When a window is maximized and I drag the title bar down, a piece of the window momentarily appears on the right monitor. This issue has been present for over a year, but I was using a different DE at the time.
What you see, is the application has still the maximized size for a moment but not yet restored the size it had before maximizing it, and then there is an animation to restore the expected size.
We could instead animate from the maximized size directly to the restored state.
Please report a bug for kwin Log in to KDE Bugtracking System
It happens even if I disable this animation. I even tried to disable all animations. I don’t think that bug is in animation