Removing a later event listener while an earlier listener for the same event is running doesn't stop it from being called

[pshaughn]

Servo currently has no "removed" flag for its event listeners, as specified in https://dom.spec.whatwg.org/#event-listener-removed . I would add it myself as part of #22783 cleanup, but there's a complicating factor: The removed flag lives on an "event listener", lowercase letters, not an EventListener, and Servo event targets currently expose a list of capital-EL Event Listeners to the dispatch algorithm. The lowercase concept is approximately Servo's struct EventListenerEntry, but it's not an exact mapping and code that returns EventListeners can't just be drop-in replaced with code that returns EventListenerEntries.

[pshaughn]

This is almost certainly a cause of other already-open issues, but I didn't happen to see any specific one that's definitely it.