Add test.
Somehow the resize doesn't fire all the time in Firefox (I'm guessing
weird about:blank stuff). But this test should still be valid, and fails
intermittently without my patch.
It's not great, but I don't have the cycles this week to dig more.
Differential Revision: https://phabricator.services.mozilla.com/D210552
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1896760
gecko-commit: 2c2afd11d06b41f59e37c420f103652c1488f27f
gecko-reviewers: smaug