Ok, I'm going to leave the solution here for anyone who may need it in the future:

https://github.com/libretro/libretro-fceumm/issues/308#issuecomment-560408467

if using the US rom (NES), set User 2 to NONE instead of auto. if using the Japan rom (Famicom), disable User 5 instead.

Further explanation:

on NES, player 2 port acts as expansion port, including arkanoid paddle in this case (using mouse) on Famicom (when loading Japanese ROMS), expansion port is on a dedicated port which is mapped to User 5 on fceumm.

this should resolve this issue and any other confusion with nes/famicom expansions

@oyyodams ca a plus de sens, mais Kodi se retrouve tout de suite avec un ecran noir dans ce cas la. Je n'ai toujour pas elucide pourquoi quand il y a le repertoire systeme externe Kodi n'aime pas ca et je ne vois rien qui me saute aux yeux dans les logs.

@henrique1775 Themes need to support snap videos, if your themes haven't been updated since 6.1 release they won't work with videos

I wanted to add that it is possible to use certain shaders that add a little horizontal blur (just horizontal) to get rid of non-integer horizontal scaling issues like rainbow banding or scroll wriggling, they dont affect speed at all. For all consoles I use:
blurs/blur5fast-horizontal.glsl
And only for megadrive:
blurs/blur7fast-horizontal.glsl
because banding and dithering in megadrive are still there with blur5. Still, with the raspi composite they look great, more crisp than original megadrive (i have one to compare)

Both set with:
Filter: non linear
Scale: 3x

But the guy I mentioned uses other, maybe its even better i didnt try yet, he says It calculates better non integer width (256 for nes snes pcengine, 304 most neo geo, 384 cps, etc. arent integers into 720 Raspberry native horizontal res)

@levingard too bad, you can't change video settings on the fly on xu4, i never found how to do it

@beladorizid we don't support games/specific pads configuration. The easiest way is to change players order when required

Oh god, this is sad. I rebooted the pi and my problem was fixed. Learn from me. If you have a problem like this, always reboot before you post.

@supernature2k
I am immensely grateful with you! I swear I made my research in the forum and I found many topics that talked about this, but none of them were conclusive (the ones I saw). I found one that I think was the one I needed but it was in french, I translated with google translator, but I wasn't much of a help.
I found difficult to find the main theme.xml (even with WinSCP) but I did it.
I am truly grateful, thanks! You all make a great job!

@paradadf thanks for the info, great news!!

Thanks for the great work you do.

Hello Substring,

In the GitHub for libretro (see link above) there are many keymaps available for many operating systems (including x11 for Linux). Does it mean that keymap shall be selected when x64 is compiled?

I tried many times to find keymaps in my SD Card (and even the kernal which is a core file of C64) so to me if those files are not visible it means they are merged to some library file (.so) ? Am I right?

@matra 😉
put like)

@never1991 try to edit this line

nevermind, got my answer. i'm dumb sorry!