Fix mobile message timeline bounce

mobile/lib/features/channels/channel_detail_page/message_list.dart

@@ -45,7 +45,7 @@ class _MessageList extends HookConsumerWidget {
       initialMessageId == null && initialThreadRootId == null,
     );
     final isAutoScrolling = useRef(false);
-    final autoScrollScheduled = useRef(false);
+    final latestRealignmentQueued = useRef(false);
     final latestEntryId = entries.isEmpty ? null : entries.last.message.id;
     final previousLatestEntryId = useRef<String?>(null);
     final didOpenInitialThread = useRef(false);
@@ -198,18 +198,6 @@ class _MessageList extends HookConsumerWidget {
       }
     }
 
-    void scheduleAutoScrollToLatest() {
-      if (autoScrollScheduled.value || isAutoScrolling.value) return;
-      autoScrollScheduled.value = true;
-      WidgetsBinding.instance.addPostFrameCallback((_) {
-        autoScrollScheduled.value = false;
-        if (!context.mounted || !followsLatest.value || hasUserScrolled.value) {
-          return;
-        }
-        scrollToLatest();
-      });
-    }
-
     bool latestIsAtBoundary() {
       // In this reversed list, item 0's leading edge is the bottom boundary.
       // Being merely visible is not enough: a user who has pulled a tall
@@ -220,6 +208,29 @@ class _MessageList extends HookConsumerWidget {
       );
     }
 
+    void realignLatestAfterLayoutChange() {
+      if (latestRealignmentQueued.value ||
+          !followsLatest.value ||
+          hasUserScrolled.value) {
+        return;
+      }
+      latestRealignmentQueued.value = true;
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        latestRealignmentQueued.value = false;
+        if (!context.mounted ||
+            !itemScrollController.isAttached ||
+            !followsLatest.value ||
+            hasUserScrolled.value ||
+            latestIsAtBoundary()) {
+          return;
+        }
+        // A dock or keyboard resize is a layout correction, not a navigation
+        // action. Keeping it instant avoids restarting a smooth scroll for
+        // every position report while the viewport settles.
+        itemScrollController.jumpTo(index: 0);
+      });
+    }
+
     useEffect(() {
       void onPositionsChanged() {
         final positions = itemPositionsListener.itemPositions.value;
@@ -232,12 +243,7 @@ class _MessageList extends HookConsumerWidget {
         }
         if (nextIsAtLatest) {
           if (!isAtLatest.value) isAtLatest.value = true;
-        } else if (followsLatest.value && !hasUserScrolled.value) {
-          // The viewport can shrink when the composer or keyboard opens.
-          // Preserve auto-follow until the user scrolls the timeline.
-          if (!isAtLatest.value) isAtLatest.value = true;
-          scheduleAutoScrollToLatest();
-        } else if (isAtLatest.value) {
+        } else if (!followsLatest.value && isAtLatest.value) {
           isAtLatest.value = false;
         }
 
@@ -261,6 +267,22 @@ class _MessageList extends HookConsumerWidget {
       );
     }, [channelId, entries.length, itemPositionsListener]);
 
+    // Composer size changes and keyboard metrics changes arrive in separate
+    // layout passes. Preserve the latest-message anchor for both, but only
+    // while the user has not deliberately left the tail.
+    useEffect(() {
+      realignLatestAfterLayoutChange();
+      return null;
+    }, [composerBottomInset]);
+
+    useEffect(() {
+      final observer = _ChannelLatestMetricsObserver(
+        onMetricsChanged: realignLatestAfterLayoutChange,
+      );
+      WidgetsBinding.instance.addObserver(observer);
+      return () => WidgetsBinding.instance.removeObserver(observer);
+    }, [itemScrollController]);
+
     useEffect(() {
       if (initialThreadRootId == null || didOpenInitialThread.value) {
         return null;
@@ -588,3 +610,12 @@ class _JumpToLatestButton extends StatelessWidget {
     );
   }
 }
+
+class _ChannelLatestMetricsObserver with WidgetsBindingObserver {
+  final VoidCallback onMetricsChanged;
+
+  _ChannelLatestMetricsObserver({required this.onMetricsChanged});
+
+  @override
+  void didChangeMetrics() => onMetricsChanged();
+}

mobile/test/features/channels/channel_detail_page_test.dart

@@ -1465,6 +1465,16 @@ void main() {
       );
       await tester.pumpAndSettle();
 
+      // History requests are deliberately scheduled after the current frame.
+      // Advance the test clock until the capped chain has settled.
+      for (
+        var frame = 0;
+        frame < 8 && messagesNotifier.fetchOlderCalls < 4;
+        frame++
+      ) {
+        await tester.pump(const Duration(milliseconds: 1));
+      }
+
       expect(messagesNotifier.fetchOlderCalls, 4);
       final unreadButton = find.byKey(
         const ValueKey('channel-jump-to-oldest-unread'),
@@ -1758,6 +1768,143 @@ void main() {
       );
     });
 
