Report focused media from the pager rather than the thumbnail rail in media-send.

feature/media-send/src/main/java/org/signal/mediasend/screens/edit/MediaEditScreen.kt

@@ -30,7 +30,9 @@ import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.produceState
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.rememberUpdatedState
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
@@ -40,6 +42,8 @@ import androidx.compose.ui.unit.dp
 import androidx.fragment.compose.AndroidFragment
 import androidx.lifecycle.ViewModelStoreOwner
 import androidx.lifecycle.viewmodel.compose.viewModel
+import kotlinx.coroutines.flow.drop
+import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.launch
 import org.signal.core.ui.WindowBreakpoint
 import org.signal.core.ui.compose.AllDevicePreviews
@@ -100,6 +104,24 @@ internal fun MediaEditScreen(
     }
   }
 
+  // Read through the latest selection rather than the one captured when the effect below started, since removals and
+  // reordering change which media a settled page refers to without restarting it.
+  val currentSelectedMedia by rememberUpdatedState(state.selectedMedia)
+
+  // Focus belongs to the pager rather than to any one piece of chrome: the thumbnail rail is not composed for
+  // documents, and a swipe still has to be reported from there.
+  LaunchedEffect(pagerState) {
+    snapshotFlow { pagerState.isScrollInProgress }
+      .filter { !it }
+      .drop(1)
+      .collect {
+        val settledMedia = currentSelectedMedia.getOrNull(pagerState.currentPage)
+        if (settledMedia != null) {
+          onEvent(MediaEditScreenEvents.FocusedMediaChanged(settledMedia))
+        }
+      }
+  }
+
   // During a camera-first flow, backing out of edit when the only selection is the capture itself should discard the
   // capture and return to the camera rather than leaving the empty editor on the back stack.
   val isOnlyCameraFirstCapture = state.cameraFirstCapture != null &&
@@ -281,9 +303,6 @@ internal fun MediaEditScreen(
               selectedMedia = state.selectedMedia,
               pagerState = pagerState,
               enabled = !isInteracting,
-              onFocusedMediaChange = {
-                onEvent(MediaEditScreenEvents.FocusedMediaChanged(it))
-              },
               onThumbnailClick = { index ->
                 if (pagerState.currentPage == index) {
                   onEvent(MediaEditScreenEvents.RemoveMedia(state.selectedMedia[index]))

feature/media-send/src/main/java/org/signal/mediasend/screens/edit/ThumbnailRow.kt

@@ -29,7 +29,6 @@ import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
-import androidx.compose.runtime.rememberUpdatedState
 import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -42,8 +41,6 @@ import androidx.compose.ui.unit.dp
 import androidx.core.net.toUri
 import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.flow.distinctUntilChanged
-import kotlinx.coroutines.flow.drop
-import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.launch
 import org.signal.core.models.media.Media
 import org.signal.core.ui.compose.DayNightPreviews
@@ -67,12 +64,14 @@ private val MAX_PADDING = 8.dp
 /**
  * Horizontally scrollable thumbnail strip that syncs with [pagerState].
  * Features fish-eye padding effect where the centered item has more padding.
+ *
+ * Dragging the strip scrolls the pager, and which media the settled page focuses is reported by the caller from
+ * [pagerState] rather than from here, since this row is only one of the chromes the pager can be swiped beneath.
  */
 @Composable
 internal fun ThumbnailRow(
   selectedMedia: List<Media>,
   pagerState: PagerState,
-  onFocusedMediaChange: (Media) -> Unit,
   onThumbnailClick: (Int) -> Unit = {},
   onReorder: (fromIndex: Int, toIndex: Int) -> Unit = { _, _ -> },
   enabled: Boolean = true
@@ -102,22 +101,6 @@ internal fun ThumbnailRow(
     pagerState.dispatchRawDelta(-scaledDelta)
   }
 
-  // Read through the latest selection rather than the one captured when the effect started, since reordering changes
-  // which media a settled page refers to without restarting the effect.
-  val currentSelectedMedia by rememberUpdatedState(selectedMedia)
-
-  LaunchedEffect(pagerState) {
-    snapshotFlow { pagerState.isScrollInProgress }
-      .filter { !it }
-      .drop(1)
-      .collectLatest {
-        val settledPage = pagerState.currentPage
-        if (settledPage in currentSelectedMedia.indices) {
-          onFocusedMediaChange(currentSelectedMedia[settledPage])
-        }
-      }
-  }
-
   // The rail's scroll position belongs to the drag rather than the pager from the moment an item is picked up until the
   // reorder it produced has landed in state. Resuming any earlier means syncing to the pre-drag order and then having to
   // correct once the new order arrives. The single catch-up afterwards is animated so the rail glides to the dropped
@@ -169,10 +152,6 @@ internal fun ThumbnailRow(
               else -> pagerState.currentPage
             }
             pagerState.animateScrollToPage(targetPage)
-
-            if (targetPage in selectedMedia.indices) {
-              onFocusedMediaChange(selectedMedia[targetPage])
-            }
           }
         }
       )
@@ -275,8 +254,7 @@ private fun ThumbnailRowPreview() {
   Previews.Preview {
     ThumbnailRow(
       selectedMedia = media,
-      pagerState = pagerState,
-      onFocusedMediaChange = { }
+      pagerState = pagerState
     )
   }
 }