Vilian,
And how do you trigger it to turn black?
The problem is shown when do you change active tabs.
Do you mean the first time you change active tabs, or at some time after the program has run for awhile?
The problem does not happen in Windows 10, only in Windows 7.
Well you really need to know the available memory on each PC when it happens. If you are only comparing one Win 10 and one Win 7 PC, it may be memory issues not the OS. I'm not saying that it can't be the OS, however, we need more information. It could also be a bug in the Ribbonbar class, but I am not sure why it would only show under one OS.
Have you tried one of the ribbonbar samples that comes with FWH, just to see if a simple small program does the same thing?
What version of FWH are you using?