The forum lost all of my post, twice, so typing it all again.
Intel IGU is the main display driver. (E3-1265L v3) With the AMD driver installed for 7970 the KDE desktop still shows fine. But when decoding video, only the top 1/16th of the display shows video. The rest of the display is a frozen frame from early in the video. Debian Testing.
Remove the AMD driver, and video display is fine.