Motion window gets dark on external monitor

I have a Samsung monitor plugged into my Retina Macbook Pro. When I have Motion running and put it on the external monitor, everything is fine. Until I switch to another program then back to Motion. When I do that the viewer window of motion (just the video content, not the menus) turn dark. Not completely black, but just enough to where all I can see are the highlights. Any ideas what could cause this? The only solution I've found is closing the Motion project (even leaving Motion running) and re-opening it.