clean up approach to avoid polling

lib/helpers/hooks/chat_scroll.dart

@@ -1,3 +1,5 @@
+import "dart:async";
+
 import "package:fast_immutable_collections/fast_immutable_collections.dart";
 import "package:flutter_hooks/flutter_hooks.dart";
 import "package:hooks_riverpod/hooks_riverpod.dart";
@@ -10,6 +12,7 @@ final class ChatScroll({
   required final IList<Event> historyItems,
   required final IList<Event> liveItems,
   required final GlobalKey centerKey,
+  required final GlobalKey anchorItemKey,
   required final ListController historyListController,
   required final ListController liveListController,
   required final ScrollController scrollController,
@@ -19,30 +22,85 @@ final class ChatScroll({
     required AsyncValue<RoomChat?> controllerData,
     required Future<void> Function() loadOlder,
     required Future<void> Function() markRead,
+    required String? contextualEvent,
+    required void Function(String id) setContextualEvent,
   }) {
-    final historyListController = useRef(ListController());
-    final liveListController = useRef(ListController());
+    final anchorId = useState<String?>(null);
+
+    final anchorItemKey = useMemoized(GlobalKey.new, [anchorId.value]);
+
+    final historyListController = useMemoized(ListController.new, [
+      anchorId.value,
+    ]);
+    final liveListController = useMemoized(ListController.new, [
+      anchorId.value,
+    ]);
     final scrollController = useScrollController();
     final centerKey = useMemoized(GlobalKey.new);
 
-    final anchorId = useState<String?>(null);
-
-    final anchorIdValue = anchorId.value;
+    final pendingAnchorTarget = useState<String?>(null);
+    final anchorMountedCompleter = useRef<Completer<BuildContext>?>(null);
 
     useEffect(() {
       if (anchorId.value == null) {
         if (controllerData case AsyncData(:final value?)
             when value.timeline.isNotEmpty) {
-          anchorId.value = value.timeline.last.eventId;
+          final hasContextualEvent = value.timeline.any(
+            (event) => event.eventId == contextualEvent,
+          );
+
+          anchorId.value = hasContextualEvent
+              ? contextualEvent
+              : value.timeline.last.eventId;
         }
       }
 
       return null;
-    }, [controllerData]);
+    }, [controllerData, contextualEvent]);
+
+    useEffect(() {
+      final target = pendingAnchorTarget.value;
+      if (target == null) return null;
+
+      final found =
+          controllerData.value?.timeline.any(
+            (event) => event.eventId == target,
+          ) ??
+          false;
+
+      if (found || controllerData is AsyncError) {
+        if (found) {
+          anchorId.value = target;
+        } else {
+          anchorMountedCompleter.value?.completeError(
+            StateError("Failed to load context for $target"),
+          );
+          anchorMountedCompleter.value = null;
+        }
+        pendingAnchorTarget.value = null;
+      }
+
+      return null;
+    }, [controllerData, pendingAnchorTarget.value]);
+
+    useEffect(() {
+      final completer = anchorMountedCompleter.value;
+      if (completer == null) return null;
+
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        final context = anchorItemKey.currentContext;
+        if (context != null && context.mounted) {
+          anchorMountedCompleter.value?.complete(context);
+          anchorMountedCompleter.value = null;
+        }
+      });
+
+      return null;
+    }, [anchorId.value]);
 
     final ({IList<Event> history, IList<Event> live}) split = useMemoized(() {
       final items = controllerData.value?.timeline;
-      final anchor = anchorIdValue;
+      final anchor = anchorId.value;
 
       if (items == null || anchor == null) {
         return (history: const .empty(), live: const .empty());
@@ -58,62 +116,58 @@ final class ChatScroll({
         history: items.take(anchorIndex).toIList().reversed.toIList(),
         live: items.skip(anchorIndex).toIList(),
       );
-    }, [controllerData, anchorIdValue]);
+    }, [controllerData, anchorId.value]);
 
-    useEffect(() {
-      const topThreshold = 500.0;
-      const bottomThreshold = 50.0;
+    useEffect(
+      () {
+        const topThreshold = 500.0;
+        const bottomThreshold = 50.0;
 
-      Future<void> checkPosition() async {
-        if (!scrollController.hasClients) return;
+        Future<void> checkPosition() async {
+          if (!scrollController.hasClients) return;
+          if (contextualEvent != null) return;
 
-        final position = scrollController.position;
+          final position = scrollController.position;
 
