fix: timeline anchoring

lib/controllers/room_chat.dart

@@ -17,27 +17,30 @@ import "package:nexus/models/room.dart";
 
 class RoomChatController extends AsyncNotifier<IList<Event>?> {
   final String roomId;
+  Future<bool>? _loadingOlder;
   RoomChatController(this.roomId);
 
   @override
   Future<IList<Event>?> build() async {
     final client = ref.watch(ClientController.provider.notifier);
-    final room = ref.watch(
+    final initialRoom = ref.watch(
       RoomsController.provider.select((rooms) => rooms[roomId]),
     );
 
-    if (room == null) return null;
+    if (initialRoom == null) return null;
+    Room room = initialRoom;
 
     if (!room.hasFetchedState) {
       final state = await client.getRoomState(.new(roomId: roomId));
 
       await ref.read(RoomsController.provider.notifier).addState(roomId, state);
+      room = ref.read(RoomsController.provider)[roomId] ?? room;
     }
 
-    // While there are under 20 events, try to load more
-    // until there's no more or the conditions are met.
+    // Load one more page when the initial timeline is short.
     if (room.hasMore && room.timeline.length < 20) {
-      loadOlder();
+      await loadOlder();
+      room = ref.read(RoomsController.provider)[roomId] ?? room;
     }
 
     return room.timeline
@@ -74,18 +77,22 @@ class RoomChatController extends AsyncNotifier<IList<Event>?> {
         ),
       );
 
-  Future<bool> loadOlder() async {
-    final timelineKeys = ref
-        .read(RoomsController.provider.select((value) => value[roomId]))
-        ?.timeline
-        .keys;
+  Future<bool> loadOlder() => _loadingOlder ??= _loadOlder().whenComplete(() {
+    _loadingOlder = null;
+  });
+
+  Future<bool> _loadOlder() async {
+    final room = ref.read(RoomsController.provider)[roomId];
+    if (room == null || !room.hasMore) return false;
+
+    final timelineKeys = room.timeline.keys;
     final response = await ref
         .watch(ClientController.provider.notifier)
         .paginate(
           .new(
             roomId: roomId,
-            maxTimelineId: timelineKeys?.isNotEmpty == true
-                ? timelineKeys?.reduce(min)
+            maxTimelineId: timelineKeys.isNotEmpty
+                ? timelineKeys.reduce(min)
                 : null,
           ),
         );

lib/widgets/room_chat.dart

@@ -72,23 +72,40 @@ class RoomChat extends HookConsumerWidget {
 
     final controllerProvider = RoomChatController.provider(roomId);
     final notifier = ref.watch(controllerProvider.notifier);
-
     final client = ref.watch(ClientController.provider.notifier);
 
-    final listController = useRef(ListController());
     final scrollController = useScrollController();
     final controllerData = ref.watch(controllerProvider);
-
-    final topEventBeforeLoad = useState<String?>(null);
+    final historyListController = useRef(ListController());
+    final liveListController = useRef(ListController());
+    final timelineCenterKey = useMemoized(GlobalKey.new);
+    final timelineBoundary = useRef<String?>(null);
+    final hasMore = ref.watch(
+      RoomsController.provider.select(
+        (rooms) => rooms[roomId]?.hasMore ?? false,
+      ),
+    );
 
     Future<void> jumpToId(String eventId) async {
-      final index = controllerData.value?.indexWhere(
-        (element) => element.eventId == eventId,
-      );
-      if (index == null) return;
+      final events = controllerData.value;
+      if (events == null || !scrollController.hasClients) return;
 
-      listController.value.animateToItem(
-        index: index,
+      final sourceIndex = events.indexWhere(
+        (event) => event.eventId == eventId,
+      );
+      final boundaryIndex = events.indexWhere(
+        (event) => event.eventId == timelineBoundary.value,
+      );
+      if (sourceIndex < 0 || boundaryIndex < 0) return;
+
+      final listController = sourceIndex <= boundaryIndex
+          ? historyListController.value
+          : liveListController.value;
+      if (!listController.isAttached) return;
+      listController.animateToItem(
+        index: sourceIndex <= boundaryIndex
+            ? boundaryIndex - sourceIndex
+            : sourceIndex - boundaryIndex - 1,
         scrollController: scrollController,
         alignment: 0.5,
         duration: (_) => .new(milliseconds: 700),
@@ -102,88 +119,6 @@ class RoomChat extends HookConsumerWidget {
       });
     }
 
