Switching desktop environment from Unity?

Use the tab key to scroll down the list, then press enter to select the DE, then tab more until the OK button, then press enter to select and return to the password screen. Yes, it sucks.

You have a few solutions here:

1. You could change the default session to be what you want using lightdm-set-defaults. If you've already set a different session, this may not work, I think it will prefer the session you chose last. The session name should match the file name in /usr/share/xsessions, without the .desktop on the end. I think maybe the default session moves to the top of the list though, so this may still work for you.

   /usr/lib/lightdm/lightdm-set-defaults --session <session>

2. You can go to /usr/share/xsessions and rename the ones you don't want to something besides <session>.desktop, like <session>.desktop.old. I think that will hide them. You could also just uninstall the desktop environments you don't want.

Lastly, please take a screen shot of this and file a bug against unity-greeter for this list not fitting properly.