Description of problem: sudo <gui-app> gives the error message
<gui-app>: Cannot open display: 
for values of gui-app equal to everything I've tried in my Rawhide-Mate installation and todays Rawhide mate and LXDE live isos.


Version-Release number of selected component (if applicable):
pam-1.3.0-1.fc25.x86_64


How reproducible:
100%

Steps to Reproduce:
1. Boot to Mate or LXDE desktop
2. Open terminal window
3. sudo leafpad or sudo pluma or sudo any-gui-app

Actual results: (shown for leafpad from LXDE live session)
[liveuser@localhost-live pam.d]$ sudo leafpad
leafpad: Cannot open display: 

Expected results:
leafpad runs as root and opens as expeced

Additional info:
Found the two lines below online. Adding these lines to /etc/pam.d/sudo made the problem vanish. However, I don't know what I'm doing here and have no idea why it works.

# Forward xauth keys between users if invoker is root or UID 1000 or higher
session optional pam_xauth.so systemuser=999