How to draw opaque background with WA_TranslucentBackground active

[d_stranz]

nothing I tried was able to change this attribute / transparency setting

Yes, I think that is correct - the attribute can only be changed when the widget has been constructed but before it is shown. Probably has something to do with the underlying window system and the way it creates the native windows wrapped by QWidget / QWindow. It is during the transition from constructing the QWidget container to showing it on screen that the underlying native window gets instantiated and configured and there are properties of the native window that can't be changed once that happens.