Fix rejection of invites not reaching sync

Always allow the user to see their own leave events, otherwise
they won't see the event if they reject an invite for a room whose
history visibility is set such that they cannot see events before
joining.

synapse/visibility.py

@@ -134,6 +134,13 @@ def allowed(event, user_id, is_peeking, ignore_list):
             if prev_membership not in MEMBERSHIP_PRIORITY:
                 prev_membership = "leave"
 
+            # Always allow the user to see their own leave events, otherwise
+            # they won't see the room disappear if they reject the invite
+            if membership == "leave" and (
+                prev_membership == "join" or prev_membership == "invite"
+            ):
+                return True
+
             new_priority = MEMBERSHIP_PRIORITY.index(membership)
             old_priority = MEMBERSHIP_PRIORITY.index(prev_membership)
             if old_priority < new_priority: