Event.target should return null after dispatch is complete, when stopped at ShadowRoot

[TakayoshiKochi]

https://dom.spec.whatwg.org/#dispatching-events doesn't specify any operation on event.target after dispatching is done (step 14 and after). This leaves event.taget as what it was when the event was last set in step 13.1.

This, in turn, means that when the event is stopped at shadow root, and the reference to the event is stored somewhere in the event handler, the last event.target would sort of leak after dispatch is complete.

Would like to reset event.target to null when the event stopped at shadow root.

[annevk]

Good catch. That seems like the correct solution. Always doing it seems too risky.

cc @whatwg/components

[TakayoshiKochi]

We noticed that we missed event.relatedTarget and should be nullified when it's possible to leak shadow tree internals. Let me update #513.