At the moment, there is a bit of a problem with fullscreen mode on OS X. The problem occurs when you have IDEA in fullscreen mode and a modal dialog needs to be displayed while you're working on a different space. The modal dialog displays on the non-IDEA space, but anytime you click on it, you're immediately taken to the IDEA space and can't dismiss the modal dialog (because it's still being displayed in the other space).
There's already an issue open for this (and Moritz's explanation is probably clearer): http://youtrack.jetbrains.com/issue/IDEA-84196
At the moment, you have to drop out of fullscreen mode to dismiss the modal dialog, which is pretty annoying. I've tried playing with OS X settings related to spaces, but I can't see anything obvious that would help.
I thought I would ask here if anyone has any better workarounds?