+    testWidgets(
+      'keeps the followed tail anchored through composer and keyboard resize',
+      (tester) async {
+        tester.view.physicalSize = const Size(400, 800);
+        tester.view.devicePixelRatio = 1;
+        addTearDown(tester.view.resetPhysicalSize);
+        addTearDown(tester.view.resetDevicePixelRatio);
+        addTearDown(tester.view.reset);
+
+        final messages = [
+          for (var i = 0; i < 20; i++)
+            _textMsg(
+              id: 'msg$i',
+              pubkey: i.isEven ? 'alice' : 'bob',
+              content: 'Message $i',
+              createdAt: 1000 + i * 1000,
+            ),
+        ];
+
+        await tester.pumpWidget(
+          _buildTestable(
+            messages: messages,
+            users: const {
+              'alice': UserProfile(pubkey: 'alice', displayName: 'Alice'),
+              'bob': UserProfile(pubkey: 'bob', displayName: 'Bob'),
+            },
+          ),
+        );
+        await tester.pumpAndSettle();
+
+        final latestMessage = find.byKey(
+          const ValueKey('channel-message-group-msg19'),
+        );
+        final composerDock = find.byKey(
+          const ValueKey('channel-composer-dock'),
+        );
+        final compactDockHeight = tester.getSize(composerDock).height;
+
+        expect(latestMessage, findsOneWidget);
+        expect(
+          tester.getBottomLeft(latestMessage).dy,
+          lessThanOrEqualTo(tester.getTopLeft(composerDock).dy + 1),
+        );
+
+        await tester.tap(find.text('Message #general'));
+        await tester.pumpAndSettle();
+
+        expect(
+          tester.getSize(composerDock).height,
+          greaterThan(compactDockHeight),
+        );
+        expect(
+          tester.getBottomLeft(latestMessage).dy,
+          lessThanOrEqualTo(tester.getTopLeft(composerDock).dy + 1),
+        );
+        expect(
+          find.byKey(const ValueKey('channel-jump-to-latest')),
+          findsNothing,
+        );
+
+        tester.view.viewInsets = const FakeViewPadding(bottom: 300);
+        await tester.pumpAndSettle();
+
+        expect(latestMessage, findsOneWidget);
+        expect(
+          tester.getBottomLeft(latestMessage).dy,
+          lessThanOrEqualTo(tester.getTopLeft(composerDock).dy + 1),
+        );
+        expect(
+          find.byKey(const ValueKey('channel-jump-to-latest')),
+          findsNothing,
+        );
+      },
+    );
+
+    testWidgets(
+      'does not realign a user-detached timeline on keyboard resize',
+      (tester) async {
+        tester.view.physicalSize = const Size(400, 600);
+        tester.view.devicePixelRatio = 1;
+        addTearDown(tester.view.resetPhysicalSize);
+        addTearDown(tester.view.resetDevicePixelRatio);
+        addTearDown(tester.view.reset);
+
+        final messages = [
+          for (var i = 0; i < 40; i++)
+            _textMsg(
+              id: 'msg$i',
+              pubkey: 'alice',
+              content: 'Message $i',
+              createdAt: 1000 + i,
+            ),
+        ];
+
+        await tester.pumpWidget(
+          _buildTestable(
+            messages: messages,
+            users: const {
+              'alice': UserProfile(pubkey: 'alice', displayName: 'Alice'),
+            },
+          ),
+        );
+        await tester.pumpAndSettle();
+
+        final messageList = find.byKey(const ValueKey('channel-message-list'));
+        await tester.drag(messageList, const Offset(0, 300));
+        await tester.pumpAndSettle();
+
+        expect(findRichText('Message 39'), findsNothing);
+        expect(
+          find.byKey(const ValueKey('channel-jump-to-latest')),
+          findsOneWidget,
+        );
+
+        tester.view.viewInsets = const FakeViewPadding(bottom: 300);
+        await tester.pumpAndSettle();
+
+        expect(findRichText('Message 39'), findsNothing);
+        expect(
+          find.byKey(const ValueKey('channel-jump-to-latest')),
+          findsOneWidget,
+        );
+        final positions = tester
+            .widget<ScrollablePositionedList>(messageList)
+            .itemPositionsNotifier!
+            .itemPositions
+            .value;
+        expect(
+          positions.any(
+            (position) =>
+                position.index == 0 && position.itemLeadingEdge.abs() < 0.01,
+          ),
+          isFalse,
+        );
+      },
+    );
+
     testWidgets('can jump back to latest after a non-drag user scroll', (
       tester,
     ) async {