I vote for some sort of tabbed design. If you weren't going with a tabbed design, how would you implement thread selection?

Drop down combo box.

but in the example given, that's messier than tabs. if it reorganizes the lines, how are you going to see something old and out of view that gets responded to?

(this is kinda stream of consciousness...)

Unless...after a certain time or number of lines idle a topic is dead and anything that resurects it starts its own thread, in a sense.

i have this above method and you have the tabbed method. we haven't talked project in a while, but you select your project tab and send a message. since that topic has been idle too long, on my end it shows up seperated from the old (maybe a line or two to refresh me) but at the bottom, but subsequent discussion on the topic still goes to your same tab. However, in the logs, all topics are grouped.

also, if not using tabs, the last line sent/received will determine the default topic.

tabs could get messy if someone is not using topics, and you try to send them something from a new topic and the respond with no topic you are back in your original tab. maybe if nothing is sent it assumes the person is responding to your last topic, but that could get messier than the way it currently works.