-    Future<void> loadOlder() async {
-      if (controllerData case AsyncData(:final value?)) {
-        topEventBeforeLoad.value = value.firstOrNull?.eventId;
-        await notifier.loadOlder();
-      }
-    }
-
-    useEffect(() {
-      ref
-          .read(controllerProvider.future)
-          .then(
-            (_) => WidgetsBinding.instance.addPostFrameCallback((_) {
-              if (scrollController.hasClients) {
-                scrollController.jumpTo(
-                  scrollController.position.maxScrollExtent - .000001,
-                );
-              }
-            }),
-          );
-
-      return null;
-    }, [scrollController.hasClients]);
-
-    useEffect(() {
-      if (controllerData case AsyncData(
-        :final value?,
-      ) when scrollController.hasClients) {
-        if (topEventBeforeLoad.value != null) {
-          WidgetsBinding.instance.addPostFrameCallback((_) {
-            if (scrollController.hasClients) {
-              final index = value.indexWhere(
-                (event) => event.eventId == topEventBeforeLoad.value,
-              );
-              if (index != -1) {
-                listController.value.jumpToItem(
-                  index: index,
-                  scrollController: scrollController,
-                  alignment: 0,
-                );
-              }
-            }
-            topEventBeforeLoad.value = null;
-          });
-        } else if (scrollController.position.atEdge &&
-            scrollController.position.pixels != 0) {
-          WidgetsBinding.instance.addPostFrameCallback((_) {
-            if (scrollController.hasClients) {
-              scrollController.jumpTo(
-                scrollController.position.maxScrollExtent,
-              );
-            }
-          });
-        }
-      }
-
-      return null;
-    }, [controllerData]);
-
-    useEffect(() {
-      Future<void> listener() async {
-        if (!scrollController.hasClients || !scrollController.position.atEdge) {
-          return;
-        }
-
-        final room = ref.watch(
-          RoomsController.provider.select((value) => value[roomId]),
-        );
-        if (room == null) return;
-
-        if (scrollController.position.pixels == 0) {
-          if (room.hasMore) {
-            await loadOlder();
-          }
-        } else {
-          await client.markRead(room);
-        }
-      }
-
-      scrollController.addListener(listener);
-      return () => scrollController.removeListener(listener);
-    }, [roomId, controllerData]);
-
     final composerNode = useFocusNode(
       onKeyEvent: (_, event) {
         if (event is KeyDownEvent && event.logicalKey == .escape) {
@@ -465,38 +400,56 @@ class RoomChat extends HookConsumerWidget {
                         padding: .symmetric(horizontal: 4),
                         child: switch (controllerData) {
                           AsyncData(:final value?) ||
-                          AsyncLoading(:final value?) => CustomScrollView(
-                            keyboardDismissBehavior:
-                                ScrollViewKeyboardDismissBehavior.onDrag,
-                            controller: scrollController,
-                            slivers: [
-                              SliverToBoxAdapter(
-                                child: Padding(
-                                  padding: .symmetric(vertical: 36),
-                                  child: Center(
-                                    child: ElevatedButton(
-                                      onPressed: controllerData is AsyncData
-                                          ? loadOlder
-                                          : null,
-                                      child: Text("Load More"),
-                                    ),
-                                  ),
-                                ),
-                              ),
-
-                              SuperSliverList.builder(
-                                listController: listController.value,
-                                itemCount: value.length,
-                                itemBuilder: (_, index) {
-                                  final event = value[index];
-                                  final previousEvent = value.getOrNull(
-                                    index - 1,
+                          AsyncLoading(:final value?) => Builder(
+                            builder: (_) {
+                              if (value.isEmpty) return nothing;
+                              var boundaryIndex = value.indexWhere(
+                                (event) =>
+                                    event.eventId == timelineBoundary.value,
                               );
-                                  return HighlightWrapper(
+                              if (boundaryIndex < 0) {
+                                boundaryIndex = value.length - 1;
+                                timelineBoundary.value =
+                                    value[boundaryIndex].eventId;
+                              }
+
+                              final historyLength = boundaryIndex + 1;
+                              final liveLength = value.length - historyLength;
+
+                              int? eventIndexForKey(Key key) {
+                                if (key is! ValueKey<String>) return null;
+                                final index = value.indexWhere(
+                                  (event) => event.eventId == key.value,
+                                );
+                                return index < 0 ? null : index;
+                              }
+
+                              int? historyChildIndex(Key key) {
+                                if (key == const ValueKey("history-loader")) {
+                                  return historyLength;
+                                }
+                                final eventIndex = eventIndexForKey(key);
