On a multi-mon virtual screen setup (win_XP), I am pushing a frame to FS mode through the primary monitor's graphics device. After that I am calling getFullScreenWindow() with respect to the secondary monitor's graphics device and it returns the frame instance that was pushed to FS mode through GD1.
Likewise, calling GD2.setFullScreenWindow(null) restores the frame (pushed into FS mode through GD1) to normal mode. This is reproducible on Win_XP since JDK1.4.2. Before that, this operation was causing a VM crash.

I have attached a sample test. Execute the sample test. You will get a frame with 3 buttons. Click the first button (gd1.setFSWindow). The frame will be pushed to FS mode. Click the second button (GD2.getFSWindow). You will see the frame instance being shown in the frame as a label. Click the third button (gd2.setFSWindow(null)). This would restore the FS frame to normal mode.