Guard showModal() and showPopover() with fully active checks

[domenic]

Closes #10659. This effectively only impacts dialogs and popovers "synthetic" documents, such as those created via document.implementation.createHTMLDocument().

• At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko
• Tests are written and can be reviewed and commented upon at:
  • TODO
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/373684393
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1924919
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=281550
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• MDN issue is filed: I've checked and MDN pages for showPopover() and showModal() do not document all the existing edge cases for exceptions, only the mainline exceptional cases. So this should not be necessary.
• 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 )
/popover.html ( diff )