When adding an overlay to the overlay controller (AddOverlay()) there is a call to the show() method of the container of the overlay.
This is annoying if you have a widget which is supposed to be drawn over the entire render window.
It is suggested not to turn on the visibility during construction in AddOverlay(). A call to SetOverlayVisibility() is now mandatory if you want to show overlays. This is also the default behavior of Qt.