On my system, after GDM displays the initial login screen, pressing the up / down arrow keys or the tab key to choose a different user causes the width of the box containing the user list to slightly increase each time. This can continue until it expands to fill the monitor horizontally.

If I actually select a user to login as, thus bringing up the password prompt, the box shrinks back to the normal size. After that, if I press escape to get back to the user selection, the bug disappears and the width does not change when I toggle between different users.