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.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=1d85889d714a96509ccc70454b1819ed9349acb6

Backed out changeset 1b76f477b19c (Bug 1666497) for unexpected reflow failures. CLOSED TREE

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&revision=1b76f477b19cf498625d48b7ee71baa66745c614&selectedTaskRun=YFJjZf1-RW-h76fFk4MwKQ.0

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=317426479&repo=autoland&lineNumber=2335

Backout: https://hg.mozilla.org/integration/autoland/rev/ec8f5ee6f607fd5e0c76bf3bd76dc0ca00a64328

Before my patch we were skipping some popuppositioned events, so it's
not unexpected that this count can go up as a result.

Backed out for bc failure on browser_appmenu.js

Backout link: https://hg.mozilla.org/integration/autoland/rev/da52c9e0a94a5726d383f1f486a9ce1eba6199e9
Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=317653140&repo=autoland&lineNumber=1529

https://hg.mozilla.org/mozilla-central/rev/741543bb9d41
https://hg.mozilla.org/mozilla-central/rev/cdec58fb4132