Both anchoring tests read the trigger's box before the click and the popup's box the
instant it turns visible. Base UI places the popup asynchronously and opening it can
shift the trigger, so both boxes could be sampled before the layout settled. The run
on 1eedaa3a43 missed by 4.2px (expected >= 446.015, got 441.799) on a tree with no UI
changes at all, having passed on 21092d633b, which differs only in a deleted python
test and a budget json.
Each assertion now re-reads both boxes under expect.poll. The conditions themselves
are unchanged: the popup must sit at or below the trigger's bottom edge in the first
test and must not overlap it in the second. Polling cannot mask a genuinely misplaced
popup, since one that never lands correctly still fails when the poll times out.
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