With Plasma dialogs & tooltips the mask is only set on the widget itself
if compositing is off. Instead is the mask explicitly passed for the
The recent changes to fix the blurbehind & contrast mask with panels
instead turned to always set the mask on the widget. For some reason
yet to be understood this can sometimes result in an offset by a pixel
between the mask used for painting and the mask used for the window effects.
Aligning the mask setting code with the one for dialogs & tooltips makes
the symptom go away for now, and also delivers consistency.