fix(mobile): keep Home bell tappable while scrolled (#9241)

## Summary
Keep the mobile Home bell tappable while scrolling. Reserve both header
buttons’ space as soon as the filters overlap the toolbar, and clip
filter overflow.

### Related issue
No matching open issue or PR found.

### Testing
- `just mobile-check` and `just mobile-test` passed.
- Android/iOS regression coverage checks the bell at five scroll
positions and Activity navigation. iOS uses platform-view hit testing
plus its callback; no physical-device verification.
- Pre-push checks passed. The additional `just ci` run passed workspace
Rust lint and desktop source checks; stopped during desktop native
dependency compilation, so full local CI is incomplete.

Signed-off-by: kenny lopez <klopez4212@gmail.com>
Co-authored-by: Terp <5856f6900e8e0a4e0ea1f4f8d2ee33e4753d2119bc122f94ecafc660b46c0511@buzz.block.builderlab.xyz>

mobile/lib/features/channels/channels_page.dart

@@ -116,6 +116,7 @@ class ChannelsPage extends HookConsumerWidget {
     this.onSearch,
     this.messagesFilters,
     this.messagesActionsBuilder,
+    this.messagesActionsWidth = 0,
     this.soloSliver,
     this.showTeam = true,
     this.showTeamHeading = false,
@@ -133,6 +134,9 @@ class ChannelsPage extends HookConsumerWidget {
   final Widget Function(bool isCollapsed)? messagesFilters;
   final List<Widget> Function(VoidCallback openSwitcher, GlobalKey avatarKey)?
   messagesActionsBuilder;
+
+  /// Width of the trailing controls that collapsed filters must leave clear.
+  final double messagesActionsWidth;
   final Widget? soloSliver;
   final bool showTeam;
   final bool showTeamHeading;
@@ -737,10 +741,22 @@ class ChannelsPage extends HookConsumerWidget {
             ? const SizedBox.expand()
             : Padding(
                 padding: EdgeInsets.only(
-                  right: 60 * headerCollapse.value / filterContentHeight,
+                  // As soon as the rows overlap, keep the entire action
+                  // cluster clear. Interpolating this inset blocks the bell
+                  // during partial collapse even when the final layout fits.
+                  right: headerCollapse.value > 0
+                      ? messagesActionsWidth + _kTopSectionInset
+                      : 0,
                 ),
-                child: messagesFilters!(
-                  headerCollapse.value >= filterContentHeight,
+                child: ClipRect(
+                  // Native glass paints its shadows outside the viewport, but
+                  // scrolled filters must not paint over the header actions.
+                  clipBehavior: headerCollapse.value > 0
+                      ? Clip.hardEdge
+                      : Clip.none,
+                  child: messagesFilters!(
+                    headerCollapse.value >= filterContentHeight,
+                  ),
                 ),
               ),
       ),

mobile/lib/features/home/home_page.dart

@@ -99,6 +99,8 @@ class HomePage extends HookConsumerWidget {
     );
     final messages = ChannelsPage(
       settingsPageBuilder: settingsPageBuilder,
+      messagesActionsWidth:
+          2 * (defaultTargetPlatform == TargetPlatform.iOS ? 44 : 48) + Grid.xs,
       messagesActionsBuilder: (openSwitcher, avatarKey) {
         final moreActions = [
           // An internal build reaches only its organization's community.

mobile/test/features/home/home_page_test.dart

@@ -342,6 +342,49 @@ void main() {
   }
 
   for (final platform in [TargetPlatform.android, TargetPlatform.iOS]) {
+    testWidgets('$platform scrolled bell opens Activity', (tester) async {
+      debugDefaultTargetPlatformOverride = platform;
+      addTearDown(() => debugDefaultTargetPlatformOverride = null);
+      tester.view.physicalSize = const Size(390, 844);
+      tester.view.devicePixelRatio = 1;
+      addTearDown(tester.view.reset);
+      await tester.pumpWidget(await buildHome(longSolo: true));
+      await tester.pumpAndSettle();
+      final scroll = tester
+          .widget<CustomScrollView>(find.byType(CustomScrollView).first)
+          .controller!;
+      final collapseHeight = tester
+          .widget<FrostedAppBar>(find.byType(FrostedAppBar))
+          .bottomHeight;
+      for (final fraction in [0.01, 0.25, 0.5, 0.75, 1.0]) {
+        scroll.jumpTo(collapseHeight * fraction);
+        await tester.pumpAndSettle();
+        expect(
+          find.byTooltip('Activity').hitTestable(),
+          findsOneWidget,
+          reason: 'Bell must remain tappable at collapse fraction $fraction',
+        );
+      }
+      if (platform == TargetPlatform.iOS) {
+        // Widget tests hit-test the platform view but cannot run UIKit's
+        // button action. Exercise the callback that native taps dispatch.
+        tester
+            .widget<IosGlassNavigationButton>(
+              find.byWidgetPredicate(
+                (widget) =>
+                    widget is IosGlassNavigationButton &&
+                    widget.icon == IosGlassNavigationIcon.bell,
+              ),
+            )
+            .onPressed!();
+      } else {
+        await tester.tap(find.byTooltip('Activity'));
+      }
+      await tester.pumpAndSettle();
+      expect(find.byType(ActivityPage), findsOneWidget);
+      debugDefaultTargetPlatformOverride = null;
+    });
+
     testWidgets('$platform bell follows unread inbox state', (tester) async {
       debugDefaultTargetPlatformOverride = platform;
       addTearDown(() => debugDefaultTargetPlatformOverride = null);