According to my own experience, su - user used to break SSH X11 authentication. There are plenty of posts on the web explaining how to use xauth list and xauth add to copy the MIT-COOKIE between sessions.
However, for some reason this is no longer necessary.
When I login using ssh -X usera@host, I see the following as usual:
-rw------- 1 usera usera 63 Jul 25 18:17 .Xauthority
When I do su - userb, the system automatically creates and uses the following:
-rw------- 1 userb userb 63 Jul 25 18:18 .xauthpNLBVw
The new .xauth file contains a copy of the MIT-COOKIE of the previous user, thus allowing me to run X11 apps. It's not necessarily a bad idea, but how is this accomplished. I've tried RHEL 5- 7 and it's the same on all those systems, even though nothing has changed that would explain this new behavior.
Any ideas? Thanks!