Mesa-git is currently broken in Siduction experimental repo.
Steam segfaults with 20.2.0~git20200708-1 and newer. Stay on the older builds for now, or downgrade to 20.1.2-1 from testing.
Towo's bug report: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3254 (https://gitlab.freedesktop.org/mesa/mesa/-/issues/3254)
Not mesa-git is broken, but binutils.
https://sourceware.org/bugzilla/show_bug.cgi?id=26263
I don't know whats broke
All I know is that steam and wine work perfect and haven't broke on 2 machines here
(http://piper-sid.net/myshit/linux/images/thumb-steam.png)
(http://piper-sid.net/myshit/linux/images/steam.png)
(http://piper-sid.net/myshit/linux/images/thumb-steam1.png)
(http://piper-sid.net/myshit/linux/images/steam1.png)
Maybe you don't use mesa 20.2-dev from our experimental.
Hardcore gamer eh, Piper? Living on the bleeding edge I am sure!