Do not calculate highlight notifs for threads unknown to the room (#2957)

Author: Germain

spec/unit/notifications.spec.ts

@@ -141,6 +141,29 @@ describe("fixNotificationCountOnDecryption", () => {
         expect(room.getThreadUnreadNotificationCount(THREAD_ID, NotificationCountType.Highlight)).toBe(0);
     });
 
+    it("does not calculate for threads unknown to the room", () => {
+        room.setThreadUnreadNotificationCount(THREAD_ID, NotificationCountType.Total, 0);
+        room.setThreadUnreadNotificationCount(THREAD_ID, NotificationCountType.Highlight, 0);
+
+        const unknownThreadEvent = mkEvent({
+            type: EventType.RoomMessage,
+            content: {
+                "m.relates_to": {
+                    rel_type: RelationType.Thread,
+                    event_id: "$unknownthread",
+                },
+                "msgtype": MsgType.Text,
+                "body": "Thread reply",
+            },
+            event: true,
+        });
+
+        fixNotificationCountOnDecryption(mockClient, unknownThreadEvent);
+
+        expect(room.getThreadUnreadNotificationCount(THREAD_ID, NotificationCountType.Total)).toBe(0);
+        expect(room.getThreadUnreadNotificationCount(THREAD_ID, NotificationCountType.Highlight)).toBe(0);
+    });
+
     it("emits events", () => {
         const cb = jest.fn();
         room.on(RoomEvent.UnreadNotifications, cb);

src/client.ts

@@ -9297,9 +9297,21 @@ export function fixNotificationCountOnDecryption(cli: MatrixClient, event: Matri
     if (oldHighlight !== newHighlight || currentCount > 0) {
         // TODO: Handle mentions received while the client is offline
         // See also https://github.com/vector-im/element-web/issues/9069
-        const hasReadEvent = isThreadEvent
-            ? room.getThread(event.threadRootId)?.hasUserReadEvent(cli.getUserId()!, event.getId()!)
-            : room.hasUserReadEvent(cli.getUserId()!, event.getId()!);
+        let hasReadEvent;
+        if (isThreadEvent) {
+            const thread = room.getThread(event.threadRootId);
+            hasReadEvent = thread
+                ? thread.hasUserReadEvent(cli.getUserId()!, event.getId()!)
+                // If the thread object does not exist in the room yet, we don't
+                // want to calculate notification for this event yet. We have not
+                // restored the read receipts yet and can't accurately calculate
+                // highlight notifications at this stage.
+                //
+                // This issue can likely go away when MSC3874 is implemented
+                : true;
+        } else {
+            hasReadEvent = room.hasUserReadEvent(cli.getUserId()!, event.getId()!);
+        }
 
         if (!hasReadEvent) {
             let newCount = currentCount;