A QVideoWidget connected to a QMediaPlayer works fine while playing but when it is paused and the window or widget goes out of focus, the video widget turns solid gray (the color of the window background). When I play the video it becomes visible again. This seems to also happen instantaneously while the video is playing: if another window is selected the video flashes gray, but then immediately displays the next frame. I am using PyQt5 but this is presumably a problem with the underlying Qt library. 

I posted my video code which exhibits the problem on my computer. It requires Python 3 and PyQt5 (pip install PyQt5) to be installed to run. It should open a window, and if you File -> Open a video file it will play.