Keep the opened image on screen when the media gallery fails to load

Opening an image from the timeline replaces it with a full-screen page
reading "error". Two things go wrong. The viewer already holds the item
that was tapped, but building the surrounding media timeline can fail and
the failure overwrites that cached item, so the picture the user asked for
disappears. And the failure page renders the exception message straight
from the SDK, which is developer text with nothing actionable in it.

Emit the failure only when there is nothing cached, so a viewer opened on
a specific event keeps showing that event and stays usable while the rest
of the gallery is unavailable. Where there really is nothing to show, the
page now uses the same alert icon, title and message as the other error
states in this screen, built from strings that already exist.

The trade-off is that the timeline error is no longer surfaced in the
cache case; it is still logged. Swapping it for a snackbar would need a
new surface in the viewer and is left out.

libraries/mediaviewer/impl/src/main/kotlin/io/element/android/libraries/mediaviewer/impl/datasource/MediaGalleryDataSource.kt

@@ -83,7 +83,9 @@ class TimelineMediaGalleryDataSource(
                 },
                 {
                     Timber.e(it, "Failed to get media timeline for room ${room.roomId}")
-                    groupedMediaItemsFlow.emit(AsyncData.Failure(it))
+                    if (cache == null) {
+                        groupedMediaItemsFlow.emit(AsyncData.Failure(it))
+                    }
                 },
             )
         }.flatMapLatest { timeline ->

libraries/mediaviewer/impl/src/main/kotlin/io/element/android/libraries/mediaviewer/impl/viewer/MediaViewerView.kt

@@ -82,7 +82,6 @@ import io.element.android.libraries.audio.api.AudioFocus
 import io.element.android.libraries.core.mimetype.MimeTypes.isMimeTypeVideo
 import io.element.android.libraries.designsystem.atomic.molecules.IconTitleSubtitleMolecule
 import io.element.android.libraries.designsystem.components.BigIcon
-import io.element.android.libraries.designsystem.components.async.AsyncFailure
 import io.element.android.libraries.designsystem.components.async.AsyncIndicator
 import io.element.android.libraries.designsystem.components.async.AsyncLoading
 import io.element.android.libraries.designsystem.components.button.BackButton
@@ -209,7 +208,6 @@ fun MediaViewerView(
             when (val dataForPage = state.listData[page]) {
                 is MediaViewerPageData.Failure -> {
                     MediaViewerErrorPage(
-                        throwable = dataForPage.throwable,
                         onDismiss = onBackClick,
                     )
                 }
@@ -506,7 +504,6 @@ private fun MediaViewerLoadingPage(
 
 @Composable
 private fun MediaViewerErrorPage(
-    throwable: Throwable,
     onDismiss: () -> Unit,
     modifier: Modifier = Modifier,
 ) {
@@ -520,9 +517,10 @@ private fun MediaViewerErrorPage(
                 .navigationBarsPadding(),
             contentAlignment = Alignment.Center
         ) {
-            AsyncFailure(
-                throwable = throwable,
-                onRetry = null
+            IconTitleSubtitleMolecule(
+                iconStyle = BigIcon.Style.AlertSolid,
+                title = stringResource(CommonStrings.common_something_went_wrong),
+                subTitle = stringResource(CommonStrings.common_something_went_wrong_message),
             )
         }
     }

libraries/mediaviewer/impl/src/test/kotlin/io/element/android/libraries/mediaviewer/impl/datasource/TimelineMediaGalleryDataSourceTest.kt

@@ -39,6 +39,7 @@ import io.element.android.libraries.matrix.ui.media.contentvalidation.NoopEventC
 import io.element.android.libraries.mediaviewer.api.MediaInfo
 import io.element.android.libraries.mediaviewer.impl.model.GroupedMediaItems
 import io.element.android.libraries.mediaviewer.impl.model.MediaItem
+import io.element.android.libraries.mediaviewer.impl.model.aMediaItemImage
 import io.element.android.libraries.mediaviewer.test.util.FileExtensionExtractorWithoutValidation
 import io.element.android.tests.testutils.WarmUpRule
 import io.element.android.tests.testutils.lambda.lambdaRecorder
@@ -171,6 +172,32 @@ class TimelineMediaGalleryDataSourceTest {
         }
     }
 
+    @Test
+    fun `test - failing to load a focused timeline keeps the item that was opened`() = runTest {
+        val room = FakeJoinedRoom(
+            createTimelineResult = { Result.failure(AN_EXCEPTION) },
+            roomCoroutineScope = backgroundScope,
+        )
+        val initialMediaItem = aMediaItemImage()
+        val sut = createTimelineMediaGalleryDataSource(
+            room = room,
+            mediaTimeline = FocusedMediaTimeline(
+                room = room,
+                eventId = AN_EVENT_ID,
+                onlyPinnedEvents = false,
+                initialMediaItem = initialMediaItem,
+            ),
+        )
+        sut.start(backgroundScope)
+        sut.groupedMediaItemsFlow().test {
+            val item = awaitItem()
+            assertThat(item.isSuccess()).isTrue()
+            assertThat(item.dataOrNull()?.imageAndVideoItems).contains(initialMediaItem)
+            expectNoEvents()
+        }
+        assertThat(sut.getLastData().isFailure()).isFalse()
+    }
+
     @Test
     fun `test - when timeline emits new data, the flow emits the data`() = runTest {
         val timelineItems = MutableStateFlow<List<MatrixTimelineItem>>(emptyList())
@@ -269,10 +296,11 @@ internal fun TestScope.createTimelineMediaGalleryDataSource(
     room: JoinedRoom = FakeJoinedRoom(
         liveTimeline = FakeTimeline(),
     ),
+    mediaTimeline: MediaTimeline? = null,
 ): TimelineMediaGalleryDataSource {
     return TimelineMediaGalleryDataSource(
         room = room,
-        mediaTimeline = LiveMediaTimeline(room),
+        mediaTimeline = mediaTimeline ?: LiveMediaTimeline(room),
         timelineMediaItemsFactory = createTimelineMediaItemsFactory(),
         mediaItemsPostProcessor = MediaItemsPostProcessor(),
     )