Prevent infinite loop in revealing algorithms

[josepharhar]

The ancestor details revealing algorithm and the hidden until found revealing algorithm may both get stuck in infinite loops because they iterate the flat tree while firing synchronous events. If the page were to listen to these events and change the flat tree in response, then the algorithm could theoretically never end.

Fixes #11436

• At least two implementers are interested (and none opposed):
  • Chromium
  • WebKit
• Tests are written and can be reviewed and commented upon at:
  • Add beforematch infinite loop test web-platform-tests/wpt#53789
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/433545121
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1980442
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=296797
• Corresponding HTML AAM & ARIA in HTML issues & PRs: Not needed
• MDN issue is filed: Not needed
• The top of this comment includes a clear commit message to use.

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

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/browsing-the-web.html ( diff )
/interaction.html ( diff )
/interactive-elements.html ( diff )