The patch in bug 1666497 (which makes regular <window>-based XUL
documents with other the XHTML-based ones) uncovers an issue in the
anchored popup tests, which starts timing out.
The underlying issue is that it changes the reflow order from popups in
XUL-only documents with respect to their anchors. Note that Firefox
already uses the "broken" code, because we moved to xhtml and
browser.xhtml doesn't use a XUL <window> anymore.
Since we reflow now the popup _after_ the anchor rather than before, we
don't need to rely on CheckForAnchorChange() to deal with changes. This
means that we need to notify on the positioning changes that happen
during reflow though.
We should be able to simplify a bit the CheckForAnchorChange() stuff
now, but it also takes care of hiding popups and such so I don't plan to
do it on this bug to minimize risk.
Differential Revision: https://phabricator.services.mozilla.com/D91012
855da86e89