changing the whole URL without the page flash

My coworker pointed out that when you're navigating through Youtube, you can go from page to page and the URL completely changes (i.e. it's not just changing stuff after the # anchor) but parts of the page, such as the side bar, remain constant (tampering with it with Inspect Element proves that it's the same DOM elements, not new copies each time)

I guess that's a thing, now! With appropriate js shims for older browsers. I don't have a reason to implement a proof of concept for myself right now, but it's a good thing to know about.