fix(mobile): lazily lay out channel member groups (#8437)

Opening a large channel member sheet mounted and laid out every person
in a grouped column. A captured Debug simulator entrance spent 242 ms in
the UI callback, including 223 ms in layout.

Build each person and agent as a lazy sliver row while preserving the
grouped cards, names, statuses, management menus, and activity actions.
Screen-reader row indices continue across both groups. A separate
formatting-only commit fixes an existing carousel-test formatter
discrepancy. No duplicate member-sheet performance PR or issue found.

### Validation

- Added a 200-member widget regression: the original mounts 100 people
initially; the fix mounts fewer than 20 nearby rows and reaches the
final person and agent.
- Added grouping, compact sizing, empty state, member tap identity,
accessible indices, and scrolled member/agent menu checks. Existing
channel-details “See all” navigation test passes.
- Mobile formatting, static analysis, and the full mobile test suite
passed on the committed tree.
- Synthetic before/after rendering is pixel-identical at the same
viewport.
- Populated native iOS 26.5 simulator workflow: open members, scroll
through people and agents to the end, then close. Final installed build
passed this workflow.

Debug, accessibility automation, profiler overhead, different Flutter
versions, and shared-host load limit timing conclusions. This removes
eager offscreen layout; it does not establish a controlled end-to-end
speedup or release-device performance.

### Visual check

Synthetic member sheet; before and after are pixel-identical at this
viewport.

![Synthetic member
groups](https://raw.githubusercontent.com/block/buzz/c327b1d973e6ec54a09358e8f6794dc0a2587304/pr-8437--members.png)

---------

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/features/channels/members_sheet.dart

@@ -19,6 +19,8 @@ import 'channel.dart';
 import 'channel_identity_names_provider.dart';
 import 'channel_management_provider.dart';
 
+part 'members_sheet/member_section.dart';
+
 class MembersSheet extends HookConsumerWidget {
   final Channel channel;
   final String? currentPubkey;
@@ -94,18 +96,19 @@ class MembersSheet extends HookConsumerWidget {
         key: const ValueKey('members-sheet-viewport'),
         constraints: BoxConstraints(maxHeight: 400 + bottomClearance),
         child: membersAsync.when(
-          data: (_) => ListView(
+          data: (_) => CustomScrollView(
             key: const ValueKey('members-sheet-list'),
+            semanticChildCount: allMembers.length,
             shrinkWrap: true,
-            padding: EdgeInsets.only(top: Grid.xxs, bottom: bottomClearance),
-            children: [
+            slivers: [
+              const SliverToBoxAdapter(child: SizedBox(height: Grid.xxs)),
               if (people.isNotEmpty)
-                SheetActionSection(
+                _MemberSection(
                   label: 'People · ${people.length}',
-                  dividerIndent: Grid.xs + 40 + Grid.xs,
-                  children: [
-                    for (final member in people)
-                      _MemberTile(
+                  itemCount: people.length,
+                  itemBuilder: (context, index) {
+                    final member = people[index];
+                    return _MemberTile(
                       member: member,
                       currentPubkey: currentPubkey,
                       profile: userCache[member.pubkey.toLowerCase()],
@@ -118,16 +121,17 @@ class MembersSheet extends HookConsumerWidget {
                       onActivityTap: onMemberTap == null
                           ? null
                           : () => onMemberTap!(context, member.pubkey),
-                      ),
-                  ],
+                    );
+                  },
                 ),
               if (bots.isNotEmpty)
-                SheetActionSection(
+                _MemberSection(
                   label: 'Agents · ${bots.length}',
-                  dividerIndent: Grid.xs + 40 + Grid.xs,
-                  children: [
-                    for (final bot in bots)
-                      _MemberTile(
+                  semanticIndexOffset: people.length,
+                  itemCount: bots.length,
+                  itemBuilder: (context, index) {
+                    final bot = bots[index];
+                    return _MemberTile(
                       member: bot,
                       currentPubkey: currentPubkey,
                       profile: userCache[bot.pubkey.toLowerCase()],
@@ -142,11 +146,12 @@ class MembersSheet extends HookConsumerWidget {
                           typingBotPubkeys.contains(bot.pubkey.toLowerCase())
                           ? () => openActivity(bot)
                           : null,
-                      ),
-                  ],
+                    );
+                  },
                 ),
               if (people.isEmpty && bots.isEmpty)
-                Center(
+                SliverToBoxAdapter(
+                  child: Center(
                     child: Text(
                       'No members found.',
                       style: context.textTheme.bodySmall?.copyWith(
@@ -154,6 +159,8 @@ class MembersSheet extends HookConsumerWidget {
                       ),
                     ),
                   ),
+                ),
+              SliverToBoxAdapter(child: SizedBox(height: bottomClearance)),
             ],
           ),
           loading: () => const Center(

