By having the up/down navigational buckets scroll highlights
into view they only come to the inside edge of the viewport. This
is not sufficient to get them far enough into view that they leave
the navigational buckets. Therefore, it's not sufficient to take
the next closest of the highlights in said buckets when attempting
to scroll further, since it may already be on screen.

Implement `getBoundingClientRect` for `TextHighlight` and use that
to check whether a highlight is off screen before deciding to
scroll to it.