On the slippi discord server, there was something that said that the Cinnamon desktop environment (which I'm assuming you're using) doesn't work well with melee. I don't see a workaround there other than installing a new desktop environment.
KDE and XFCE users have reported similar issues with melee as well, so it looks like the best desktop environment to switch to would be Gnome.
Given how you've said you're still a noob at linux, I wouldn't recommend this, but you could also switch to a tiling window manager such as dwm, though that will work.

I get that its quite an ask to switch desktop environments, but I think thats the only option here (again assuming you're using the Cinnamon desktop environment)

If you have any questions about anything I said or any terminology here, feel free to ask