What goes on here is that there's a couple of unfortunate style change
sequences which end up making us not do the EffectsInfo dance correctly.
Twitter uses (maybe didn't use to, which would explain the regression) a
visibility: hidden, out-of-flow iframe for a bit (which we correctly
throttle). But then they switch to an in-flow, visible, zero-height
iframe. That we should _not_ throttle. However, we end up not getting to
the display list code at all, because nsBlockFrame decides that we don't
need to descend into an empty line[1].
It seems less error prone to re-use the IntersectionObserver timing and
computation to determine whether the iframe is visible. That completely
matches in-process iframes, too.
Removing the empty frame border and putting them on an empty line in
dom/base/test/test_bug1639328.html is enough to reproduce the issue
without this patch.
[1]: https://searchfox.org/mozilla-central/rev/fe2743c6c5c708061c7f6504b26958fcc815bb4a/layout/generic/nsBlockFrame.cpp#7569-7579
Differential Revision: https://phabricator.services.mozilla.com/D207479
0b3521ccca
