Retroarch on windows can go fullscreen in super resolution as long as it’s 2560x240, if it’s 2560x224 it doesn’t work (only if you change the desktop resolution to that beforehand but that makes frontends unusable)
soooo what I’d like to suggest is that when retroarch is in super resolution, it identifies the horizontal and vertical resolution of the game you’re loading, then adjust the vertical to what’s needed then centralize it
for example in umk3 for lr-mame the vertical is 256, i set custom viewport height to 256 then the Y position up a bit to center it, doesn’t crop much at all, the resolution is still 2560x240, also the aspect ratio isn’t far from 4:3 and the scanlines match, i saved game overrides in this case but mame has so many games with different resolutions, every time i want to experiment a new game i have to go into settings and change custom viewport height up and down until i see that (1x)