That actually doesn’t really work in real life - since when you’re at the last post and you click reply - the window opens and covers up much of the most recent post. Nobody would actually scroll down while they are composing a post.
So what I"m talking about here is a usability tested form of this feature where its actually usable. Ideally the page would auto-scroll as a new message is posted so that you can automatically see it as it comes in, even if you’re in the middle of composing a message.
In this way - you can adjust your post as need be while you’re still in the window.