Comment # 4 on bug 979190 from Egbert Eich

(In reply to M.Hanny Sabbagh from comment #2)
> I just installed LightDM instead of GDM, the session was GNOME Shell, I
> started Steam by clicking it's icon from the applications menu (and running
> from command line gave the same result).
> 
> I am not sure about other applications, it's just Steam for now.

Please be so kind to try other applications. 
Apparently, you were able to start a terminal and use the command line. So not
all X applications seem to be affected.
Thus this seems to be more of a Steam issue than a problem with lightdm.

When you go back to GDM, does it still work?