Just had some idea that could work and it seems to work very fine…
What I saw in Hwnd is that CreateWindow defaults x and y to 0. I changed this to make y default to 3000. This seems a bit strange, but it works. No more flickering in VertControl, RepeatControl, etc. Makes display more friendly on the eyes 😀
Of course the setting y = 3000 is a bit extreme, but it works. Maybe a negative value also works. In the past, working with C++ on BeOS, I used off-screen area to construct display elements. I never realised that under Windows this could also be done.