When a PDF page transitions from a rendered to non-rendered state the
text layer element is removed and existing highlights in that layer
briefly become disconnected from the document. As a result they have an
empty/zero client rect as returned by `element.getBoundingClientRect()`.
This caused the bucket-building logic to think that the anchor
had moved to the top of the screen as the `top` and `bottom` coordinates
of the anchor box became zero. This could lead to anchors which should
be in the "below screen" bucket briefly jumping to the "above screen"
bucket. Once re-anchoring completes and the highlight is anchored into a
placeholder element for the non-rendered page, its anchor box becomes
non-empty again and the anchor "jumps back" to the "below screen" box.

This commit adds logic to ignore any highlights which have zero height,
including highlight elements that are disconnected from the document.