1 Configure basic libretro emulator options
14 Configure additional options for mame-libretro
1 Aspect Ratio
this basically makes all libretro mame games display at 1:1 ‘PAR’ resolution (ie, every pixel is treated as a square), which fixes your issue with vertical games. I think this should probably be the default for mame libretro, but really we should fix the core so it renders these games properly…
Thank you very much, I’ll give this a go a bit later!