My guess is that it was done using shadows to not interfere with the
native look, but actually this just works even with native-looking menus
(like the <select> menulist), because the background-color for those is
set on the menupopup, rather than the ::part(content).
So those have effectively 1px of extra padding (due to the transparent
border), but that seems barely perceptible, and worth the consistency
and simplification.
Differential Revision: https://phabricator.services.mozilla.com/D164716
8bbf7f3f5b