fix: timeline anchoring

lib/controllers/room_chat.dart

@@ -37,8 +37,7 @@ class RoomChatController extends AsyncNotifier<IList<Event>?> {
       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) {
       await loadOlder();
       room = ref.read(RoomsController.provider)[roomId] ?? room;

lib/widgets/room_chat.dart

@@ -126,12 +126,6 @@ class RoomChat extends HookConsumerWidget {
       });
     }
 
-    Future<void> loadOlder() async {
-      if (controllerData case AsyncData(value: final _?)) {
-        await notifier.loadOlder();
-      }
-    }
-
     final composerNode = useFocusNode(
       onKeyEvent: (_, event) {
         if (event is KeyDownEvent && event.logicalKey == .escape) {
@@ -429,26 +423,29 @@ class RoomChat extends HookConsumerWidget {
                               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;
                                 }
-                                if (key is! ValueKey<String>) return null;
-                                final eventIndex = value.indexWhere(
-                                  (event) => event.eventId == key.value,
-                                );
-                                return eventIndex < 0 ||
+                                final eventIndex = eventIndexForKey(key);
+                                return eventIndex == null ||
                                         eventIndex >= historyLength
                                     ? null
                                     : historyLength - eventIndex - 1;
                               }
 
                               int? liveChildIndex(Key key) {
-                                if (key is! ValueKey<String>) return null;
-                                final eventIndex = value.indexWhere(
-                                  (event) => event.eventId == key.value,
-                                );
-                                return eventIndex < historyLength
+                                final eventIndex = eventIndexForKey(key);
+                                return eventIndex == null ||
+                                        eventIndex < historyLength
                                     ? null
                                     : eventIndex - historyLength;
                               }
@@ -508,7 +505,7 @@ class RoomChat extends HookConsumerWidget {
                                           hasMore: hasMore,
                                           isLoading:
                                               controllerData is AsyncLoading,
-                                          onLoad: loadOlder,
+                                          onLoad: notifier.loadOlder,
                                         );
                                       }
                                       final event =