Comment 87 for bug 1968040

Thanks Tolga. It looks like the root cause of the issue on your machine is a lack of display bandwidth when trying to drive 2 screens through the same cable:

May 11 21:08:51 messier32 kernel: i915 0000:00:02.0: [drm:intel_dp_mst_compute_config [i915]] Trying to find VCPI slots in DSC mode
May 11 21:08:51 messier32 kernel: i915 0000:00:02.0: [drm:intel_dp_mst_compute_config [i915]] DSC Source supported min bpp 18 max bpp 24
May 11 21:08:51 messier32 kernel: i915 0000:00:02.0: [drm:intel_dp_mst_compute_config [i915]] DSC Sink supported min bpp 0 max bpp 0
May 11 21:08:51 messier32 kernel: i915 0000:00:02.0: [drm:intel_dp_mst_find_vcpi_slots_for_bpp.constprop.0.isra.0 [i915]] failed finding vcpi slots:-22
May 11 21:08:51 messier32 kernel: i915 0000:00:02.0: [drm:intel_atomic_check [i915]] [ENCODER:106:DP-MST C] config failure: -22

I'm not sure if that's an "expected" or "normal" error which we should expect to see on some setups forever, or if it's a kernel bug. If it's "expected" then ideally mutter/gnome-shell should fail more gracefully but I don't know how that's possible given how limited the atomic API is. You can never find out _what_ failed, only that something failed. I hate atomic KMS.

Did the workaround work for you?

  MUTTER_DEBUG_ENABLE_ATOMIC_KMS=0
  MUTTER_DEBUG_FORCE_KMS_MODE=simple

in /etc/environment