Oh, try sitting on the homepage. You’ll get a N new topics. Click to see. message. There’s something weird where it won’t show updates to threads that aren’t either new or that you’re tracking, so I hope that gets fixed soon.
And topic pages themselves autoupdate too, and it’s pretty obvious when it happens - to see this, create an account on try.discourse.org, open a thread in a second browser, and reply to the thread in your first browser.