XFCE + Hybrid Graphics = No Window Manager or Desktop

[AutoDMC II]

Hello, everybody.

TL;DR: __GLX_VENDOR_LIBRARY_NAME=nvidia seems to break XFCE; I get only a black screen with a mouse pointer. Works fine without that flag in /etc/environment

My machine has my main monitor (USB-C/DP monitor) plugged into the USB-C port of my motherboard; my secondary monitor is plugged into my NVIDIA 3090. My main monitor is an Apple Studio Monitor (from my MacOS days), and I want to continue using the webcam and audio equipment, so simply using a DP/USB-C cable won't fly.

Set up Hybrid Graphics per the Wiki Page (https://wiki.gentoo.org/wiki/Hybrid_graphics) as the little integrated silicon on my chip hooked to the primary monitor isn't there for gaming.

After following the instructions on Hybrid Graphics, running startxfce4 gave me black screens with a mouse pointer and no WM or anything else. Have to control-alt-f2 and kill XFCE to get anywhere.

After a lot of double checking, debugging, forum reading, searching, and recompiling XFCE, I discovered that putting __GLX_VENDOR_LIBRARY_NAME=nvidia into /etc/environment stalls the XFCE window manager. The WM runs, it's process is in ps, just nothing displays.

prime-run works fine to manually push a OpenGL apps to the dedicated GPU, but I really don't want to have to micromanage that.

Anyone have any ideas on how to continue troubleshooting this?

(The most fun bit of this whole 3 day adventure is that no logs anywhere complain about the GLX environment causing the WM to fail; many a red herring was followed.)