countMessagesBelow measured the turn straddling the fold by its messages'
boxes. After a programmatic jump the straddling turn is still
content-visibility skipped for one frame — its messages report empty rects —
so they were dropped as "above the fold" and the count published short. The
ResizeObserver only re-measured when the turn's remembered size differed from
its real one, which is why repeat clicks on the same rail mark disagreed.
An unsettled measure (a straddling message with no box) re-schedules one frame
later; the retry publishes whatever is measurable so an empty turn never stalls
the count.
Part of #117298 (atom 2).
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