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