When highlighting something that does not correspond to a
`PaintableBox`, we still need to apply `AccumulatedVisualContext`s. In
that case, search for the closest `PaintableBox` ancestor and apply its
`AccumulatedVisualContext`
This bug is most easily observed by highlighting a text node on a page
that has a scrollbar and is scrolled down at least a bit.
This fixes a regression from 009ddd4823.
