I see now that runcommand is supposed to automatically launch runcommand-onstart.sh if it exists. I added a line to the script to output a text string to /dev/shm/runcommand.log, but it doesn't appear. I don't think runcommand-onstart.sh is actually running. I'm running the unsupported Buster version of Retropie, so maybe I stumbled on something funky. Will keep playing with this. EDIT: I didn't change anything, but the script is suddenly working and I can see my custom string in runcommand.log. Excellent. Will tinker with my ROM filenames and see if I could get this to work.

@dankcushions Yes I think that is what it looks like. But it's much easier to see in motion. It's not related to the resolution (I tried several).

Report the issue here: https://github.com/libretro/snes9x2010

Ok, I tried turning off dithering, and it didn't seem to make any difference. I'm still getting artifacts, like echo-type sounds, sounds that go on longer than they should etc. I have listened carefully to games that I am very familiar with on other systems, and this definitely only seems to affect Hatari.