After the upgrade to 5.1.10, the video quality in VLC and kaffeine (using the
vlclib) has significantly degraded. It seems as if the vertical resolution has
been reduced to 50%, then stretched to fill the frame.

vlc was not updated (3.0.7.1 "Vetinari"), so I suspect that the different
behavior comes from the kernel update.

Here are links that show screenshots I created on the openSUSE KDE Plasma
desktop. In both cases, the same program was being shown; I reverted to the
previous kernel in the meantime using "start with read-only snapshot".

Have a closer look at the TV station logo in the upper left corner to spot the
difference most easily.

With kernel 5.1.7: http://www.mizapf.eu/files/vlc_5.1.7.png
With kernel 5.1.10: http://www.mizapf.eu/files/vlc_5.1.10.png

Here is one final screenshot that shows another point of different behavior.
When vlc is started, the video output is only half-height for about 1 second or
slightly less. Then, the video is stretched to the window height. I created
this snapshot by launching VLC and then quickly creating a screenshot.

http://www.mizapf.eu/files/vlc_5.1.10_sec1.png

I just updated to the newest release 5.1.15, but the reduced quality is still
there.