I hate hate HATE new windows. It reminds me of IE or MS outlook manage this particular problem. I like this idea of a smaller chat like window, it's really blurring the line between chat and email once again. (Like google did when they started chaining emails together. Remember when that wasn't even a thing! I do, cuz I was amazed when google started doing it.) I think google really gets me when it comes to how I like to communicate via email..
Gmail's innovation wasn't threading, but conversation view - displaying all the messages in a thread (linearly)in a large scrollable region rather than one at a time.