- Switch to gdm and wait for the lock screen to show up in gdm
- Press some keys
- In the now revealed user list select a user
- Now in the password entry already contains the characters entered in the lock screen

While this is a useful behavior in the regular session lock screen, it is much less useful and even annoying in the case of the login screen, since if you use the keyboard to unlock the screen, you now have to delete some characters before you can start entering your password.