Refine when selectedcontent elements are updated

[annevk]

This is a follow-up to #12263 that addresses the remaining cases where selectedcontent elements were updated too little or too much:

• Inserting an option element that causes another option element to become selected now updates selectedcontent elements. This happens when the select element has no selected option, e.g., after setting selectedIndex to -1.
• Inserting a select element that already contains option elements now updates its selectedcontent elements once, rather than again from the post-connection steps of its selected option element.
• The microtasks queued to update selectedcontent elements after option elements are removed or moved are now coalesced per select element, and do nothing if the selectedcontent elements were updated in the meantime. The same goes for moving a selectedcontent element.
• The microtask queued by the selectedcontent moving steps now determines the select element when it runs, rather than when it is queued.
• Moving a selectedcontent element no longer updates it when its select element and disabledness do not change, e.g., when an ancestor of the select element is moved.
• Removing a selectedcontent element now recalculates its disabledness.
• Setting the selected IDL attribute of an option element only updates selectedcontent elements when selectedness changes.

This also removes "ask for a reset" and "update descendant selectedcontent elements for an option", which would otherwise each have a single caller.

Tests: web-platform-tests/wpt#63153

• At least two implementers are interested (and none opposed):
  • WebKit
  • …
• Tests are written and can be reviewed and commented upon at:
  • WebKit export of https://bugs.webkit.org/show_bug.cgi?id=325898 web-platform-tests/wpt#63153
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=325898
• Corresponding HTML AAM & ARIA in HTML issues & PRs: not needed
• MDN issue is filed: not needed, as this only changes when selectedcontent elements are updated
• The top of this comment includes a clear commit message to use.

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

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/form-elements.html ( diff )

[annevk]

@josepharhar @jnjaeschke I noticed a couple more cases. Once some of my WebKit work lands I'll upload tests for this as well.