redefine dialog closedby IDL to return computed value

[keithamus]

This attempts to fix #10945 by redefining the IDL reflection for closedby to instead use getter and setter steps, using the computed closedby steps to determine the getter value.

• At least two implementers are interested (and none opposed):
  • Chromium: This is the current behaviour of Chrome
  • Gecko: Prototyping the feature, the prototype has this behaviour. @zcorpan commented about preferring this option here: Clarify IDL reflection for closedby attribute #10945 (comment), which I'll take as a signal.
  • WebKit: @annevk also pointed out 2 is prefererable here: Clarify IDL reflection for closedby attribute #10945 (comment) so I'll take that as a signal.
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/1171011d28302ebc41ee5240fa3a88b98d4ad730/html/semantics/interactive-elements/the-dialog-element/dialog-closedby.html#L76-L91
• Implementation bugs are filed:
  • Chromium: This is the current behaviour of Chrome.
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1936940
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=284592
• 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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/interactive-elements.html ( diff )