Ah, darn.  So, just to clarify, I think the following behavior will be possible:

1) Click send for draft 'BBB' to a person with whom I have a separate thread with another draft 'AAA'
2) Composer is cleared of message 'BBB', but original 'AAA' draft is not deleted yet
3) Message doesn't send successfully
4) Composer remains empty
5) Go back to ThreadList, go back to the assimilated thread
6) Composer is now populated with draft 'AAA' (because it never got deleted, because message never sent)

I'm not suggesting we address this, just wanted to point out this may happen, I think?