Write down why a mark waits to be seen

Four things that cost time to find. An IntersectionObserver threshold is
a fraction of the element, so a ratio silently excludes blocks taller
than the window — exactly the ones most worth noticing. An element that
crosses the viewport in one frame produces no entry at all. A block's
index moves when anything above it is inserted, so comparing indices
reports most of a document as having moved over one added paragraph.

And a correction: the note about layout tests blamed happy-dom for
returning zero. There is no happy-dom, and no DOM — `document` is
undefined in the test runtime, so the answer is to keep the part worth
testing separable rather than to reach for a fake one.