The layer addition hang appears to be due to the failure of the "change handlers should only be called for events that happen after they are registered" invariant I mentioned.

Basically what is happening is that on a layer switch, the layer selectors each register a new handler (after unregistering the old one), but those handlers immediately get called for the event that has already happened, repeating the process and creating an infinite treadmill.

The obvious fix is to defer addition of new change handlers to the list until after all the existing handlers have been called. Of course that is not a sufficient guarantee for the recursive case, but hopefully there are no recursive changes involved here.

-mental