Hi
>    I _think_
> it's when you view the senders of some message in the currently viewed
> method, you get the senders spliced into the viewed list, indented a
> bit.
>
to get this behaviour the Preference "trace messages" needs to be enabled.
checking....
Right now this only works for implementors, senders spawns a new window. 
4.4 #12332

Cheers

Herbert