KDE's Desktop effects (compositing) has a setting for when to keep window thumbnails. It says that setting this to always "breaks minimization". Does anyone know how it breaks minimization?
In order to be able to draw the windows (offscreen) so that it can create the thumbnails, KWin just shows them as minimized, while they aren't really.
So, the windows get minimized, as far as the user can tell, but they aren't really.
This could produce problems with applications that want to know whether they are minimized or not, but in reality, you shouldn't have any problems by choosing this option.