I too get that black border with desktop windows, XP Home, 6.15.
If the window is opened, maximised and then filled with /background, and then made smaller, it's fine. If the window is opened and is a normal size in the middle of the screen, then has a background picture put on it and is then maximised, the black border in your screenshot above is displayed.
I have not (yet) tested if this happens in earlier versions and don't know if this is intended behaviour.
Regards,