mobile/lib/features/channels/members_sheet/member_section.dart

@@ -0,0 +1,75 @@
+part of '../members_sheet.dart';
+
+/// The member card is split into independently laid-out rows so a large group
+/// cannot force every offscreen member to mount during the sheet entrance.
+class _MemberSection extends StatelessWidget {
+  const _MemberSection({
+    required this.label,
+    required this.itemCount,
+    required this.itemBuilder,
+    this.semanticIndexOffset = 0,
+  });
+
+  final String label;
+  final int itemCount;
+  final IndexedWidgetBuilder itemBuilder;
+  final int semanticIndexOffset;
+
+  @override
+  Widget build(BuildContext context) => SliverPadding(
+    padding: const EdgeInsets.symmetric(vertical: Grid.xxs),
+    sliver: SliverMainAxisGroup(
+      slivers: [
+        SliverToBoxAdapter(
+          child: Padding(
+            padding: const EdgeInsets.only(left: Grid.half, bottom: Grid.xxs),
+            child: Text(
+              label,
+              style: context.textTheme.labelMedium?.copyWith(
+                color: context.colors.onSurfaceVariant,
+                fontWeight: FontWeight.w600,
+              ),
+            ),
+          ),
+        ),
+        SliverList(
+          delegate: SliverChildBuilderDelegate(
+            (context, index) => Material(
+              color: context.colors.surfaceContainerHighest,
+              borderRadius: BorderRadius.vertical(
+                top: index == 0
+                    ? const Radius.circular(Radii.container)
+                    : Radius.zero,
+                bottom: index == itemCount - 1
+                    ? const Radius.circular(Radii.container)
+                    : Radius.zero,
+              ),
+              clipBehavior: Clip.antiAlias,
+              child: ListTileTheme.merge(
+                contentPadding: const EdgeInsets.symmetric(horizontal: Grid.xs),
+                minLeadingWidth: 22,
+                horizontalTitleGap: Grid.xs,
+                child: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: [
+                    if (index > 0)
+                      Divider(
+                        height: 1,
+                        thickness: 1,
+                        indent: Grid.xs + 40 + Grid.xs,
+                        endIndent: Grid.xs,
+                        color: context.colors.onSurface.withValues(alpha: 0.07),
+                      ),
+                    itemBuilder(context, index),
+                  ],
+                ),
+              ),
+            ),
+            childCount: itemCount,
+            semanticIndexOffset: semanticIndexOffset,
+          ),
+        ),
+      ],
+    ),
+  );
+}

mobile/test/features/channels/channel_detail_page_test.dart

@@ -3497,23 +3497,13 @@ void main() {
       await tester.tap(seeAllRow);
       await tester.pumpAndSettle();
       // Verify each group when it enters the lazy sheet viewport.
-      expect(
-        find.byWidgetPredicate(
-          (widget) => widget is AppListCard && widget.label == 'People · 6',
-        ),
-        findsOneWidget,
-      );
+      expect(find.text('People · 6'), findsOneWidget);
       await tester.drag(
         find.byKey(const ValueKey('members-sheet-list')),
         const Offset(0, -500),
       );
       await tester.pumpAndSettle();
-      expect(
-        find.byWidgetPredicate(
-          (widget) => widget is AppListCard && widget.label == 'Agents · 1',
-        ),
-        findsOneWidget,
-      );
+      expect(find.text('Agents · 1'), findsOneWidget);
       expect(find.text('Agent'), findsOneWidget);
       expect(find.text('Bot'), findsNothing);
     });

