Test Instructions:
Open a modal box in any form (JForm or not) where there's a select list. For example: when adding or editing an article. Successful Tests:

Summary:
Squeezebox modal causes select lists to disappear

Details:
I just recently noticed this issue, but it seems to be in previous versions,
too.
When I'm in a form where select lists are shown, and I open a Squeezebox (for
example a user field), the select lists disappear. When I close the modal box,
the lists reappear again, but it van be annoying, especially in the frontend,
when form fields just disappear.

The problem seems to be with either Mootools or the Squeezebox.
It doesn't matter if the form is a JForm or a simple HTML form, the problem
seems to be with any select list.

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=28742