This commits sets up the Table of Contents to bold the active section
when the section is scrolled.
Unfortunately, because our content does not have actual sections but
instead has a flat list of headings and paragraphs, we can't use
IntersectionObserver in the conventional way as it is optimized to find
intersections of elements that are *within* the viewport and the
callback will not reliably fire during certain scenarios (e.g. with fast
scrolling or when the headings are not currently within the viewport).
Furthermore, iterating through a list of elements and calling
`getBoundingClientRect()` can be expensive and can also cause
significant forced synchronous layouts that block the main thread.
The best compromise in terms of performance and function that I've found
is to use a combination of a throttled scroll event listener and
IntersectionObserver's ability to asyncronously find the
boundingClientRect of all elements off the main thread when `.observe`
is called which is the approach this patch takes. Although this is an
unorthodox way to use IntersectionObserver, performance profiles
recorded while holding the "down" arrow and scrolling for 10 seconds
with a 6x CPU throttle are comparable between master and this patch:
master: https://phabricator.wikimedia.org/F34930737
this patch: https://phabricator.wikimedia.org/F34930738
Bug: T297614
Change-Id: I4077d86a1786cc1f4a7d85b20b7cf402960940e7
