Just fixed it. The last state used before posting a message is the state you get when you get redirected after posting. There was already some code to manage this behavior, but was buggy.
I've tested again and now, the threads appear collapsed by default.
So when I create a new thread, after submitting, the thread appears collapsed. When I go to an existing thread, it's collapsed, I expand it to answer one of the message, and after submitting the thread is collapsed again.
