Hi, excuse me for the delay. I'm attaching an (ugly) small test app showing the problem.
Scroll the using the mousewheel. Look at the bottom part of frame #1 when scrolling upwards and the upper part of frame #2 when scrolling down. You'll see that the background color in the frames is wrong. If you then drag a bit the scrollbar by its thumb everything repaints as it should.
Thanks…
Mario