How does 4chan's thread updater work?

How does 4chan's thread updater work?

In Futaba, their thread update function makes an XMLHttp request and reloads the new page and scrolls you back down to the spot of the screen you were viewing. However, this means that if you had any text in the form field, it'll be gone due to the reloading of the page.

However, in 4chan the form field is untouched when a thread is updated. Whatever it's doing, it seems a lot more fluid and seamless.

So how does it work, any ideas? I tried looking around the source but it's a little too zomgoptimized for me to make sense of it.

>>1
I seriously hope that by "the source" you mean https://github.com/4chan/4chan-JS/blob/master/extension.js but your post reads dumb enough that you might not. Not that you need to read any code to guess what's it doing.

By the way and just out of curiosity, have you never visited Gmail (or any other webmail service in the last year) or Twitter or GitHub itself? Is this the first time this question pops in your head?