Getting X11 forwarding through ssh working after running su

Getting X11 forwarding through ssh working after running su

$ xauth list $DISPLAY
You'll get something like

somehost.somedomain:10 mit-magic-cookie-1 4d22408a71a55b41ccd1657d377923ae

Then, after having done su, tell the new user what the cookie is:

$ xauth add somehost.somedomain:10 MIT-MAGIC-COOKIE-1 4d22408a71a55b41ccd1657d377923ae