balsa 2.0.15 and earlier 2.x versions hang reproducibly when changing the
sort order while a message in a thread is selected. 
I'm using the pre-compiled redhat 9 rpm from http://balsa.gnome.org

To reproduce:
1. Select the second (or later) message of a thread in the message index.
2. Change sort order by clicking e.g. on the size tab

Result: program hangs and has to be killed manually.

The hang only happens if a threaded message is selected and not all threads
are expanded (i.e. at least one thread must be collapsed). 

Attaching gdb to the hung precess gives the following traceback:

+ Trace 41025