Reset window.name also when opener is set to null

[annevk]

Helps with #5680.

#5569 might allow for further simplification, but given that #313 is not solved I'm not tackling that here.

We don't have test coverage for session history yet so I haven't made changes there. See also #5679. (This could maybe be folded in if there's no perceived problem with Safari/Firefox's behavior.)

#5350 might obsolete a lot of this, but the specifics haven't really been decided upon yet. It might well make sense to use equivalent logic, but the implications of doing that are bigger:

1. Agent clusters would be replaced. (Or perhaps they are not guaranteed to survive top-level navigations anyway? I guess it's a bit unclear and might depend on what is in the bfcache.)
2. The opener browsing context's reference to this browsing context would also be severed.

• At least two implementers are interested (and none opposed):
  • Safari (presumably, they ship this)
  • Firefox
• Tests are written and can be reviewed and commented upon at:
  • https://phabricator.services.mozilla.com/D81335 and https://phabricator.services.mozilla.com/D88417
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=706350
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=444222
  • Safari: n/a

cc @andypaicu @jakearchibald @rakina @artines1

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/browsing-the-web.html ( diff )

[domenic]

Looks reasonable and the change is simple. I'll ping the Chromium bug linked in case folks have thoughts; it might be nice to wait a business day or two before merging in case they do.

Also I'll note that the tests as written use BroadcastChannel so we can't really tell if Firefox is proposed to behave the same as Safari or not. Perhaps consider rewriting them to use stash.

[annevk]

The tests have been updated to use a stash. I forgot about this PR unfortunately, but it still seems good to land now it is rebased. Firefox is about to ship as well (we baked it on Nightly first for a while and found no issues).

Any final thoughts?

[domenic]

Still looks good. And it seems the tests have made it to wpt.fyi and entirely pass in both Firefox and Safari, so no concerns. https://wpt.fyi/results/html/browsers/windows/clear-window-name.https.html?label=master&label=experimental&aligned