[DRAFT] Propagate events into the event's source's tree where appropriate.

[alice]

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When an event is triggered by a source in a different tree from the target, propagate events into the source's tree (where appropriate) so that event listeners can be added in that tree without needing to have access into the target's tree.

• Add the concept of a "source" for an Event.
• Modify shadow root's "get the parent" algorithm such that events propagate into the event's source's tree where appropriate.

https://github.com/whatwg/html/pull/11349 pulls the event source concept into the HTML spec.

See https://github.com/WICG/webcomponents/issues/1098 for more information and worked examples.

• [ ] At least two implementers are interested (and none opposed):
  • …
• [ ] Tests are written and can be reviewed and commented upon at:
  • Some tests are added in https://chromium-review.googlesource.com/c/chromium/src/+/6490687 (also a WIP), but more are needed.
• [ ] Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for aborting and events): …
  • Node.js (only for aborting and events): …
• [ ] MDN issue is filed: …
• [ ] The top of this comment includes a clear commit message to use.

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