The attached project illustrates a simple MDI window application with two subwindows. One subwindow is blank and the other has a QCameraViewfinder displaying live video from the USB camera. To observe the problem drag the empty subwindow for partial overlap over the video window.

This works correctly on the Mac, i.e., the live video continues to draw correctly behind the front occluding window. On Windows there are only traces of video at the edges of the viewfinder.