Collapse disowned into opener browsing context

[annevk]

Tests: TODO. Ideas:

• Create a popup. Set opener to null. Then name target it. Opener should be back now pointing to whatever name-targeted it.
• Create a popup. Set opener to "blah". This writes a property to the Window object. Can we still get the original opener getter somehow and see the actual opener or is this lost?

Fixes #5569.

• At least two implementers are interested (and none opposed):
  • N/A
• Tests are written and can be reviewed and commented upon at:
  • HTML: set opener to a string web-platform-tests/wpt#38447
• Implementation bugs are filed:
  • N/A
• MDN issue is filed: N/A

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

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/browsing-the-web.html ( diff )
/document-sequences.html ( diff )
/nav-history-apis.html ( diff )

[domenic]

• Can we still get the original opener getter somehow and see the actual opener or is this lost?

If you have a same-origin iframe handy, this should work:

const getter = Object.getOwnPropertyDescriptor(frames[0], "opener").get;

const actualOpener = getter.call(popupWindow);

[domenic]

Normative changes look good, but we should keep the note in some form.

[annevk]

I'm no longer convinced my first suggested test case is good. I think you might end up with two popups due to #313.

I created a test case for the second case: web-platform-tests/wpt#38447.