Generally filling up the buffer is fine. It's perfectly normal that after viewing a certain number of documents one has to manually decide where to go.

You know what you need. A button that goes back but instead of going to the prior document goes to the first document with a different domain than the current one.

Yeah, I'm not convinced it's really the site's fault... I can understand the logic of wanting the back button to go back a channel, but also I generally want to go "out" of a page when I hit the back button, so there's no real one-size-fits-all answer.

But speaking only for myself, if history.pushState was removed from my browser I would probably be happier in the end. JavaScript SPA's abuse it 10x more often than they use it appropriately, I'd rather that if you're an SPA anyway, you implement your own in-app navigation and let my browser's mean "get me out of this app". Lord knows SPA's already reinvent every other web standard, why not one more...