Thanks. I'll probably write up a blog post of how it's done eventually but the short version is we use HTML5 replaceState to update the URL as you scroll down a

Thanks. I'll probably write up a blog post of how it's done eventually but the short version is we use HTML5 replaceState to update the URL as you scroll down a topic stream, so you have a unique URL to go back to and share. There is a little more intelligence as links within a topic just replace the state, but links to other topics push the state. Also infinite scrolling downwards is easy to implement. Upwards is a huge headache.