Allow a Window as the target of "fire a focus event"

[zcorpan]

"Fire a focus event" was defined as taking an element, but the focus update steps invoke it with a Window object when the entry is a Document, and a Window has no node document to get the view attribute's value from.

Fixes #12866.

• At least two implementers are interested (and none opposed):
  • Gecko (already passes the test)
  • …
• Tests are written and can be reviewed and commented upon at:
  • HTML: test view and target of focus events fired at a Window web-platform-tests/wpt#62344
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: N/A
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• The top of this comment includes a clear commit message to use.

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

[Ms2ger]

While you're here: I'm not too fond of the single-letter variables

[annevk]

Modulo my nit and @Ms2ger's nit. So we never call this with a Document object then?

[zcorpan]

@annevk no, it's only called from focus update steps with either an element or a Window.

[zcorpan]

Are WebKit and Chromium ok with switching from Event to FocusEvent in the Window case?