Comment 8 for bug 918072

I think I know what's happening. I have the same problem in a ASUS EEE-PC 1005HA, with 1024x600 screen resolution.

When Lightdm is opened, the "OK" button, where is possible confirm the selected session, doesn't appear because there are no space at screen. Then, it's impossible select another session because I can't confirm the selection.

Then, I've tryed an workaround:
1) sudo apt-get install dconf-tools
2) open dconf-editor
3) go to "com/canonical/unity-greeter" and change the xft-dpi option to a lower value. The font's now appear smaller
4) To test without need restart lightdm, run "lightdm –test-mode"

Now It's possible change the session, but I can't click at first button yet, and sounds like the button's clickable areas are now wrong. For example, the second button click area is almost at button base.

I think, then, this is a problem at lightdm theme.