When a fingerprint device is present but no fingerprints are enrolled for
the
user, pam_fprintd causes GDM greeter instability at login. The symptom is a
visible black flash on the login screen. Greeter with user list shows ->
flash to tty -> flash to user list again. Then I can log in.

On kernel 6.12.63, the GDM greeter restarts (two gnome-shell instances spawn
in sequence). On kernel 6.12.73, it escalates to a full Xwayland crash that
takes down the entire GDM greeter session.

The root cause appears to be pam_fprintd returning a failure rather than
PAM_IGNORE when no fingerprints are enrolled, causing GDM to restart or
crash
its greeter compositor.

Hardware: Intel Meteor Lake (Goodix MOC Fingerprint Sensor)
Display server: Wayland (GDM/GNOME Shell 48.7)
OS: Debian trixie

Workaround: disable fingerprint authentication in pam-auth-update.

Confirmed with:
  fprintd-list jan
  → "User jan has no fingers enrolled for Goodix MOC Fingerprint Sensor"