Panel: Add floating margins to avoid overflowing content when floating

This is because we're adding the floating margin *inside* the panel's window,
which means that the actual length of the panel ends up being (content length -
floating margins). I'm just adding them back when we calculate the size of the
panel.

BUG: 462130
FIXED-IN: 5.26.5


(cherry picked from commit 9d90c710)