diff --git a/src/app/hooks/useRoomsListener.ts b/src/app/hooks/useRoomsListener.ts
index 33bb80f7c..2f413ac80 100644
--- a/src/app/hooks/useRoomsListener.ts
+++ b/src/app/hooks/useRoomsListener.ts
@@ -16,16 +16,18 @@ import {
  * need to memoize it — changing the handler identity never re-attaches the
  * per-room listeners.
  *
- * The emitting {@link Room} is appended as a FINAL extra argument after the
+ * The emitting {@link Room} is PREPENDED as the first argument, before the
  * event's own args: several room-level SDK events (e.g.
  * `RoomEvent.UnreadNotifications`) don't include the room in their payload,
- * which callers need for per-room updates. Handlers that don't care can simply
- * ignore it.
+ * which callers need for per-room updates. Prepending (not appending) is
+ * load-bearing — some SDK events emit with VARIABLE arity
+ * (UnreadNotifications fires with 0, 1, or 2 args), so a trailing extra arg
+ * would land in a different positional slot per emit.
  */
 export function useRoomsListener<E extends RoomEmittedEvents>(
   mx: MatrixClient,
   event: E,
-  handler: (...args: [...Parameters<RoomEventHandlerMap[E]>, Room]) => void,
+  handler: (room: Room, ...args: Parameters<RoomEventHandlerMap[E]>) => void,
 ): void {
   const handlerRef = useRef(handler);
   handlerRef.current = handler;
@@ -39,9 +41,9 @@ export function useRoomsListener<E extends RoomEmittedEvents>(
     const attach = (room: Room) => {
       if (attached.has(room.roomId)) return;
       // Per-room trampoline: forwards to the current ref value with the
-      // emitting room appended.
+      // emitting room PREPENDED (stable slot regardless of emit arity).
       const roomHandler = (...args: unknown[]) =>
-        (handlerRef.current as (...a: unknown[]) => void)(...args, room);
+        (handlerRef.current as (...a: unknown[]) => void)(room, ...args);
       attached.set(room.roomId, roomHandler);
       // `event`/`roomHandler` are correlated through E but TS can't prove it
       // for the open generic, so we assert at the boundary.
diff --git a/src/app/pages/client/ClientNonUIFeatures.tsx b/src/app/pages/client/ClientNonUIFeatures.tsx
index a86cfe66d..750e9d20a 100644
--- a/src/app/pages/client/ClientNonUIFeatures.tsx
+++ b/src/app/pages/client/ClientNonUIFeatures.tsx
@@ -6,6 +6,7 @@ import {
   Room,
   RoomEvent,
   RoomEventHandlerMap,
+  Thread,
   ThreadEvent,
 } from 'matrix-js-sdk';
 import { focusAssistActiveAtom } from '../../state/focusAssist';
@@ -446,8 +447,10 @@ function MessageNotifications() {
     };
   }, [mx, notificationSelected, selectedRoomId, deliverNotification]);
 
-  const handleNewReply = useCallback<RoomEventHandlerMap[ThreadEvent.NewReply]>(
-    (thread, mEvent) => {
+  const handleNewReply = useCallback(
+    // useRoomsListener prepends the emitting Room; the thread's own room lookup
+    // below is kept as the authority (identical object in practice).
+    (_room: Room, thread: Thread, mEvent: MatrixEvent) => {
       if (mx.getSyncState() !== 'SYNCING') return;
       const room = mx.getRoom(thread.roomId);
       if (!room || room.isSpaceRoom()) return;
@@ -457,8 +460,7 @@ function MessageNotifications() {
       // Suppress when the user is actively looking at this thread (or the inbox).
       if (
         document.hasFocus() &&
-        (notificationSelected ||
-          (selectedRoomId === thread.roomId && activeThreadId === thread.id))
+        (notificationSelected || (selectedRoomId === thread.roomId && activeThreadId === thread.id))
       ) {
         return;
       }
diff --git a/src/app/state/room/roomToUnread.ts b/src/app/state/room/roomToUnread.ts
index c7cf0edd1..4ab31f95a 100644
--- a/src/app/state/room/roomToUnread.ts
+++ b/src/app/state/room/roomToUnread.ts
@@ -4,7 +4,6 @@ import {
   IRoomTimelineData,
   MatrixClient,
   MatrixEvent,
-  NotificationCount,
   Room,
   RoomEvent,
   SyncState,
@@ -282,18 +281,15 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
 
   // RoomEvent.UnreadNotifications is emitted room-level only (never re-emitted
   // client-side), so the main Timeline pathway misses thread-count changes and
-  // room badges lag. useRoomsListener appends the emitting Room as the final
-  // arg, making this a surgical per-room PUT (not a full RESET per emit) with
-  // muted-thread subtraction re-applied. Room-mute keeps its DELETE semantics.
+  // room badges lag. useRoomsListener PREPENDS the emitting Room (the SDK emits
+  // this event with variable arity — 0/1/2 args — so only a leading slot is
+  // positionally stable), making this a surgical per-room PUT with muted-thread
+  // subtraction re-applied. Room-mute keeps its DELETE semantics.
   useRoomsListener(
     mx,
     RoomEvent.UnreadNotifications,
     useCallback(
-      (
-        _unreadNotifications: NotificationCount | undefined,
-        _threadId: string | undefined,
-        room: Room,
-      ) => {
+      (room: Room) => {
         if (room.isSpaceRoom()) return;
         if (getNotificationType(mx, room.roomId) === NotificationType.Mute) {
           setUnreadAtom({ type: 'DELETE', roomId: room.roomId });
@@ -333,7 +329,7 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
         if (mEvent.getType() === StateEvent.SpaceChild) {
           setUnreadAtom({
             type: 'RESET',
-            unreadInfos: getUnreadInfos(mx),
+            unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
           });
         }
       },