The spec pinned Base UI's collision behaviour, not our code: it only passes
while the template popup happens to fit under the trigger at 1280x900, and
#41315's taller Add Auto Router form broke that premise for the second time
in three weeks. #41527 tried to scroll the trigger into the upper half, but
the dialog content is shorter than its max height, so nothing scrolls and CI
still fails 3/3 with the trigger at y=487
The guarantee #38554 introduced is that the popup never covers the trigger,
and the sibling spec keeps asserting that at a viewport with no room below
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