I've just upgraded to JDK 1.8. I had previously upgraded to Netbeans 8. After experiencing the problem in my application I created a test project, with the default Netbeans Java FX application, with a default start class.

Run that hello world class, minimise the resulting screen, then restore it and everything within the frame of the window is black.

Sometimes just mousing over the content area will result in the content reappearing. Other times I need to click on the black space to get it to reappear.