fix: reset live timeline on room navigation and reconnect

The Matrix SDK for sliding sync deliberately does not call resetLiveTimeline()
on limited:true (the handling is in a commented-out TODO block). This means
events from a previous visit remain in the live timeline and new events get
appended after them, with the backward pagination token pointing to the gap
between old and new events rather than to before all events. The result is
that new messages appear out of order or the gap is unreachable via the UI.

Two fixes:

1. useSlidingSyncActiveRoom: reset the live timeline synchronously before
   the subscription request is sent on room navigation. This ensures the
   fresh initial:true response populates a clean timeline, not one polluted
   with events from the previous visit. Also removes the 100ms delay that
   was added as an incomplete workaround for the same issue.

2. SlidingSyncManager.onLifecycle: in RequestFinished state (fires before
   any room data listeners run), reset the live timeline for active-room
   subscriptions that receive initial:true or limited:true. This covers the
   reconnect/background case where the pos token expires and all active rooms
   get a fresh initial:true in the same sync cycle.

Author: Just-Insane

src/app/hooks/useSlidingSyncActiveRoom.ts

@@ -24,16 +24,7 @@ export const useSlidingSyncActiveRoom = (): void => {
     const manager = getSlidingSyncManager(mx);
     if (!manager) return undefined;
 
-    // Wait for the room to be initialized from list sync before subscribing
-    // with the full timeline limit. This prevents timeline ordering issues where
-    // the room might be receiving events from list expansion while we're also
-    // trying to load a large timeline, causing events to be added out of order.
-    const timeoutId = setTimeout(() => {
-      manager.subscribeToRoom(roomId);
-    }, 100);
-
-    return () => {
-      clearTimeout(timeoutId);
-    };
+    manager.subscribeToRoom(roomId);
+    return undefined;
   }, [mx, roomId]);
 };

src/client/slidingSync.ts

@@ -8,6 +8,7 @@ import {
   MSC3575List,
   MSC3575RoomData,
   MSC3575RoomSubscription,
+  MSC3575SlidingSyncResponse,
   MSC3575_WILDCARD,
   RoomMemberEvent,
   SlidingSync,
@@ -350,6 +351,27 @@ export class SlidingSyncManager {
         return;
       }
 
+      // Before room data is processed, reset live timelines for active rooms that
+      // are receiving a full refresh (initial: true) or a post-gap update
+      // (limited: true). The SDK deliberately does not call resetLiveTimeline() for
+      // sliding sync, so events from previous visits accumulate in the live
+      // timeline alongside new events. Resetting here — before the SDK's
+      // onRoomData listener runs — ensures the fresh batch lands on a clean
+      // timeline with a correct backward pagination token.
+      if (state === SlidingSyncState.RequestFinished && resp && !err) {
+        const rooms = (resp as MSC3575SlidingSyncResponse).rooms ?? {};
+        Object.entries(rooms)
+          .filter(([, roomData]) => roomData.initial || roomData.limited)
+          .filter(([roomId]) => this.activeRoomSubscriptions.has(roomId))
+          .forEach(([roomId]) => {
+            const room = this.mx.getRoom(roomId);
+            if (!room) return;
+            const timelineSet = room.getUnfilteredTimelineSet();
+            if (timelineSet.getLiveTimeline().getEvents().length === 0) return;
+            timelineSet.resetLiveTimeline();
+          });
+      }
+
       if (err || !resp || state !== SlidingSyncState.Complete) return;
 
       // Track what changed in this sync cycle