Add info about mutter primary gpu

docs/guides/hybrid-graphics.md

@@ -44,6 +44,12 @@ If using the iGPU causes the screen to be black after waking up from suspend, th
 - Add `i915.enable_guc=3` to [your kernel parameters](https://wiki.t2linux.org/guides/postinstall/#add-necessary-kernel-parameters). If that has a problem, try setting the value to 2 instead of 3.
 - Turn the screen off and on after the backlight turns on. For GNOME: type your password then press enter, press Command + L to lock (this should turn off the backlight), then press any key.
 
+## Using iGPU as primary gpu (Mutter)
+
+Mutter-based desktop environments (e.g. GNOME) pick one GPU to use as the "primary GPU", and it's not necessarily the same as the one connected to the display. Even if apple-gmux is configured with `force_igd=y`, Mutter's primary GPU might be the AMD GPU, which makes the AMD GPU active when screen contents change.
+
+See [Mutter Multi-GPU documentation](https://gitlab.gnome.org/GNOME/mutter/-/blob/main/doc/multi-gpu.md) for more info, including how to change the primary GPU.
+
 ## Use on Windows
 
 The iGPU only works on Windows if there's no driver for it installed. Windows likes installing drivers.