We want the link that the user has clicked inside the TOC to be "active"
(e.g. bolded) regardless of whether the browser's scroll position
corresponds to that section. Therefore, we need to temporarily ignore
section observer until the browser has finished scrolling to the section
(if needed).
However, because the scroll event happens asyncronously after the user
clicks on a link and may not even happen at all (e.g. the user has
scrolled all the way to the bottom and clicks a section that is already
in the viewport), determining when we should resume section observer is
a bit tricky.
Because a scroll event may not even be triggered after clicking the
link, we instead allow the browser to perform a maximum number of
repaints before resuming sectionObserver. Per T297614#7687656, Firefox
wasn't consistently activating the table of contents section that the
user clicked even after waiting 2 frames. After further investigation,
it sometimes waits up to 3 frames before painting the new scroll
position so we have that as the limit.
Bug: T297614
Change-Id: If3632529f58c15348a7200258f4f5999ea0dadc4
