TL;DR: They used an iframe for the content. But this brings with it the navigation issues that were the reason we abandoned frames: The URL in location bar no

TL;DR: They used an iframe for the content. But this brings with it the navigation issues that were the reason we abandoned frames: The URL in location bar no longer reflects the location of the specific page, so it can't be bookmarked easily. Also, when opening in a link in a new tab, we get the reverse of this problem in that the other frames are lost. You could solve this with JavaScript that updates the location, I suppose, but then the result is essentially an SPA.