavigator
@@ -28,6 +32,7 @@ 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
 import androidx.window.core.ExperimentalWindowCoreApi
 import androidx.window.core.layout.WindowHeightSizeClass
 import androidx.window.core.layout.WindowWidthSizeClass
@@ -57,7 +62,7 @@ enum class WindowSizeClass(
 ) {
   COMPACT_PORTRAIT(Navigation.BAR),
   COMPACT_LANDSCAPE(Navigation.BAR),
-  MEDIUM_PORTRAIT(Navigation.BAR),
+  MEDIUM_PORTRAIT(Navigation.RAIL),
   MEDIUM_LANDSCAPE(Navigation.RAIL),
   EXTENDED_PORTRAIT(Navigation.RAIL),
   EXTENDED_LANDSCAPE(Navigation.RAIL);
@@ -160,6 +165,7 @@ fun AppScaffold(
   detailContent: @Composable () -> Unit = {},
   navRailContent: @Composable () -> Unit = {},
   bottomNavContent: @Composable () -> Unit = {},
+  paneExpansionDragHandle: (@Composable ThreePaneScaffoldScope.(PaneExpansionState) -> Unit)? = null,
   listContent: @Composable () -> Unit
 ) {
   val isForcedCompact = WindowSizeClass.checkForcedCompact()
@@ -176,9 +182,10 @@ fun AppScaffold(
     return
   }
 
-  if (windowSizeClass.isMedium()) {
-    Row {
-      Box(modifier = Modifier.weight(1f)) {
+  NavigableListDetailPaneScaffold(
+    navigator = navigator,
+    listPane = {
+      AnimatedPane {
         ListAndNavigation(
           listContent = listContent,
           navRailContent = navRailContent,
@@ -186,31 +193,15 @@ fun AppScaffold(
           windowSizeClass = windowSizeClass
         )
       }
-
-      Box(modifier = Modifier.weight(1f)) {
+    },
+    detailPane = {
+      AnimatedPane {
         detailContent()
       }
-    }
-  } else {
-    NavigableListDetailPaneScaffold(
-      navigator = navigator,
-      listPane = {
-        AnimatedPane {
-          ListAndNavigation(
-            listContent = listContent,
-            navRailContent = navRailContent,
-            bottomNavContent = bottomNavContent,
-            windowSizeClass = windowSizeClass
-          )
-        }
-      },
-      detailPane = {
-        AnimatedPane {
-          detailContent()
-        }
-      }
-    )
-  }
+    },
+    paneExpansionDragHandle = paneExpansionDragHandle,
+    paneExpansionState = rememberPaneExpansionState()
+  )
 }
 
 @Composable
@@ -248,6 +239,13 @@ private fun ListAndNavigation(
 private fun AppScaffoldPreview() {
   Previews.Preview {
     AppScaffold(
+      navigator = rememberListDetailPaneScaffoldNavigator<Any>(
+        scaffoldDirective = calculatePaneScaffoldDirectiveWithTwoPanesOnMediumWidth(
+          currentWindowAdaptiveInfo()
+        ).copy(
+          horizontalPartitionSpacerSize = 10.dp
+        )
+      ),
       listContent = {
         Box(
           contentAlignment = Alignment.Center,