fix(mobile): lazily lay out new-DM recipients (#8435)

The new-message picker laid out every recipient inside an outer scroll
view. Searching or changing selection could therefore mount the whole
directory in one frame. Use a bounded sliver viewport so offscreen
recipients stay lazy while the search field and selection chips still
scroll above the keyboard.

### Validation

- Current-head CI passed, including all Flutter test shards, Android,
and native/release Swift jobs. Code and security reviews found no
issues. After merging main and retaining both test imports, all 81
channel-page tests and the full mobile check/test suite passed.

- Added a 200-person widget regression: the original mounts all 200
rows; the fix keeps fewer than 30 mounted through open, search,
selection and removal, and can scroll to the final recipient. Covers
no-match and closing too.
- Strengthened the keyboard/chip test to verify the eight-recipient
limit and that removing a chip makes the ninth recipient reachable.
- All 79 channel-page tests passed; the strengthened limit test and
Flutter analysis passed.
- Native iOS simulator build passed. On a separately cloned paired
simulator, verified populated directory/search, eight selected
recipients, limit message, removal, no-match, scrolling to the final
recipient and close without creating a conversation. Debug/AX
instrumentation and a newer Flutter baseline make this functional
validation, not a controlled release-speed benchmark.
- Normal pre-push mobile checks passed, including formatting, analysis
and the full Flutter test suite. Required CI exposed existing formatting
in `message_carousel_preview_cases.dart`; a separate formatter-only
commit corrects it.
- The full local repository gate hit an unrelated Rust failure in
`buzz-acp::signals_cancel_startup_turn_and_pending_stdin_without_hanging`
(exit status `None` rather than `Some(143)`). No Rust code changed. The
gate stopped at unit tests, so later build stages did not run.

### Related issue

No duplicate recipient-layout fix found in the existing issue/PR search.

### Before and after

The synthetic 200-person picker renders identically before and after
(with a reserved keyboard inset). One image represents both; mounted-row
assertions establish the performance mechanism separately.

![Recipient picker before and
after](https://raw.githubusercontent.com/block/buzz/c9980118b75e64ee8ba0b559b7cc981ee59f809b/pr-8435--dm-directory.png)

---------

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

mobile/lib/features/channels/channels_page/sheets.dart

@@ -535,12 +535,14 @@ class _NewDirectMessageSheet extends HookConsumerWidget {
       ),
       child: SafeArea(
         top: false,
-        child: SingleChildScrollView(
+        // The sheet bounds this viewport even with the keyboard visible.
-          child: Column(
+        // Keep rows as slivers so search and chip updates do not lay out the
-            mainAxisSize: MainAxisSize.min,
+        // entire directory. The field still scrolls with long chip selections.
-            crossAxisAlignment: CrossAxisAlignment.start,
+        child: CustomScrollView(
-            children: [
+          shrinkWrap: true,
-              GestureDetector(
+          slivers: [
+            SliverToBoxAdapter(
+              child: GestureDetector(
                 behavior: HitTestBehavior.translucent,
                 onTap: isSubmitting.value ? null : queryFocusNode.requestFocus,
                 child: SizedBox(
@@ -664,24 +666,27 @@ class _NewDirectMessageSheet extends HookConsumerWidget {
                   ),
                 ),
               ),
-              const SizedBox(height: Grid.xs),
+            ),
+            const SliverToBoxAdapter(child: SizedBox(height: Grid.xs)),
             Builder(
               key: const Key('new-dm-results'),
               builder: (context) {
                 if (selectedUsers.value.length >= 8) {
-                    return const SizedBox(
+                  return const SliverToBoxAdapter(
+                    child: SizedBox(
                       height: 96,
                       child: Center(
                         child: Text(
                           'DMs support up to nine people, including you.',
                         ),
                       ),
+                    ),
                   );
                 }
-                  if (directoryAsync.isLoading &&
+                if (directoryAsync.isLoading && directoryAsync.asData == null ||
-                          directoryAsync.asData == null ||
                     isSearchTransitionPending) {
-                    return const SizedBox(
+                  return const SliverToBoxAdapter(
+                    child: SizedBox(
                       height: 280,
                       child: Center(
                         child: BuzzLoadingIndicator(
@@ -689,10 +694,12 @@ class _NewDirectMessageSheet extends HookConsumerWidget {
                           semanticLabel: 'Loading people',
                         ),
                       ),
+                    ),
                   );
                 }
                 if (directoryAsync.hasError) {
-                    return SizedBox(
+                  return SliverToBoxAdapter(
+                    child: SizedBox(
                       height: 280,
                       child: Center(
                         child: Column(
@@ -719,10 +726,12 @@ class _NewDirectMessageSheet extends HookConsumerWidget {
                           ],
                         ),
                       ),
+                    ),
                   );
                 }
                 if (availableResults.isEmpty) {
-                    return SizedBox(
+                  return SliverToBoxAdapter(
+                    child: SizedBox(
                       height: 280,
                       child: Center(
                         child: Text(
@@ -731,11 +740,10 @@ class _NewDirectMessageSheet extends HookConsumerWidget {
                               : 'No matching users.',
                         ),
                       ),
+                    ),
                   );
                 }
-                  return ListView.builder(
+                return SliverList.builder(
-                    physics: const NeverScrollableScrollPhysics(),
-                    shrinkWrap: true,
                   itemCount: availableResults.length,
                   itemBuilder: (context, index) {
                     final user = availableResults[index];
@@ -793,16 +801,17 @@ class _NewDirectMessageSheet extends HookConsumerWidget {
               },
             ),
             if (submitError.value case final error?) ...[
-                const SizedBox(height: Grid.xxs),
+              const SliverToBoxAdapter(child: SizedBox(height: Grid.xxs)),
-                Text(
+              SliverToBoxAdapter(
+                child: Text(
                   error,
                   style: context.textTheme.bodySmall?.copyWith(
                     color: context.colors.error,
                   ),
                 ),
-              ],
-            ],
               ),
+            ],
+          ],
         ),
       ),
     );

mobile/test/features/channels/channels_page_test.dart

@@ -59,6 +59,7 @@ import 'package:buzz/shared/widgets/skeleton.dart';
 import '../../helpers/thread_ids.dart';
 
 part 'channels_page_test/presence_tests.dart';
+part 'channels_page_test/dm_recipient_layout_tests.dart';
 part 'channels_page_test/discovery_journey_tests.dart';
 part 'channels_page_test/unread_readiness_tests.dart';
 
@@ -3360,6 +3361,16 @@ void main() {
     );
   });
 
+  dmRecipientLayoutTests(
+    (overrides) => buildTestable(
+      keyboardInset: 300,
+      overrides: [
+        channelsProvider.overrideWith(() => _FakeNotifier(testChannels)),
+        ...overrides,
+      ],
+    ),
+  );
+
   testWidgets('new message sheet lists and selects relay members', (
     tester,
   ) async {
@@ -3537,7 +3548,7 @@ void main() {
     addTearDown(tester.view.resetDevicePixelRatio);
 
     final directoryUsers = [
-      for (var index = 0; index < 8; index++)
+      for (var index = 0; index < 9; index++)
         DirectoryUser(
           pubkey: 'person-$index',
           displayName: 'Long recipient name number $index',
@@ -3562,7 +3573,7 @@ void main() {
     await tester.tap(find.text('New direct message'));
     await tester.pumpAndSettle();
 
-    for (final user in directoryUsers) {
+    for (final user in directoryUsers.take(8)) {
       final result = find.byKey(Key('new-dm-person-${user.pubkey}'));
       await tester.ensureVisible(result);
       await tester.pumpAndSettle();
@@ -3579,6 +3590,31 @@ void main() {
       find.text('DMs support up to nine people, including you.'),
       findsOneWidget,
     );
+    expect(find.byKey(const Key('new-dm-person-person-8')), findsNothing);
+    final lastChip = find.byKey(const Key('new-dm-selected-person-7'));
+    await tester.ensureVisible(lastChip);
+    await tester.pumpAndSettle();
+    await tester.tap(lastChip);
+    await tester.pumpAndSettle();
+    expect(find.byKey(const Key('new-dm-selected-person-7')), findsNothing);
+    final ninthRecipient = find.byKey(const Key('new-dm-person-person-8'));
+    final listScroll = find
+        .descendant(
+          of: find.byType(BottomSheet).last,
+          matching: find.byWidgetPredicate(
+            (widget) =>
+                widget is Scrollable &&
+                widget.axisDirection == AxisDirection.down,
+          ),
+        )
+        .last;
+    await tester.scrollUntilVisible(
+      ninthRecipient,
+      100,
+      scrollable: listScroll,
+    );
+    await tester.pumpAndSettle();
+    expect(ninthRecipient.hitTestable(), findsOneWidget);
     expect(tester.takeException(), isNull);
   });
 