mobile/test/features/channels/members_sheet_test.dart

@@ -0,0 +1,213 @@
+import 'package:buzz/features/channels/agent_activity/working_bots_provider.dart';
+import 'package:buzz/features/channels/channel.dart';
+import 'package:buzz/features/channels/channel_identity_names_provider.dart';
+import 'package:buzz/features/channels/channel_management_provider.dart';
+import 'package:buzz/features/channels/members_sheet.dart';
+import 'package:buzz/features/profile/user_status.dart';
+import 'package:buzz/features/profile/user_status_cache_provider.dart';
+import 'package:buzz/shared/identity_names/identity_names.dart';
+import 'package:buzz/shared/profile/user_cache_provider.dart';
+import 'package:buzz/shared/profile/user_profile.dart';
+import 'package:buzz/shared/theme/theme.dart';
+import 'package:buzz/shared/theme/buzz_icons.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:hooks_riverpod/hooks_riverpod.dart';
+
+class _Cache extends UserCacheNotifier {
+  @override
+  Map<String, UserProfile> build() => {};
+  @override
+  Future<bool> preload(List<String> pubkeys) async => true;
+}
+
+class _Statuses extends UserStatusCacheNotifier {
+  @override
+  Map<String, UserStatus?> build() => {};
+  @override
+  void track(List<String> pubkeys) {}
+}
+
+String _key(int index) => index.toRadixString(16).padLeft(64, '0');
+
+Future<void> _pump(
+  WidgetTester tester,
+  List<ChannelMember> members, {
+  ValueChanged<String>? onTap,
+}) async {
+  final names = IdentityNameSources.empty.scope(
+    members.map((m) => m.pubkey),
+    fallbackNames: {for (final m in members) m.pubkey: m.displayName!},
+  );
+  await tester.pumpWidget(
+    ProviderScope(
+      overrides: [
+        channelMembersProvider('channel').overrideWith((ref) async => members),
+        channelIdentityNamesProvider('channel').overrideWith((ref) => names),
+        userCacheProvider.overrideWith(_Cache.new),
+        userStatusCacheProvider.overrideWith(_Statuses.new),
+        workingBotPubkeysProvider('channel').overrideWith((ref) => {}),
+      ],
+      child: MaterialApp(
+        theme: AppTheme.light(),
+        home: Scaffold(
+          body: Align(
+            alignment: Alignment.bottomCenter,
+            child: MembersSheet(
+              channel: Channel(
+                id: 'channel',
+                name: 'Test',
+                channelType: 'stream',
+                visibility: 'open',
+                description: '',
+                createdBy: _key(0),
+                createdAt: DateTime(2026),
+                memberCount: members.length,
+              ),
+              currentPubkey: _key(0),
+              onMemberTap: (_, key) => onTap?.call(key),
+            ),
+          ),
+        ),
+      ),
+    ),
+  );
+  await tester.pumpAndSettle();
+}
+
+List<ChannelMember> _roster(int people, int bots) => [
+  for (var i = 0; i < people + bots; i++)
+    ChannelMember(
+      pubkey: _key(i),
+      role: i == 0
+          ? 'owner'
+          : i < people
+          ? 'member'
+          : 'bot',
+      joinedAt: DateTime(2026),
+      displayName: i < people ? 'Person $i' : 'Agent $i',
+    ),
+];
+
+void main() {
+  testWidgets('large groups mount nearby rows and can reach the final agent', (
+    tester,
+  ) async {
+    tester.view.physicalSize = const Size(390, 844);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.resetPhysicalSize);
+    addTearDown(tester.view.resetDevicePixelRatio);
+    await _pump(tester, _roster(100, 100));
+    expect(find.text('People · 100'), findsOneWidget);
+    expect(find.text('You'), findsOneWidget);
