Center PWA dialogs again by dropping the stale iOS offset that pushed a
flex-centered (position: relative) dialog down by half the viewport.
Seat the mobile overlay panel flush to the bottom (drop the float margin),
drop its bottom border in favor of a scroll shadow on the content, and raise
it above sheets (z-60) so an overlay opened over a surface sheet renders on
top instead of behind it.
Bohdan Triapitsyn
7adbaca797