Comment 2 for bug 1522105

Just holding the lock while calling Observer::surface_exists but that can lead to deadlocks. Indeed CursorController installs an observer, in which it's surface_exists handler calls back into the scene.

Taking a copy of the list of surfaces and then releasing a lock can lead to calling Observer::surface_removed before Observer::surface_exists which could lead to the observer having stale and/or invalid Surface pointers around leading to other subtle runtime failures.

Maybe a secondary lock that serializes add_observer and surface_removed is enough.