fix(mobile): harden channel discovery refresh

Signed-off-by: Tom Brow <tomb@block.xyz>
Co-authored-by: Codex <noreply@openai.com>
Ai-assisted: true

mobile/lib/features/channels/channel_directory.dart

@@ -0,0 +1,43 @@
+part of 'channels_provider.dart';
+
+const _channelDirectoryPageSize = 500;
+const _maxChannelDirectoryPages = 100;
+
+Future<List<NostrEvent>> _fetchChannelDirectoryMetas(
+  RelaySessionNotifier session,
+) async {
+  final directoryMetas = <NostrEvent>[];
+  final seenDirectoryChannelIds = <String>{};
+  int? directoryUntil;
+  String? directoryBeforeId;
+  for (var pageIndex = 0; pageIndex < _maxChannelDirectoryPages; pageIndex++) {
+    final page = await session.fetchHistory(
+      NostrFilter(
+        kinds: const [39000],
+        limit: _channelDirectoryPageSize,
+        until: directoryUntil,
+        extensions: {'before_id': ?directoryBeforeId},
+      ),
+    );
+    directoryMetas.addAll(page);
+
+    var madeProgress = false;
+    for (final event in page) {
+      final channelId = event.getTagValue('d');
+      if (channelId != null && seenDirectoryChannelIds.add(channelId)) {
+        madeProgress = true;
+      }
+    }
+    if (!madeProgress || page.length < _channelDirectoryPageSize) break;
+
+    final last = page.last;
+    directoryUntil = last.createdAt;
+    directoryBeforeId = last.id;
+    if (pageIndex == _maxChannelDirectoryPages - 1) {
+      throw StateError(
+        'Channel directory exceeded $_maxChannelDirectoryPages pages',
+      );
+    }
+  }
+  return directoryMetas;
+}

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

@@ -9,6 +9,10 @@ class _BrowseChannelsSheet extends ConsumerWidget {
     final channels = channelsAsync.asData?.value
         .where((channel) => channel.canJoin)
         .toList();
+    channels?.sort(
+      (left, right) =>
+          left.name.toLowerCase().compareTo(right.name.toLowerCase()),
+    );
 
     return SafeArea(
       top: false,
@@ -19,45 +23,64 @@ class _BrowseChannelsSheet extends ConsumerWidget {
           Grid.gutter,
           Grid.xs,
         ),
-        child: ListView(
+        child: CustomScrollView(
           shrinkWrap: true,
-          children: [
-            Text(
-              'Join an open channel to add it to your conversations.',
-              style: context.textTheme.bodyMedium?.copyWith(
-                color: context.colors.onSurfaceVariant,
+          slivers: [
+            SliverToBoxAdapter(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  Text(
+                    'Join an open channel to add it to your conversations.',
+                    style: context.textTheme.bodyMedium?.copyWith(
+                      color: context.colors.onSurfaceVariant,
+                    ),
+                  ),
+                  const SizedBox(height: Grid.xs),
+                ],
               ),
             ),
-            const SizedBox(height: Grid.xs),
             if (channelsAsync.isLoading && channels == null)
-              const Padding(
-                padding: EdgeInsets.all(Grid.sm),
-                child: Center(child: BuzzLoadingIndicator()),
+              const SliverToBoxAdapter(
+                child: Padding(
+                  padding: EdgeInsets.all(Grid.sm),
+                  child: Center(child: BuzzLoadingIndicator()),
+                ),
               )
             else if (channelsAsync.hasError && channels == null)
-              Padding(
-                padding: const EdgeInsets.symmetric(vertical: Grid.sm),
-                child: Text(
-                  'Could not load open channels.',
-                  textAlign: TextAlign.center,
-                  style: context.textTheme.bodyMedium?.copyWith(
-                    color: context.colors.onSurfaceVariant,
+              SliverToBoxAdapter(
+                child: Padding(
+                  padding: const EdgeInsets.symmetric(vertical: Grid.sm),
+                  child: Text(
+                    'Could not load open channels.',
+                    textAlign: TextAlign.center,
+                    style: context.textTheme.bodyMedium?.copyWith(
+                      color: context.colors.onSurfaceVariant,
+                    ),
                   ),
                 ),
               )
             else if (channels == null || channels.isEmpty)
-              Padding(
-                padding: const EdgeInsets.symmetric(vertical: Grid.sm),
-                child: Text(
-                  'No open channels available to join.',
-                  textAlign: TextAlign.center,
-                  style: context.textTheme.bodyMedium?.copyWith(
-                    color: context.colors.onSurfaceVariant,
+              SliverToBoxAdapter(
+                child: Padding(
+                  padding: const EdgeInsets.symmetric(vertical: Grid.sm),
+                  child: Text(
+                    'No open channels available to join.',
+                    textAlign: TextAlign.center,
+                    style: context.textTheme.bodyMedium?.copyWith(
+                      color: context.colors.onSurfaceVariant,
+                    ),
                   ),
                 ),
               )
             else
-              _JoinableChannelList(channels: channels, closeAfterJoin: true),
+              SliverList.builder(
+                itemCount: channels.length,
+                itemBuilder: (context, index) => _JoinableChannelTile(
+                  channel: channels[index],
+                  closeAfterJoin: true,
+                ),
+              ),
           ],
         ),
       ),
@@ -67,12 +90,8 @@ class _BrowseChannelsSheet extends ConsumerWidget {
 
