Can't start X11 applications after `su` or `su -` to another user

Issue

• X11 connection rejected because of wrong authentication (all Red Hat Enterprise Linux versions).

• Error: Can't open display:
  (and no display is listed after "display:"). Red Hat Enterprise Linux 7 only

• Environment variable for DISPLAY is not forwarded to another user when running su $USER su - $USER or sudo su - $USER (Red Hat Enterprise Linux 7 only).