The latest version of Discourse has a different scrollbar. In the right sidebar, you’ll see a timeline, with the first date being the thread start date, the last one being the last post date. You can drag the slider around to quickly jump to a certain post in the thread. You’ll also see the reply button has moved to the sidebar.
Hope this isn’t too confusing, it seems pretty easy to use. And before anyone asks, no I can’t put it back how it was… this change is coming direct from the Discourse forum developers.