fix(mediaviewer): Fix wrong image shown after opening from a group

Opening the first image of a group (or any image within the first two
positions of the currently loaded media) triggered an older-history
prefetch that prepends older items to the pager's item list and
shifts the pager to compensate. That shift correction was emitted as
a one-shot SharedFlow event, collected in a long-lived LaunchedEffect
whose closure captured the item count from the composition active
when the effect first launched. By the time the event fired, the item
list had already grown, but the effect's clamp (`items.size` / later
`pagerState.pageCount`) still reflected the stale, smaller count, so
the pager settled on the wrong, small index - showing an unrelated,
often much older image a moment after the tap.

Moved the shift into UiState itself (MediaViewerViewModel.PendingShift)
so the Compose side always applies it from the very same state
snapshot as the already-updated item list, removing the possibility
of a stale item count at the point the correction is applied.

Assisted-by: Claude Code:claude-sonnet-5

Signed-off-by: Marcel Hibbe <dev@mhibbe.de>

Author: mahibi

app/src/main/java/com/nextcloud/talk/mediaviewer/activities/MediaViewerScreen.kt

@@ -120,13 +120,13 @@ fun MediaViewerScreen(
     }
     val coroutineScope = rememberCoroutineScope()
 
-    // See MediaViewerViewModel.indexShiftEvents: prepending older groups shifts every existing
-    // index, so the pager must silently jump to keep the same item on screen.
-    LaunchedEffect(viewModel) {
-        viewModel.indexShiftEvents.collect { shift ->
-            if (shift != 0) {
-                pagerState.scrollToPage((pagerState.currentPage + shift).coerceIn(0, items.size - 1))
+    LaunchedEffect(uiState.pendingShift?.id) {
+        val shift = uiState.pendingShift
+        if (shift != null) {
+            if (shift.amount != 0) {
+                pagerState.scrollToPage((pagerState.currentPage + shift.amount).coerceIn(0, items.size - 1))
             }
+            viewModel.consumePendingShift(shift.id)
         }
     }
 

app/src/main/java/com/nextcloud/talk/mediaviewer/viewmodels/MediaViewerViewModel.kt

@@ -30,7 +30,6 @@ import com.nextcloud.talk.shareditems.repositories.SharedItemsRepository
 import com.nextcloud.talk.utils.CapabilitiesUtil
 import com.nextcloud.talk.utils.FileUtils
 import io.reactivex.Observable
-import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.collect
@@ -52,13 +51,16 @@ import kotlin.coroutines.resumeWithException
 class MediaViewerViewModel @Inject constructor(private val sharedItemsRepository: SharedItemsRepository) :
     ViewModel() {
 
+    data class PendingShift(val id: Long, val amount: Int)
+
     data class UiState(
         val groups: List<MediaViewerGroup> = emptyList(),
         val currentGlobalIndex: Int = 0,
         val loadingOlder: Boolean = false,
         val canLoadOlder: Boolean = true,
         val cachedFilePaths: Map<Long, String> = emptyMap(),
-        val downloadingMessageIds: Set<Long> = emptySet()
+        val downloadingMessageIds: Set<Long> = emptySet(),
+        val pendingShift: PendingShift? = null
     ) {
         val flattenedItems: List<MediaViewerItem> get() = groups.flatMap { it.items }
         val currentItem: MediaViewerItem? get() = flattenedItems.getOrNull(currentGlobalIndex)
@@ -71,14 +73,7 @@ class MediaViewerViewModel @Inject constructor(private val sharedItemsRepository
 
     private val _uiState = MutableStateFlow(UiState())
     val uiState: StateFlow<UiState> = _uiState
-
-    // HorizontalPager indexes into the flattened item list; prepending older groups shifts every
-    // existing index by the number of items prepended. This is a one-shot event (not part of
-    // UiState) so the pager can silently jump to the shifted position - via
-    // pagerState.scrollToPage(pagerState.currentPage + shift) - the moment it fires, keeping the
-    // same item on screen instead of visually jumping to whatever is now at the old index.
-    private val _indexShiftEvents = MutableSharedFlow<Int>(extraBufferCapacity = 1)
-    val indexShiftEvents = _indexShiftEvents
+    private var nextShiftId = 0L
 
     private lateinit var user: User
     private lateinit var repositoryParameters: SharedItemsRepository.Parameters
@@ -112,6 +107,10 @@ class MediaViewerViewModel @Inject constructor(private val sharedItemsRepository
         onPageSettled(globalIndex)
     }
 
+    fun consumePendingShift(id: Long) {
+        _uiState.update { if (it.pendingShift?.id == id) it.copy(pendingShift = null) else it }
+    }
+
     private fun ensureCachedAround(globalIndex: Int) {
         val items = _uiState.value.flattenedItems
         for (index in (globalIndex - PREFETCH_RADIUS)..(globalIndex + PREFETCH_RADIUS)) {
@@ -215,17 +214,16 @@ class MediaViewerViewModel @Inject constructor(private val sharedItemsRepository
             }
 
             oldestKnownMessageId = olderItems.first().messageId
-            val previousCount = _uiState.value.flattenedItems.size
             _uiState.update { current ->
                 val combined = (olderItems + current.flattenedItems).toMediaViewerGroups()
                 current.copy(
                     groups = combined,
                     currentGlobalIndex = current.currentGlobalIndex + olderItems.size,
                     loadingOlder = false,
-                    canLoadOlder = sharedItems?.moreItemsExisting == true
+                    canLoadOlder = sharedItems?.moreItemsExisting == true,
+                    pendingShift = PendingShift(id = nextShiftId++, amount = olderItems.size)
                 )
             }
-            _indexShiftEvents.tryEmit(_uiState.value.flattenedItems.size - previousCount)
         }
     }