Comment # 1 on bug 958366 from Dominique Leuenberger

potentially gdm 3.24.2 finally addresses this.. from the .changelog:

- Update to version 3.24.2:
  + Update session type when we know username (and user's session).
  + Fix bug leading to reauthentication happening outside the
    session.
  + Make sure greeter is killed and reaped when gdm is restarted.