This fixes the content cropping. The issue is that when there are
multiple subdialogs, we set the --inner-height of the latest (smaller in
this case) dialog on the dialog stack, overriding the inner-height for
other frames as well.
With XUL layout we flexed anyways instead of honoring the height:
var(--inner-height), which papered over the problem because the dialog
parent had a min-height.
Rework the window modal offset so that we don't need to set the
--inner-height variable on the parent.
Differential Revision: https://phabricator.services.mozilla.com/D166532
7b6b803400
