This looks like the same problem I was having running any gui app as root from a terminal window using sudo or su in both LXDE and Mate. 

https://bugzilla.redhat.com/show_bug.cgi?id=1403041

Solution was to add the following line to /etc/pam.d/su and /etc/pam.d/sudo:

session optional pam_xauth.so systemuser=999