mobile/test/features/channels/channels_page_test/dm_recipient_layout_tests.dart

@@ -0,0 +1,110 @@
+part of '../channels_page_test.dart';
+
+void dmRecipientLayoutTests(Widget Function(List<Override>) harness) {
+  testWidgets(
+    'recipient directory lays out only visible rows and reaches end',
+    (tester) async {
+      tester.view.physicalSize = const Size(390, 800);
+      tester.view.devicePixelRatio = 1;
+      addTearDown(tester.view.resetPhysicalSize);
+      addTearDown(tester.view.resetDevicePixelRatio);
+      final users = [
+        for (var i = 0; i < 200; i++)
+          DirectoryUser(pubkey: 'person-$i', displayName: 'Person $i'),
+      ];
+      await tester.pumpWidget(
+        harness([
+          relayDirectoryUsersProvider.overrideWith((ref) async => users),
+          relayDirectorySearchProvider.overrideWith(
+            (ref, query) async => users,
+          ),
+        ]),
+      );
+      await tester.pumpAndSettle();
+      await tester.tap(find.byTooltip('Create or start conversation'));
+      await tester.pumpAndSettle();
+      await tester.tap(find.text('New direct message'));
+      await tester.pumpAndSettle();
+
+      if (Platform.environment['COMMUNITY_SCREENSHOTS'] case final directory?) {
+        final boundary = tester
+            .element(find.byType(BottomSheet).last)
+            .findAncestorRenderObjectOfType<RenderRepaintBoundary>()!;
+        await tester.runAsync(() async {
+          final image = await boundary.toImage(pixelRatio: 2);
+          final bytes = await image.toByteData(format: ui.ImageByteFormat.png);
+          await Directory(directory).create(recursive: true);
+          await File(
+            '$directory/dm-directory.png',
+          ).writeAsBytes(bytes!.buffer.asUint8List());
+          image.dispose();
+        });
+      }
+
+      final rows = find.byWidgetPredicate(
+        (widget) =>
+            widget is ListTile &&
+            widget.key is ValueKey<String> &&
+            (widget.key! as ValueKey<String>).value.startsWith(
+              'new-dm-person-',
+            ),
+      );
+      void expectBoundedRows() {
+        expect(rows.evaluate().length, inExclusiveRange(0, 30));
+        expect(find.byKey(const Key('new-dm-person-person-199')), findsNothing);
+      }
+
+      expectBoundedRows();
+      final search = find.byKey(const Key('new-dm-search'));
+      await tester.enterText(search, 'Person');
+      await tester.pump(const Duration(milliseconds: 300));
+      await tester.pumpAndSettle();
+      expectBoundedRows();
+      await tester.tap(find.byKey(const Key('new-dm-person-person-0')));
+      await tester.pumpAndSettle();
+      expect(find.bySemanticsLabel('Remove Person 0'), findsOneWidget);
+      expectBoundedRows();
+      await tester.tap(find.bySemanticsLabel('Remove Person 0'));
+      await tester.pumpAndSettle();
+      expectBoundedRows();
+
+      await tester.enterText(search, 'no such recipient');
+      await tester.pump(const Duration(milliseconds: 300));
+      await tester.pumpAndSettle();
+      expect(find.text('No matching users.'), findsOneWidget);
+      expect(rows, findsNothing);
+      await tester.enterText(search, '');
+      await tester.pump(const Duration(milliseconds: 300));
+      await tester.pumpAndSettle();
+      expectBoundedRows();
+
+      final scrollable = find
+          .descendant(
+            of: find.byType(BottomSheet).last,
+            matching: find.byWidgetPredicate(
+              (widget) =>
+                  widget is Scrollable &&
+                  widget.axisDirection == AxisDirection.down,
+            ),
+          )
+          .last;
+      await tester.scrollUntilVisible(
+        find.byKey(const Key('new-dm-person-person-199')),
+        300,
+        scrollable: scrollable,
+        maxScrolls: 100,
+      );
+      await tester.pumpAndSettle();
+      expect(
+        find.byKey(const Key('new-dm-person-person-199')).hitTestable(),
+        findsOneWidget,
+      );
+      expect(rows.evaluate().length, lessThan(30));
+      expect(tester.takeException(), isNull);
+
+      await tester.tap(find.byTooltip('Close sheet'));
+      await tester.pumpAndSettle();
+      expect(find.text('New message'), findsNothing);
+    },
+  );
+}