Bug#716770: Can not select user in gdm3 to log into when you have many users

[opensource at tigusoft.pl]

Package: gdm3
Version:  3.4.1-8

If you have many local users, gdm3 will show a list of them, but it can not be 
scrolled.
There seems to be no option to even enter username (in gdm3) manually!

Such user would find him unable to log in (without help of textmode console or 
some administratice action).

E.g. seen this when locking screen (in xfce4 4.8.0.3) and selecting New Login.