Restorer relies on focusout event firing when element is removed

[smhigley]

Safari does not fire focusout or any other events when a node is removed, which means focus isn't restored when the restorer source is unmounted, even though focus returns to the body.

This can be reproduced in the Restorer stories in Safari, and also in FUI in components with popups (e.g. Popover, Dialog, Menu, etc.).

[smhigley]

Actually jumped the gun a little here -- the current focusout approach doesn't work in any non-Chromium browser, and the spec is unclear/currently debated as to which approach is correct.

ref:

• https://bugs.webkit.org/show_bug.cgi?id=243749
• https://bugzilla.mozilla.org/show_bug.cgi?id=559561
• whatwg/html#3847
• whatwg/html#3162