The experience you're describing still doesn't need to break the back button. Going back means going back, not closing a window I never opened. If that's an awkward experience, don't build one that works that way.

Fair ‘nuff, and I agree, but would they be able to argue that they never explicitly “broke” the back button?

I remember when JavaScript became a big Web site driver. The arguments against using it to fetch and build content usually included broken back button functionality.

I don’t think a lot of folks really paid much attention to it, though.