Close dialog elements when the open attribute is removed

[josepharhar]

Fixes https://github.com/whatwg/html/issues/5802

This patch adds attribute change steps to run the dialog closing steps whenever the open attribute is removed in order to prevent a bad state where the dialog is hidden but modal which renders the rest of the page inert.

• [ ] At least two implementers are interested (and none opposed):
  • Chrome
  • …
• [x] Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/pull/44408
• [ ] Implementation bugs are filed:
  • Chromium: https://chromium-review.googlesource.com/c/chromium/src/+/5269905
  • Gecko: …
  • WebKit: …
• [x] MDN issue is filed: MDN already discourages modifying the open attribute, and I think its a good idea to keep it that way.
• [x] 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-control-infrastructure.html ( diff ) /interactive-elements.html ( diff )

[josepharhar]

I hope other implementers are interested and that it's web-compatible.

I will ship this behind a flag, and I will let yall know if any issues come up. I don't have a usecounter for it but I don't see why this change would be a problem.

[annevk]

Seems reasonable modulo web compatibility. @lukewarlow might also be interested in this.

[lukewarlow]

This feels like a big win for developers and having looked at the changes I don't think it's likely to cause any issues. Especially because it's unlikely developers will be relying on this behaviour (seems like its always undesirable)