Sequence of Window.sizeToScene immediately followed by Window.centerOnScreen is problematic for the gtk port. The centerOnScreen in this sequence needs the correct window with and height, so it needs the sizeToScene to finish synchronously with all its notifications (which set the correct window with and height from the known content width and height). We have no good way how to wait for these notifications produced by sizeToScene so we will need to synthesize them manually. For this we need to know the size of the window decorations. But in general these are not known until the window is displayed, so we will need some correction mechanism for any mistakes done in centerOnScreen.