The Alias option on the right-click menu in the Buddy List has an
unfortunate interaction with window managers configured with focus
follows mouse. Selecting Alias highlights the current buddy name so
that it can be replaced, but that highlighting is immediately lost if
the Buddy List loses focus. Since the right-click menu is frequently
longer than the Buddy List menu (or at least the vertical space
remaining in the Buddy List menu), selecting Alias in a window manager
with focus follows mouse highlights the buddy name and immediately
unhighlights it due to the window losing focus. It makes the menu option
appear to do nothing.
The workaround is to make the window large enough so that the mouse is
still over the Buddy List menu after selecting Alias from the menu.
The best fix would probably be to leave the highlighting of the buddy
name persistant even if the window loses focus, but I don't know if this
would break something else.