Add the focusreset event

[noamr]

Fire a focusreset event at the Document, if focus was reset to the viewport due to a focused element being removed, and wasn't since restored.

The timing of the event is in the mutation observer microtask, together with slotchange events.

Together with whatwg/dom#1512

We keep a "has pending focus reset" flag on the document (see the DOM PR) when a focused element is removed, and check it shortly after (in the mutation observer microtask).

The asynchronous check is there to avoid being able to observe intermediate DOM state while removing a node tree.
The firing happens in a microtask rather in the rendering steps to make sure DOM changes in the focusreset handler are picked up in the next or current render - e.g. focusreset might be called in the middle of a ResizeObserver callback.

Fixes #12842.

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • Mark focusreset tests as non-tentative web-platform-tests/wpt#63595
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/572677865
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2081188
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=327387
• MDN issue is filed: Document focusreset mdn/mdn#876
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/document-lifecycle.html ( diff )
/dom.html ( diff )
/indices.html ( diff )
/infrastructure.html ( diff )
/interaction.html ( diff )
/webappapis.html ( diff )

[noamr]

@emilio or @zcorpan, would you like to take an initial look?
This is ready including tests etc.