document the 'mark-as-read-detection' system

src/services/notifications_fetcher/notifications_fetcher.service.js

@@ -27,17 +27,18 @@ const fetchAndUpdate = ({ store, credentials, older = false }) => {
     }
     const result = fetchNotifications({ store, args, older })
 
-    // load unread notifications repeatedly to provide consistency between browser tabs
+    // If there's any unread notifications, try fetch notifications since
+    // the newest read notification to check if any of the unread notifs
+    // have changed their 'seen' state (marked as read in another session), so
+    // we can update the state in this session to mark them as read as well.
+    // The normal maxId-check does not tell if older notifications have changed
     const notifications = timelineData.data
     const readNotifsIds = notifications.filter(n => n.seen).map(n => n.id)
-    if (readNotifsIds.length) {
-      const possibleMax = Math.max(...readNotifsIds)
-      if (possibleMax !== timelineData.maxId) {
-        args['since'] = possibleMax
-        fetchNotifications({ store, args, older })
-      }
+    const numUnseenNotifs = notifications.length - readNotifsIds.length
+    if (numUnseenNotifs > 0) {
+      args['since'] = Math.max(...readNotifsIds)
+      fetchNotifications({ store, args, older })
     }
-
     return result
   }
 }