 class _JoinableChannelList extends StatelessWidget {
   final List<Channel> channels;
-  final bool closeAfterJoin;
 
-  const _JoinableChannelList({
-    required this.channels,
-    this.closeAfterJoin = false,
-  });
+  const _JoinableChannelList({required this.channels});
 
   @override
   Widget build(BuildContext context) {
@@ -85,10 +104,7 @@ class _JoinableChannelList extends StatelessWidget {
       mainAxisSize: MainAxisSize.min,
       children: [
         for (final channel in sortedChannels)
-          _JoinableChannelTile(
-            channel: channel,
-            closeAfterJoin: closeAfterJoin,
-          ),
+          _JoinableChannelTile(channel: channel, closeAfterJoin: false),
       ],
     );
   }

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

@@ -494,7 +494,7 @@ class _EmptyState extends StatelessWidget {
                   ),
                 ),
                 const SizedBox(height: Grid.xs),
-                _JoinableChannelList(channels: channels),
+                _JoinableChannelList(channels: channels.take(3).toList()),
               ],
             ],
           ),

mobile/lib/features/channels/channels_provider.dart

@@ -18,10 +18,10 @@ import 'unread_badge/is_high_priority_event.dart';
 import 'unread_badge/observed_unread_event.dart';
 import 'unread_badge/should_notify_for_event.dart';
 
+part 'channel_directory.dart';
+
 const _channelTypeOrder = {'stream': 0, 'forum': 1, 'dm': 2};
 const _unreadCatchUpLimit = 1000;
-const _channelDirectoryPageSize = 500;
-const _maxChannelDirectoryPages = 100;
 const _participatedRootIdsPrefix = 'buzz-thread-participation.v1';
 const _authoredRootIdsPrefix = 'buzz-thread-authored.v1';
 
@@ -57,6 +57,7 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
   String? _memberSnapshotRelayBaseUrl;
   String? _memberSnapshotPubkey;
   Map<String, List<ChannelMember>> _memberSnapshotsByChannelId = const {};
+  List<NostrEvent> _directoryMetas = const [];
 
