(In reply to Olivier Fourdan from comment #10)
> (In reply to Olivier Fourdan from comment #9)
> >   glamor_composite_choose_shader:       Unsupported source picture format.
> >     glamor_composite_with_shader:       glamor_composite_choose_shader failed
> 
> I see the same with intel and yet rendering is correct there, so it's not
> that.

Unless the issue is with the fallback code in glamor, i.e.:

https://cgit.freedesktop.org/xorg/xserver/tree/glamor/glamor_render.c#n1699

Calling the fallback code unconditionally (i.e. "goto fail;" at the beginning
of glamor_composite() ) still exhibits the issue on nvidia/nouveau and not on
intel, but then it doesn't seem to be specific to nouveau... I am confused now.