I would love to bring this up again, had the idea before but I found it here and seen that it was suggested long time ago and had no feedback.
This would be great, sometimes you just want to chat with 3 or more people and it's really annoying to keep all conversations in one window, you get them all mess up and other times you are talking to someone but need to change tab, then when you are back to PM tab the conversation is not there and you have to PM the guy again, now imagine the guy was a random guy who wanted to trade something and had a random name that you cannot memorize that easily, you checked other tab and took you just 1 minute but when you want to go back to conversation you lost the name.
It would be great to have a mini window for PMs, so you can speak with people out of the normal chat window.