Bug 1568778 - Do suppress adjustments when switching an undisplayed element to be abspos. r=dholbert

One of the issues with the test-case in this bug is that the page consumes a ton
of CPU due to scroll anchor adjustments being triggered from scroll events,
which in turn cause other scroll events to fire.

This happens in Chrome as well (just scroll to the bottom of the test-case, and
do `addEventListener("scroll", () => console.log("scroll"))` on devtools. But I
think it's worth fixing. This patch fixes it and overall I think it's a slightly
better approach to suppress adjustments than what we're doing.

Differential Revision: https://phabricator.services.mozilla.com/D39339

--HG--
extra : moz-landing-system : lando