GDM login screen shown on VR display instead of main monitor.
Mutter, when used by the GDM login screen, happily creates outputs for attached VR HMD devices that are marked with the 'non-desktop' KMS connector property by the kernel. This results in the login screen being shown between the eyes inside an attached VR HMD instead of on the main monitor on my system. Meanwhile, the monitor only shows an empty background.