From 6116c6411e2f84b7eab5146c68e8aa144f962b25 Mon Sep 17 00:00:00 2001
From: 7w1 <noreply@sl.sable.moe>
Date: Tue, 5 May 2026 13:34:45 -0500
Subject: [PATCH] fix sliding sync recursion by avoiding notif fixup reentry

---
 .../fix-sliding-sync-call-stack-crash.md      |  5 +++
 src/app/state/room/roomToUnread.ts            |  3 +-
 src/app/utils/room.ts                         | 12 +++++--
 src/client/initMatrix.ts                      | 33 ++++++++++++-------
 4 files changed, 38 insertions(+), 15 deletions(-)
 create mode 100644 .changeset/fix-sliding-sync-call-stack-crash.md

diff --git a/.changeset/fix-sliding-sync-call-stack-crash.md b/.changeset/fix-sliding-sync-call-stack-crash.md
new file mode 100644
index 000000000..2ca6fdc8d
--- /dev/null
+++ b/.changeset/fix-sliding-sync-call-stack-crash.md
@@ -0,0 +1,5 @@
+---
+default: patch
+---
+
+Fix the call stack size crash on load when sliding sync is enabled.
diff --git a/src/app/state/room/roomToUnread.ts b/src/app/state/room/roomToUnread.ts
index 208df289c..24ded9f80 100644
--- a/src/app/state/room/roomToUnread.ts
+++ b/src/app/state/room/roomToUnread.ts
@@ -312,7 +312,8 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
         if (room.getMyMembership() !== (KnownMembership.Join as string)) return;
 
         const unreadInfo = getUnreadInfo(room, {
-          applyFixup: shouldApplyUnreadFixup(),
+          // Counts are already updated before this event would recurse if true
+          applyFixup: false,
           mDirects,
         });
         if (unreadInfo.total === 0 && unreadInfo.highlight === 0) {
diff --git a/src/app/utils/room.ts b/src/app/utils/room.ts
index 2d68b5d86..aa7ea1487 100644
--- a/src/app/utils/room.ts
+++ b/src/app/utils/room.ts
@@ -287,11 +287,17 @@ type UnreadInfoOptions = {
   mDirects?: Set<string>;
 };
 
+const unreadInfoFixupInProgress = new WeakSet<Room>();
+
 export const getUnreadInfo = (room: Room, options?: UnreadInfoOptions): UnreadInfo => {
   const userId = room.client.getUserId();
-  if (userId && options?.applyFixup) {
-    // Reconcile known notification-count drift (notably with Sliding Sync / mixed receipts).
-    room.fixupNotifications(userId);
+  if (userId && options?.applyFixup && !unreadInfoFixupInProgress.has(room)) {
+    unreadInfoFixupInProgress.add(room);
+    try {
+      room.fixupNotifications(userId);
+    } finally {
+      unreadInfoFixupInProgress.delete(room);
+    }
   }
 
   let total = room.getUnreadNotificationCount(NotificationCountType.Total);
diff --git a/src/client/initMatrix.ts b/src/client/initMatrix.ts
index 1e6ca9c84..9e0496ee3 100644
--- a/src/client/initMatrix.ts
+++ b/src/client/initMatrix.ts
@@ -56,11 +56,17 @@ type MatrixClientWithWritableFetchRoomEvent = MatrixClient & {
   fetchRoomEvent: (roomId: string, eventId: string) => Promise<FetchRoomEventResult>;
 };
 
-// Replace fetchRoomEvent for first sync so thread roots don't each trigger GET /event
-// Uses cached timeline events when present otherwise a stub that fetches when user opens thread
-function installStartupFetchRoomEventPatch(mx: MatrixClient): void {
+type StartupFetchRoomEventPatchOptions = {
+  stubOnCacheMiss: boolean;
+};
+
+function installStartupFetchRoomEventPatch(
+  mx: MatrixClient,
+  options: StartupFetchRoomEventPatchOptions
+): void {
   fetchRoomEventStartupCleanupByClient.get(mx)?.();
 
+  const { stubOnCacheMiss } = options;
   const mxWritable = mx as MatrixClientWithWritableFetchRoomEvent;
   const origFetchRoomEvent = mx.fetchRoomEvent.bind(mx);
   let restored = false;
@@ -84,12 +90,17 @@ function installStartupFetchRoomEventPatch(mx: MatrixClient): void {
   mxWritable.fetchRoomEvent = (roomId: string, eventId: string) => {
     if (restored) return origFetchRoomEvent(roomId, eventId);
     const cachedEvent = mx.getRoom(roomId)?.findEventById(eventId);
-    // Reuse sync payload instead of another GET when we already have the root.
-    const payload: FetchRoomEventResult = cachedEvent?.event ?? {
-      event_id: eventId,
-      room_id: roomId,
-    };
-    return Promise.resolve(payload);
+    if (cachedEvent) {
+      return Promise.resolve(cachedEvent.event);
+    }
+    if (stubOnCacheMiss) {
+      const payload: FetchRoomEventResult = {
+        event_id: eventId,
+        room_id: roomId,
+      };
+      return Promise.resolve(payload);
+    }
+    return origFetchRoomEvent(roomId, eventId);
   };
 
   mx.on(ClientEvent.Sync, onSync);
@@ -518,7 +529,7 @@ export const startClient = async (mx: MatrixClient, config?: StartClientConfig):
       (filterDefinition.room.timeline as { lazy_load_members?: boolean }).lazy_load_members = true;
     }
 
-    installStartupFetchRoomEventPatch(mx);
+    installStartupFetchRoomEventPatch(mx, { stubOnCacheMiss: true });
 
     let syncStarted: Promise<void>;
     try {
@@ -693,7 +704,7 @@ export const startClient = async (mx: MatrixClient, config?: StartClientConfig):
   });
 
   try {
-    installStartupFetchRoomEventPatch(mx);
+    installStartupFetchRoomEventPatch(mx, { stubOnCacheMiss: false });
     await mx.startClient({
       lazyLoadMembers: true,
       slidingSync: manager.slidingSync,