This was new. X forwarding broke after I su'ed to another user. What previously worked says
Connection lost to X server `localhost:10.0'
after a su. It's the magic cookie that isn't there for the new user. So, how to add the cookie for the new user?
We're on 10.
Hostname/unix:10 MIT-MAGIC-COOKIE-1 f4bb4714280c22018cacd461277dd760
Copy the entire line for the display and su to your user.
xauth add Hostname/unix:10 MIT-MAGIC-COOKIE-1 f4bb4714280c22018cacd461277dd760
And xclock works!