Make focus events composed

[annevk]

Since focus events are UI events, they should be composed. https://html.spec.whatwg.org/multipage/interaction.html#fire-a-focus-event needs to be updated and I guess we should add some tests.

(Note that #1583 captures other issues around focus.)

cc @whatwg/components

[TakayoshiKochi]

This sounds reasonable to me.

[hayatoito]

This sounds reasonable, however, I think there are several checkpoints before changing the behavior:

@TakayoshiKochi

• Could you do double-check that this change doesn't have any impact on https://w3c.github.io/webcomponents/spec/shadow/#focus ?
• Also it could be a breaking change. Could you confirm that Blink wouldn't break existing Web sites? I think it is unlikely, but that could be.

ping also @rniwa.

[annevk]

@hayatoito implementations already have this behavior. See the tests I wrote (linked from the PR).

[hayatoito]

Ah, I see. Please forget my previous comment. I thought I forgot to make it composed.

I had updated UI events spec at w3c/uievents#91.
Wasn't that enough?

[annevk]

No, the UI Events specification doesn't really define these events or when they get dispatched. HTML defines that at https://html.spec.whatwg.org/#fire-a-focus-event. I filed w3c/uievents#185 to simply stop defining these in UI Events to make this less confusing.

[hayatoito]

I got it. Thank you for the explanation.