   /// The member snapshot already returned while loading the channel list.
   ///
@@ -84,6 +85,7 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
       _memberSnapshotRelayBaseUrl = relayBaseUrl;
       _memberSnapshotPubkey = pubkey;
       _memberSnapshotsByChannelId = const {};
+      _directoryMetas = const [];
     }
     final connected = Completer<void>();
     final sessionState = ref.read(relaySessionProvider);
@@ -128,10 +130,12 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
   Future<List<Channel>> _fetch({
     bool subscribeLive = false,
     bool fetchLastMessage = true,
+    bool fetchDirectory = true,
   }) async {
     final channels = await _fetchChannels(
       subscribeLive: subscribeLive,
       fetchLastMessage: fetchLastMessage,
+      fetchDirectory: fetchDirectory,
     );
     _hasLoaded = true;
     return channels;
@@ -140,6 +144,7 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
   Future<List<Channel>> _fetchChannels({
     bool subscribeLive = false,
     bool fetchLastMessage = true,
+    bool fetchDirectory = true,
   }) async {
     final myPk = ref.read(myPubkeyProvider);
     if (myPk == null) throw StateError('No signing identity available');
@@ -186,40 +191,13 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
     // kind:39000 query by the caller's access, but the client still rejects
     // private channels and DMs below so discovery fails closed if that contract
     // ever regresses. The composite cursor preserves tied-timestamp rows.
-    final directoryMetas = <NostrEvent>[];
-    final seenDirectoryChannelIds = <String>{};
-    int? directoryUntil;
-    String? directoryBeforeId;
-    for (
-      var pageIndex = 0;
-      pageIndex < _maxChannelDirectoryPages;
-      pageIndex++
-    ) {
-      final page = await session.fetchHistory(
-        NostrFilter(
-          kinds: const [39000],
-          limit: _channelDirectoryPageSize,
-          until: directoryUntil,
-          extensions: {'before_id': ?directoryBeforeId},
-        ),
-      );
-      directoryMetas.addAll(page);
-
-      var madeProgress = false;
-      for (final event in page) {
-        final channelId = event.getTagValue('d');
-        if (channelId != null && seenDirectoryChannelIds.add(channelId)) {
-          madeProgress = true;
-        }
-      }
-      if (!madeProgress || page.length < _channelDirectoryPageSize) break;
-
-      final last = page.last;
-      directoryUntil = last.createdAt;
-      directoryBeforeId = last.id;
-      if (pageIndex == _maxChannelDirectoryPages - 1) {
-        throw StateError(
-          'Channel directory exceeded $_maxChannelDirectoryPages pages',
+    if (fetchDirectory) {
+      try {
+        _directoryMetas = await _fetchChannelDirectoryMetas(session);
+      } catch (error) {
+        debugPrint(
+          '[ChannelsNotifier] channel directory refresh failed; retaining '
+          'cached discovery: $error',
         );
       }
     }
@@ -227,7 +205,7 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
     // Merge and dedupe by `d` tag. Kind:39000 is parameterized-replaceable,
     // but stale revisions from before the relay's d_tag backfill can linger.
     final latestMetaPerId = <String, NostrEvent>{};
-    for (final event in [...memberMetas, ...directoryMetas]) {
+    for (final event in [...memberMetas, ..._directoryMetas]) {
       if (event.kind != 39000) continue;
       final id = event.getTagValue('d');
       if (id == null) continue;
@@ -576,8 +554,8 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
   }
 
   /// Subscribe per-channel to live events (requires `#h` tag for relay
-  /// channel-scoped fan-out). Also starts a 60s WS backstop poll to detect
-  /// newly created channels we don't yet have subscriptions for.
+  /// channel-scoped fan-out). Also starts a 60s WS backstop poll to reconcile
+  /// membership changes without repeatedly downloading the global directory.
   Future<void> _subscribeLive(List<Channel> channels) {
     final channelIds = {
       for (final channel in channels)
@@ -919,6 +897,7 @@ class ChannelsNotifier extends AsyncNotifier<List<Channel>> {
       final channels = await _fetch(
         subscribeLive: sessionState.status == SessionStatus.connected,
         fetchLastMessage: false,
+        fetchDirectory: false,
       );
       for (var i = 0; i < channels.length; i++) {
         final prev = prevLastMessage[channels[i].id];

mobile/test/features/channels/channels_page_test.dart

@@ -1420,6 +1420,46 @@ void main() {
     expect(find.text('No open channels available to join.'), findsOneWidget);
   });
 
+  testWidgets('browse action lazily builds a large channel directory', (
+    tester,
+  ) async {
+    final channels = List.generate(
+      500,
+      (index) => Channel(
+        id: 'directory-$index',
+        name: 'channel-${index.toString().padLeft(3, '0')}',
+        channelType: 'stream',
+        visibility: 'open',
+        description: '',
+        createdBy: 'abc',
+        createdAt: DateTime(2025),
+        memberCount: 0,
+      ),
+    );
+    await tester.pumpWidget(
+      buildTestable(
+        disableAnimations: true,
+        overrides: [
+          channelsProvider.overrideWith(() => _FakeNotifier(channels)),
+        ],
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byTooltip('Create or start conversation'));
+    await tester.pump();
+    await tester.tap(
+      find.byKey(const Key('quick-action-browse-channels-card')),
+    );
+    await tester.pumpAndSettle();
+
+    expect(
+      find.byKey(const Key('browse-channel-directory-0')),
+      findsAtLeast(1),
+    );
+    expect(find.byKey(const Key('browse-channel-directory-499')), findsNothing);
+  });
+
   testWidgets('create channel sheet lists type and visibility radio options', (
     tester,
   ) async {

mobile/test/features/channels/channels_provider_test.dart

@@ -120,7 +120,7 @@ void main() {
     expect(session.metadataPageRequestCount, 2);
   });
 
