Handle command buttons inside of popover light dismiss

[lukewarlow]

Handle commandfor buttons in popover light dismiss code

This accounts for commandfor buttons within the "nearest inclusive target popover" algorithm. It does this by adding a new algorithm that given a node returns an associated popover this looks at both commandfor buttons but also buttons and input buttons with popovertarget.

This includes two additional changes not currently implemented in browsers:

• Exclude commandfor buttons when they're reset or submit buttons with a form owner from light dismiss logic.

• Exclude commandfor buttons without a popover command from light dismiss logic.

Without these two change you end up in situations where light dismiss doesn't work as expected.

Fixes #11607

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • WebKit export of https://bugs.webkit.org/show_bug.cgi?id=308308 web-platform-tests/wpt#57936 - test for disabled buttons including commandfor (accounted for in chromium and firefox already but not WebKit)
  • https://wpt.live/html/semantics/popovers/popover-light-dismiss-command.html - general test for commandfor buttons and popover light dismiss
  • TODO write tests for the form ownership and non-popover command scenarios.
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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