+    expect(find.byType(ListTile).evaluate().length, lessThan(20));
+    expect(find.text('Person 99'), findsNothing);
+    final scrollable = find.descendant(
+      of: find.byKey(const ValueKey('members-sheet-list')),
+      matching: find.byType(Scrollable),
+    );
+    await tester.scrollUntilVisible(
+      find.text('Person 99'),
+      300,
+      scrollable: scrollable,
+      maxScrolls: 100,
+    );
+    expect(find.text('Person 99'), findsOneWidget);
+    await tester.scrollUntilVisible(
+      find.text('Agent 199'),
+      300,
+      scrollable: scrollable,
+      maxScrolls: 100,
+    );
+    expect(find.text('Agent 199'), findsOneWidget);
+    expect(find.byType(ListTile).evaluate().length, lessThan(20));
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('short list keeps grouping, self label and person tap identity', (
+    tester,
+  ) async {
+    String? selected;
+    final semantics = tester.ensureSemantics();
+
+    await _pump(tester, _roster(2, 1), onTap: (key) => selected = key);
+    expect(find.text('People · 2'), findsOneWidget);
+    expect(find.text('Agents · 1'), findsOneWidget);
+    final agentIndex = tester.widget<IndexedSemantics>(
+      find.ancestor(
+        of: find.text('Agent 2'),
+        matching: find.byType(IndexedSemantics),
+      ),
+    );
+    expect(agentIndex.index, 2);
+    expect(
+      tester
+          .widget<CustomScrollView>(
+            find.byKey(const ValueKey('members-sheet-list')),
+          )
+          .semanticChildCount,
+      3,
+    );
+    expect(find.text('You'), findsOneWidget);
+    expect(find.bySemanticsLabel(RegExp('Person 1')), findsWidgets);
+    await tester.tap(find.text('Person 1'));
+    expect(selected, _key(1));
+    semantics.dispose();
+    expect(
+      tester.getSize(find.byKey(const ValueKey('members-sheet-list'))).height,
+      lessThan(400),
+    );
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('owner actions stay available after lazy scrolling', (
+    tester,
+  ) async {
+    await _pump(tester, _roster(40, 1));
+    final scrollable = find.descendant(
+      of: find.byKey(const ValueKey('members-sheet-list')),
+      matching: find.byType(Scrollable),
+    );
+    await tester.scrollUntilVisible(
+      find.text('Person 39'),
+      300,
+      scrollable: scrollable,
+      maxScrolls: 50,
+    );
+    final row = find.ancestor(
+      of: find.text('Person 39'),
+      matching: find.byType(ListTile),
+    );
+    await tester.tap(
+      find.descendant(of: row, matching: find.byIcon(BuzzIcons.ellipsis)),
+    );
+    await tester.pumpAndSettle();
+    expect(find.text('Role'), findsOneWidget);
+    expect(find.text('Remove from channel'), findsOneWidget);
+    // Inspect only: do not invoke a membership mutation.
+    Navigator.of(tester.element(find.text('Role'))).pop();
+    await tester.pumpAndSettle();
+    await tester.scrollUntilVisible(
+      find.text('Agent 40'),
+      100,
+      scrollable: scrollable,
+    );
+    final agent = find.ancestor(
+      of: find.text('Agent 40'),
+      matching: find.byType(ListTile),
+    );
+    await tester.tap(
+      find.descendant(of: agent, matching: find.byIcon(BuzzIcons.ellipsis)),
+    );
+    await tester.pumpAndSettle();
+    expect(find.text('View activity'), findsOneWidget);
+    expect(find.text('Role'), findsNothing);
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('empty roster retains empty message', (tester) async {
+    await _pump(tester, []);
+    expect(find.text('No members found.'), findsOneWidget);
+    expect(tester.takeException(), isNull);
+  });
+}