DFGUI - GUI acting weird inside Game View when Fullscreen, Build is fine

I've experienced a really strange behaviour recently regarding DFGUI. I know, DFGUI has been discontinued and there's the new uGUI, but I can't afford switching over just yet. When updating from Unity 4 to Unity 5.0.1f1, some of my DFGUI Objects were behaving incorrectly when in "Maximize on Play" mode. At first, I though it's because of the transition (DFGUI isn't really U5 ready due the discontinuing), but then I made a Standalone Build, run it and saw that everything was working correctly - even when re/maximizing.

Pictures can tell better.


In-Game (before hitting Play):


Scene View, before hitting play:


In-Game (after hitting play):


Scene View, after hitting play:


It works without problems in "Maximize on Play" mode when unplugging the Game tab first and then hitting play. Neat workaround, but not really a solution.

Anyone else has experienced the same? Any idea whats going on and how to fix that?
(I also asked in the official DFGUI forums, but no response yet and it's pretty unlikely.)