+                                return eventIndex == null ||
+                                        eventIndex >= historyLength
+                                    ? null
+                                    : historyLength - eventIndex - 1;
+                              }
+
+                              int? liveChildIndex(Key key) {
+                                final eventIndex = eventIndexForKey(key);
+                                return eventIndex == null ||
+                                        eventIndex < historyLength
+                                    ? null
+                                    : eventIndex - historyLength;
+                              }
+
+                              Widget eventRow(
+                                Event event,
+                                Event? previousEvent,
+                              ) => HighlightWrapper(
                                 EventRenderer(
                                   event,
-                                      onTapReply: () =>
-                                          jumpToId(event.replyTo!),
+                                  onTapReply: () => jumpToId(event.replyTo!),
                                   getEventOptions: getEventOptions,
                                   isGrouped:
                                       previousEvent?.content
@@ -507,16 +460,60 @@ class RoomChat extends HookConsumerWidget {
                                       "${event.sender}${event.pmp?.id}" ==
                                           "${previousEvent?.sender}${previousEvent?.pmp?.id}",
                                 ),
+                                key: ValueKey(event.eventId),
                                 isHighlighted:
                                     highlightedEvent.value == event.eventId,
                               );
+
+                              return CustomScrollView(
+                                reverse: true,
+                                center: timelineCenterKey,
+                                keyboardDismissBehavior:
+                                    ScrollViewKeyboardDismissBehavior.onDrag,
+                                controller: scrollController,
+                                slivers: [
+                                  SliverToBoxAdapter(
+                                    child: SizedBox(height: composerSize.value),
+                                  ),
+                                  SuperSliverList.builder(
+                                    listController: liveListController.value,
+                                    itemCount: liveLength,
+                                    findChildIndexCallback: liveChildIndex,
+                                    itemBuilder: (_, index) => eventRow(
+                                      value[historyLength + index],
+                                      value.getOrNull(
+                                        historyLength + index - 1,
+                                      ),
+                                    ),
+                                  ),
+                                  SuperSliverList.builder(
+                                    key: timelineCenterKey,
+                                    listController: historyListController.value,
+                                    itemCount: historyLength + 1,
+                                    findChildIndexCallback: historyChildIndex,
+                                    itemBuilder: (_, index) {
+                                      if (index == historyLength) {
+                                        return _HistoryLoader(
+                                          key: const ValueKey("history-loader"),
+                                          hasMore: hasMore,
+                                          isLoading:
+                                              controllerData is AsyncLoading,
+                                          onLoad: notifier.loadOlder,
+                                        );
+                                      }
+                                      final event =
+                                          value[historyLength - index - 1];
+                                      return eventRow(
+                                        event,
+                                        value.getOrNull(
+                                          historyLength - index - 2,
+                                        ),
+                                      );
                                     },
                                   ),
-
-                              SliverPadding(
-                                padding: .only(bottom: composerSize.value),
-                              ),
                                 ],
+                              );
+                            },
                           ),
                           AsyncData() => nothing,
                           AsyncLoading() => Loading(),
@@ -564,3 +561,42 @@ class RoomChat extends HookConsumerWidget {
     );
   }
 }
+
+class _HistoryLoader extends HookWidget {
+  final bool hasMore;
+  final bool isLoading;
+  final Future<void> Function() onLoad;
+
+  const _HistoryLoader({
+    required this.hasMore,
+    required this.isLoading,
+    required this.onLoad,
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    useEffect(() {
+      if (hasMore) {
+        Future.microtask(onLoad);
+      }
+      return null;
+    }, const []);
+
+    return Padding(
+      padding: .symmetric(vertical: 36),
+      child: Center(
+        child: ElevatedButton(
+          onPressed: hasMore && !isLoading ? onLoad : null,
+          child: isLoading
+              ? SizedBox(
+                  height: 20,
+                  width: 20,
+                  child: CircularProgressIndicator(strokeWidth: 2),
+                )
+              : Text(hasMore ? "Load More" : "Start of timeline"),
+        ),
+      ),
+    );
+  }
+}