2015 retina MacBook with macOS Sierra 10.12.1 installed. Since upgrading to Sierra I get a black screen when playing back mp4 videos. Sound plays back, if I load the clips I can see those, and sometimes with enough fussing I can get it to actually play the video. Playback in VLC works fine, but it uses 3x as much power so I'd like to figure out what's going on. Thanks.