Add attribute changed steps to dialog element

[lukewarlow]

Add attribute changed steps to dialog element

When the dialogs open attribute is removed:

1. Remove dialog from the document's open dialogs list.
2. Destroy and nullify dialog's close watcher

This also adds an assertion to the start of
'set the dialog close watcher' that dialog's close watcher is null.

Fixes #10953

• At least two implementers are interested (and none opposed):
  • Chromium - Already implements this.
  • …
• Tests are written and can be reviewed and commented upon at:
  • This behaviour is already enforced by multiple Web platform tests. e.g. https://wpt.live/html/semantics/interactive-elements/the-dialog-element/dialog-remove-open-attr-crash.html
  • Added a new test that doesn't rely on Assertions: Add test for dialog attribute changed steps web-platform-tests/wpt#50393
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1936940 (meta bug)
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=284592 (meta bug)
• Corresponding HTML AAM & ARIA in HTML issues & PRs: n/A
• [ x] 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 )

[domenic]

• This behaviour is already enforced by multiple Web platform tests. e.g. https://wpt.live/html/semantics/interactive-elements/the-dialog-element/dialog-remove-open-attr-crash.html

This test would also pass if we removed the assert, i.e. fixed the regression introduced recently.

Marking "do not merge yet" until we get more clarity in #10953 as to what the right path forward is.

[domenic]

Removing "do not merge yet" as I'm convinced by the arguments in #10953, but of course we still need to check all the boxes to merge.

For the tests box, could you point to more tests that enforce this behavior that aren't crash tests? Since assertions are actually comments; they don't cause crashes.

[lukewarlow]

I've opened web-platform-tests/wpt#50393 which fails in a browser which follows the current spec:

• I tested by locally modifiying chrome to use an ordered list for the open dialogs list, and removed the attribute changed set. In that build both the escape case and the click outside case result in the dialog being closed when it shouldn't be.