fix(sliding-sync): reset live timeline on reconnect to prevent event ordering corruption

When the sliding sync pos token expires (app backgrounded) and the server
re-sends initial:true for an already-open room, the SDK's processRoomData
treats any post-gap 'stranded' event as a known anchor and prepends gap-fill
events ahead of earlier history, producing out-of-order timelines
(Today -> Yesterday -> Today).

Register onInitialRoomData in the constructor and call slidingSync.on() in
attach() before mx.startClient() (i.e. before SlidingSyncSdk.onRoomData is
registered). When initial:true arrives for a room that already has events,
reset the live timeline so _eventIdToTimeline is cleared, all received events
go through the newEvents path, and chronological order is preserved.

Author: Just-Insane

src/client/slidingSync.ts

@@ -277,6 +277,13 @@ export class SlidingSyncManager {
     (roomId: string, data: MSC3575RoomData) => void
   >();
 
+  /**
+   * Listener registered in attach() that resets the live timeline before the SDK
+   * processes initial:true room data. Must fire before SlidingSyncSdk.onRoomData,
+   * which is registered in mx.startClient() (called after attach()).
+   */
+  private readonly onInitialRoomData: (roomId: string, roomData: MSC3575RoomData) => void;
+
   /** Wall-clock time recorded in attach() — used to compute true initial-sync latency. */
   private attachTime: number | null = null;
 
@@ -425,6 +432,17 @@ export class SlidingSyncManager {
       }
     };
 
+    // Reset the live timeline when the server re-sends initial:true (reconnect after
+    // pos token expiry). Must be assigned in the constructor so the reference is stable
+    // for removeListener(); registered in attach() before mx.startClient() so it fires
+    // ahead of SlidingSyncSdk.onRoomData.
+    this.onInitialRoomData = (roomId: string, roomData: MSC3575RoomData) => {
+      if (!roomData.initial) return;
+      const room = this.mx.getRoom(roomId);
+      if (!room || room.getLiveTimeline().getEvents().length === 0) return;
+      room.getUnfilteredTimelineSet().resetLiveTimeline();
+    };
+
     this.onMembershipLeave = (_event, member) => {
       if (member.userId !== this.mx.getUserId()) return;
       if (member.membership !== KnownMembership.Leave && member.membership !== KnownMembership.Ban)
@@ -474,6 +492,10 @@ export class SlidingSyncManager {
       attributes: { 'sync.transport': 'sliding', 'sync.proxy': this.proxyBaseUrl },
     });
 
+    // Register BEFORE mx.startClient() so this fires ahead of SlidingSyncSdk.onRoomData
+    // (see constructor comment on onInitialRoomData for the full explanation).
+    this.slidingSync.on(SlidingSyncEvent.RoomData, this.onInitialRoomData);
+
     this.slidingSync.on(SlidingSyncEvent.Lifecycle, this.onLifecycle);
     this.mx.on(RoomMemberEvent.Membership, this.onMembershipLeave);
     const connection = (
@@ -513,6 +535,7 @@ export class SlidingSyncManager {
     this.disposed = true;
     // Stop the SDK's internal polling loop and abort any in-flight requests.
     this.slidingSync.stop();
+    this.slidingSync.removeListener(SlidingSyncEvent.RoomData, this.onInitialRoomData);
     this.slidingSync.removeListener(SlidingSyncEvent.Lifecycle, this.onLifecycle);
     this.mx.removeListener(RoomMemberEvent.Membership, this.onMembershipLeave);
     const connection = (