The three fixed-position popovers computed top/left inline while rendering,
which reads the trigger's rect exactly once. Scroll a sheet afterwards and the
trigger walks off while the panel stays nailed to the viewport; on iOS the
keyboard slides up over a panel that was placed before it existed.
useAnchoredPosition measures in a layout effect and re-measures on scroll
(capture phase, so the sheets' own scroll containers count), on resize and on
visualViewport changes. It also reports how much room is actually left, so a
list opened next to the keyboard shrinks instead of hiding underneath it.
CustomDateTimePicker clamps a fixed 268x360 calendar horizontally, which no
generic box can express, so it takes the bare re-measure signal instead.
Closes#1999Closes#2000
Maurice