It is useful for web authors do determine when their <dialog> elements open and close. popover already has ToggleEvent which is dispatched when a popover opens or closes, but <dialog> does not. The current way to detect when a <dialog> opens is to register a mutation observer to check for open, however, this is quite a lot of work where an event would be easier.

I propose we add dispatching of ToggleEvent for <dialog> also. To be explicit: when showModal is called, <dialog> should dispatch a ToggleEvent with newState=open. When a dialog is closed (via form or button or close signal) it should dispatch a ToggleEvent with newState=closed.

Spec PR