Windows use this effect when they're closed, so almost each time a window is closed it flickers before disappearing. And in System Settings where you hover around options it flickers, not always but frequently enough to get noticed. On X session everything is ok.
this happens here too ; windows flicker or "blink" just before closing when the "Fade away" effect is enabled. Arch Linux, Plasma 5.10, Intel IGP with modesettings driver.
I'm no longer able to see it with Qt 5.9 windows.
Yes, this bug is gone, atleast i can't reproduce it.
It seems to be fixed here too.