risks of hidden overflow in modal popups

originally wrongly posted in #1213252: Allow media file's fields to be edited in a modal or after upload in the media widget (comment #95) :

Reinventing the wheel with jQuery pseudo-popup is a major failure (my window manager does its job pretty well thus I don't need jQuery workarounds that window$ users need). I can leave with this though, but I'm getting an annoying problem which make me think that the overflow: hidden CSS properties must be removed from media.css.
It already encountered quite a few times the case where the cancel/submit buttons are hidden because they overflowed.
You can reproduce this when using the view library with non-square thumbnails from the media browser modal popup, then you'll get the cancel/submit button pushed below to an inaccessible place.
That's very annoying when your good old scrollbar is hidden too.

It may be also be solved by overriding the core .item-list .pager clear: both property, but hiding the overflow is almost always a bad choice anyway, especially when the box may contains controls.