-  test('fails loudly when channel discovery exceeds its page cap', () async {
+  test('directory page-cap failure does not fail channel loading', () async {
     final session = _FakeRelaySession(
       memberships: const [],
       metadataPageBuilder: (pageIndex) => List.generate(
@@ -135,16 +135,77 @@ void main() {
     final container = _buildContainer(session: session);
     addTearDown(container.dispose);
 
-    await expectLater(
-      container.read(channelsProvider.future),
-      throwsA(
-        isA<StateError>().having(
-          (error) => error.message,
-          'message',
-          contains('Channel directory exceeded'),
-        ),
-      ),
+    expect(await container.read(channelsProvider.future), isEmpty);
+    expect(session.metadataPageRequestCount, 100);
+  });
+
+  test(
+    'directory failure retains discovery while membership refreshes',
+    () async {
+      final session = _FakeRelaySession(
+        memberships: [_membership(_channelA, myPk)],
+        metadata: [
+          _meta(id: _channelA, name: 'general'),
+          _meta(id: _channelB, name: 'discoverable'),
+        ],
+      );
+      final container = _buildContainer(session: session);
+      addTearDown(container.dispose);
+
+      expect(
+        (await container.read(
+          channelsProvider.future,
+        )).map((channel) => channel.id),
+        unorderedEquals([_channelA, _channelB]),
+      );
+
+      session.memberships = [
+        _membership(_channelA, myPk),
+        _membership(_channelD, myPk),
+      ];
+      session.metadata = [
+        _meta(id: _channelA, name: 'general'),
+        _meta(id: _channelB, name: 'discoverable'),
+        _meta(id: _channelD, name: 'newly joined'),
+      ];
+      session.directoryFailures = 1;
+
+      await container.read(channelsProvider.notifier).refresh();
+
+      final refreshed = container.read(channelsProvider).requireValue;
+      expect(
+        refreshed.map((channel) => channel.id),
+        unorderedEquals([_channelA, _channelB, _channelD]),
+      );
+      expect(
+        refreshed.firstWhere((channel) => channel.id == _channelD).isMember,
+        isTrue,
+      );
+    },
+  );
+
+  test('reconnect backstop does not refetch the channel directory', () async {
+    final session = _FakeRelaySession(
+      memberships: [_membership(_channelA, myPk)],
+      metadata: [_meta(id: _channelA, name: 'general')],
     );
+    final container = _buildContainer(session: session);
+    addTearDown(container.dispose);
+
+    await container.read(channelsProvider.future);
+    final initialDirectoryRequests = session.metadataPageRequestCount;
+    final initialMembershipRequests = session.membershipRequestCount;
+
+    session.setStatus(SessionStatus.reconnecting);
+    session.setStatus(SessionStatus.connected);
+    await _waitUntil(
+      () => session.membershipRequestCount > initialMembershipRequests,
+    );
+    for (var i = 0; i < 10; i++) {
+      await Future<void>.delayed(Duration.zero);
+    }
+
+    expect(session.metadataPageRequestCount, initialDirectoryRequests);
   });
 
   test('deduplicates joined channels from directory discovery', () async {
@@ -908,6 +969,8 @@ class _FakeRelaySession extends RelaySessionNotifier {
   final List<NostrEvent> hiddenDmEvents;
   final List<NostrEvent> recentMessages;
   int membershipFailures;
+  int directoryFailures = 0;
+  int membershipRequestCount = 0;
   int metadataPageRequestCount = 0;
 
   final List<NostrFilter> historyFilters = [];
@@ -958,6 +1021,7 @@ class _FakeRelaySession extends RelaySessionNotifier {
   }) async {
     historyFilters.add(filter);
     if (filter.kinds.contains(39002) && filter.tags['#p'] != null) {
+      membershipRequestCount++;
       if (membershipFailures > 0) {
         membershipFailures--;
         throw Exception('membership fetch failed');
@@ -977,6 +1041,10 @@ class _FakeRelaySession extends RelaySessionNotifier {
     if (filter.kinds.contains(39000)) {
       final ids = filter.tags['#d']?.toSet();
       if (ids == null) {
+        if (directoryFailures > 0) {
+          directoryFailures--;
+          throw Exception('directory fetch failed');
+        }
         final requestIndex = metadataPageRequestCount++;
         final maxRequests = maxMetadataPageRequests;
         if (maxRequests != null && requestIndex >= maxRequests) {