This problem was already reported for GDM 3.4.1-2+b1 in bug #688741 and is still present in the final version of Jessie (GDM 3.14). Maybe we should merge #688741 and #759360 (or close #688741 and continue with #759360).
There's also a bug report in Gnome bug tracker regarding this issue ( https://bugzilla.gnome.org/show_bug.cgi?id=684773) Looking for a workaround I was able to disable the user list by setting "disable-user-list=true" in /etc/gdm3/greeter.dconf-defaults, which is fine for some scenarios but not for my computer at home.
