Technical Limitations of Client-Side-Decorations
I was just made aware of the fact that you can open Alt+F3 in Plasma popups and that helped to produce this wonderful screenshot (sorry Eike and Plasma devs for destroying your apps in such a horrible way):
As we see all windows have KWin decorations and they should not have them. Yakuake has a kind of own CSD – you see the controls at the bottom. For Yakuake it’s totally fine to use the own controls as it has a very special usecase. Also all the Plasma windows should not have decorations as they are part of the workspace and by that not traditional windows.
So this is a technical and unsolvable problem for all windows which want to have CSD: the window manager may reparent the window and the window has no control over it.