Yes, Wally is working of porting PCSX to SDL2.
Mupen uses GLES and EGL. PCSX uses SDL 1.2.Ok this might also be the reason why Muppen 64 runs Fullscreen (and i had some issues on my Pyra to let it run because i missd some packages, ), its also OpenGL ^^ , but the Playstation Emulator only in Maximised Window Mode (Which is also at least most of the Screen)..
https://github.com/spurious/SDL-mirror/blob/release-1.2.15/src/video/x11/SDL_x11modes.c#L1014-L1015
./AlphaFlight --fullscreen