fix(timeline): reset live timeline on room re-navigation to prevent stale linked chain

When the user navigates away from a room, the list subscription may deliver a
limited:true response that appends a gap to the existing paginated chain
(T0\u2192T1\u2192T2\u2192live). On re-navigation the server does not resend initial:true
(room was already delivered via list), so onInitialRoomData never fired and the
stale chain was rendered in useTimelineSync, causing events to appear out of
order or anchored at the wrong position.

Fix: subscribeToRoom() adds the room to a pendingTimelineResets Set when it has
an existing timeline. onInitialRoomData (registered before SlidingSyncSdk in
attach(), so it fires first) checks the set and calls resetLiveTimeline() before
the active-room snapshot events are processed. Both operations land in the same
React render cycle, so there is no blank-screen window between the reset and the
fresh 50-event pa
When the user  cd /Users/evie/git/Sable && cat > /tmp/commit_msg.txt << 'COMMIT_EOF'
fix(timeline): reset live timeline on room re-navigation to prevent stale linked chain

When the user navigates away from a room, the list subscription may deliver a
limited:true response that appends a gap to the existing paginated chain
(T0->T1->T2->live). On re-navigation the server does not resend initial:true
(room was already delivered via list), so onInitialRoomData never fired and the
stale chain was rendered in useTimelineSync, causing events to appear out of
order or anchored at the wrong position.

Fix: subscribeToRoom() adds the room to a pendingTimelineResets Set when it has
an existing timeline. onInitialRoomData (registered before SlidingSyncSdk in
attach(), so it fires first) checks the set and calls resetLiveTimeline() before
the active-room snapshot events are processed. Both operations land in the same
React render cycle, so there is no blank-screen window between the reset and the
fresh 50-event page appearing.
COMMIT_Efix(timeline): reset live timeline on room re-navigation to prevent s.t
Wh q'
 git status

Author: Just-Insane

src/client/slidingSync.ts

@@ -244,6 +244,14 @@ export class SlidingSyncManager {
 
   private readonly activeRoomSubscriptions = new Set<string>();
 
+  /**
+   * Rooms that subscribeToRoom() has marked as needing a live-timeline reset
+   * on the next RoomData event. Consumed by onInitialRoomData before the SDK
+   * processes the incoming events, so the reset and new events land in the
+   * same React render (no blank-screen window).
+   */
+  private readonly pendingTimelineResets = new Set<string>();
+
   private readonly listPageSize: number;
 
   private readonly roomTimelineLimit: number;
@@ -432,15 +440,39 @@ 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.
+    // Fires before SlidingSyncSdk.onRoomData (registered in attach() ahead of
+    // mx.startClient()) so any timeline resets happen before events are added.
+    // Must be assigned in the constructor so the reference is stable for removeListener().
+    //
+    // Handles two cases:
+    //   1. Reconnect: server re-sends initial:true for all rooms after pos token expiry.
+    //      The live timeline may have since drifted; reset it so the fresh snapshot lands
+    //      cleanly without a stale linked chain.
+    //   2. Room navigation: subscribeToRoom() adds the room to pendingTimelineResets when
+    //      the user actively navigates there. The server may not resend initial:true if the
+    //      room was already delivered via list subscription, leaving old paginated timelines
+    //      (T0→T1→T2→live) with a limited:true gap. This handler fires before the 50-event
+    //      active-room snapshot is processed, so the reset and new events land in the same
+    //      React render cycle with no blank-screen window.
     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();
+      if (!room) return;
+
+      // Case 1: reconnect — server re-delivers initial:true for all known rooms.
+      if (roomData.initial) {
+        if (room.getLiveTimeline().getEvents().length > 0) {
+          room.getUnfilteredTimelineSet().resetLiveTimeline();
+        }
+        return;
+      }
+
+      // Case 2: room navigation — subscribeToRoom() marked this room for reset.
+      if (this.pendingTimelineResets.has(roomId)) {
+        this.pendingTimelineResets.delete(roomId);
+        if (room.getLiveTimeline().getEvents().length > 0) {
+          room.getUnfilteredTimelineSet().resetLiveTimeline();
+        }
+      }
     };
 
     this.onMembershipLeave = (_event, member) => {
@@ -890,6 +922,14 @@ export class SlidingSyncManager {
       // encrypted default.
       this.slidingSync.useCustomSubscription(roomId, UNENCRYPTED_SUBSCRIPTION_KEY);
     }
+    // Mark the room for a live-timeline reset on the next RoomData event. The
+    // reset is deferred (handled by onInitialRoomData which fires before the
+    // SDK processes events) so it happens in the same sync cycle as the new
+    // events, avoiding a blank-screen window. See onInitialRoomData for the
+    // full explanation of why a reset is required here.
+    if (room && room.getLiveTimeline().getEvents().length > 0) {
+      this.pendingTimelineResets.add(roomId);
+    }
     this.activeRoomSubscriptions.add(roomId);
     this.slidingSync.modifyRoomSubscriptions(new Set(this.activeRoomSubscriptions));
     Sentry.metrics.gauge('sable.sync.active_subscriptions', this.activeRoomSubscriptions.size, {
@@ -959,6 +999,8 @@ export class SlidingSyncManager {
       this.slidingSync.removeListener(SlidingSyncEvent.RoomData, pendingListener);
       this.pendingRoomDataListeners.delete(roomId);
     }
+    // Discard any deferred reset so it doesn't fire if the room is re-subscribed later.
+    this.pendingTimelineResets.delete(roomId);
     this.activeRoomSubscriptions.delete(roomId);
     this.slidingSync.modifyRoomSubscriptions(new Set(this.activeRoomSubscriptions));
     Sentry.metrics.gauge('sable.sync.active_subscriptions', this.activeRoomSubscriptions.size, {