Read the tute again carefully. If all 3rd party panels are affected a simple CFG tweak should fix it.

If that doesn't do it this problem is most common when the windscreen frame or canopy covers the top left corner of the main panel BMP. These are the pixels you paint True Black.

PS. It might help to paint the 4 pixels in the corner rather than just the one. Most panels use 1024 x 768 sized BMPs these days.