[CentOS] CentOS 7.1 user login screen can't scroll up/down

After we upgraded our CentOS 7.0 desktops to CentOS 7.1, a critical error in the graphical login screen has appeared on all 7.1 machines:

We have 100+ users defined in /etc/passwd, and a list of names is presented on the initial login screen. However, it's impossible to scroll up or down in this user list to select the desired user. The middle mouse button seems to be disabled, so scrolling has become impossible! One can use the left and right mouse buttons to select one of the users in view, but no one else.

This seems definitely to be a critical bug in CentOS 7.1.

Workarounds:

1. Log in non-graphically using Ctrl-Alt-F2, then run the command startx.

2. Trim down /etc/passwd to 1 (or a few) relevant users.