This one shouldn't be too hard to reproduce. It happens with a default configuration on my XP sp3 system:
[*]Open an image that's smaller than your screen
[*]Press F5 to see it full screen. Note that only your image is visible - no other screen elements.
[*]Press F5 again to return to the previous window size. So far so good..
[*]Now press Space to advance to the next image
[*]Press F5, and notice how the menu and the status line are visible in fullscreen mode.
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While I have your attention.. :) the following is a bit too generic and minor to bring up separately, but there's quite a lot of flashing and blinking of scroll bars and other window elements going on, and last-minute adjustments to the centering of images, when I do simple things like advancing to the next image or pressing F6 and going back to reduced-to-window view. When I press F5 to switch from maximized window to full screen mode, I get a brief glimpse at my desktop or any other open windows. In all, it's as if the viewer is rapidly making adjustments to itself and updating the screen in real-time, instead of quietly preparing these things in the background, which would give the program a more 'quiet' and solid look & feel.