-        if (position.extentAfter <= topThreshold) {
-          await loadOlder();
-        } else if (position.extentBefore <= bottomThreshold) {
-          await markRead();
+          if (position.extentAfter <= topThreshold) {
+            await loadOlder();
+          } else if (position.extentBefore <= bottomThreshold) {
+            await markRead();
+          }
         }
+
+        scrollController.addListener(checkPosition);
+
+        WidgetsBinding.instance.addPostFrameCallback((_) => checkPosition());
+
+        return () => scrollController.removeListener(checkPosition);
+      },
+      [scrollController, controllerData, loadOlder, markRead, contextualEvent],
+    );
+
+    double? resolveOffset(String itemId) {
+      final historyIndex = split.history.indexWhere(
+        (item) => item.eventId == itemId,
+      );
+      if (historyIndex != -1) {
+        // TODO: Replace SuperSliverView because of the bug that requires this: #94
+        // ignore: invalid_use_of_visible_for_testing_member
+        return historyListController.getOffsetToReveal(historyIndex, 0.5);
       }
 
-      scrollController.addListener(checkPosition);
+      final liveIndex = split.live.indexWhere((item) => item.eventId == itemId);
+      if (liveIndex != -1) {
+        // ignore: invalid_use_of_visible_for_testing_member
+        return liveListController.getOffsetToReveal(liveIndex, 0.5);
+      }
 
-      WidgetsBinding.instance.addPostFrameCallback((_) => checkPosition());
-
-      return () {
-        scrollController.removeListener(checkPosition);
-      };
-    }, [scrollController, controllerData, loadOlder, markRead]);
+      return null;
+    }
 
     Future<void> jumpToId(String itemId) async {
       if (!scrollController.hasClients) return;
 
-      final historyIndex = split.history.indexWhere(
-        (item) => item.eventId == itemId,
-      );
-
-      double? offset;
-      if (historyIndex != -1) {
-        // TODO: Replace SuperSliverView because of the bug that requires this: #94
-        // ignore: invalid_use_of_visible_for_testing_member
-        offset = historyListController.value.getOffsetToReveal(
-          historyIndex,
-          0.5,
-        );
-      } else {
-        final liveIndex = split.live.indexWhere(
-          (item) => item.eventId == itemId,
-        );
-
-        if (liveIndex != -1) {
-          // ignore: invalid_use_of_visible_for_testing_member
-          offset = liveListController.value.getOffsetToReveal(liveIndex, 0.5);
-        }
-      }
-
-      if (offset == null) {
-        // not in current timeline
-      }
+      final offset = resolveOffset(itemId);
 
       if (offset != null) {
         await scrollController.animateTo(
@@ -121,6 +175,23 @@ final class ChatScroll({
           duration: const Duration(milliseconds: 700),
           curve: Curves.easeInOut,
         );
+        return;
+      }
+
+      final completer = Completer<BuildContext>();
+      anchorMountedCompleter.value = completer;
+      pendingAnchorTarget.value = itemId;
+      setContextualEvent(itemId);
+
+      final context = await completer.future;
+
+      if (context.mounted) {
+        await Scrollable.ensureVisible(
+          context,
+          alignment: 0.5,
+          duration: const Duration(milliseconds: 700),
+          curve: Curves.easeInOut,
+        );
       }
     }
 
@@ -128,8 +199,9 @@ final class ChatScroll({
       historyItems: split.history,
       liveItems: split.live,
       centerKey: centerKey,
-      historyListController: historyListController.value,
-      liveListController: liveListController.value,
+      anchorItemKey: anchorItemKey,
+      historyListController: historyListController,
+      liveListController: liveListController,
       scrollController: scrollController,
       jumpToId: jumpToId,
     );

lib/widgets/room_chat/chat_timeline.dart

@@ -28,6 +28,7 @@ class const ChatTimeline({
     required Future<void> Function(String) jumpToId,
     required IList<PopupMenuEntry> Function(Event) getEventOptions,
     required String? highlightedEvent,
+    required Key key,
   }) => HighlightWrapper(
     EventRenderer(
       event,
@@ -35,7 +36,7 @@ class const ChatTimeline({
       getEventOptions: getEventOptions,
       isGrouped: isGrouped(event, previousEvent),
     ),
-    key: ValueKey(event.eventId),
+    key: key,
     isHighlighted: highlightedEvent == event.eventId,
   );
 
@@ -59,6 +60,9 @@ class const ChatTimeline({
           jumpToId: jumpToId,
           getEventOptions: getEventOptions,
           highlightedEvent: highlightedEvent,
+          key: index == 0
+              ? scroll.anchorItemKey
+              : ValueKey(scroll.liveItems[index].eventId),
         ),
       ),
 
@@ -72,6 +76,7 @@ class const ChatTimeline({
           jumpToId: jumpToId,
           getEventOptions: getEventOptions,
           highlightedEvent: highlightedEvent,
+          key: ValueKey(scroll.historyItems[index].eventId),
         ),
       ),
     ],

lib/widgets/room_chat/room_chat.dart

@@ -74,6 +74,8 @@ final class const RoomChat({
     final scroll = ChatScroll.use(
       controllerData: controllerData,
       loadOlder: notifier.loadOlder,
+      contextualEvent: contextualEvent.value,
+      setContextualEvent: (id) => contextualEvent.value = id,
       markRead: () async {
         final room = ref.read(
           RoomsController.provider.select((rooms) => rooms[roomId]),