This already exists - in the list of posts! It's a small red-brown sheet of paper with an arrow pointing to it! This icon is to the left of the post title. I'm using Mozilla, so when replying to posts I middle click on all these arrows to open all threads with new messages on new tabs.
Yes, I have noticed that - but I was refering to the threads. You are refering to the "overview" ?
Some threads have lost of posts and when pressing the mentioned button the browser should scroll to (or load the page - then scroll to) the "first unread post".