When selecting a specific person using the "Show messages by" option, one will see all of that persons messages on that same page (I believe), but not all of that persons messages on that thread (as was intended, I guess).
In any case, I believe that it is better be defined per thread and not per web page.
Do you agree?
This is also a very minor bug, and I guess that there aren't many users who are using it anyhow.