Or better yet, just... Navigate to a new (real) web page? Maybe passing some state via URL parameters? At least then the back button has a reasonable chance of working correctly.
It's not hard to get perfect back button behavior with SPAs. You just have to care enough to do it _and_ have written code decent enough that it can be shimmed in.
I believe I was only talking about a single page load.
Instead of a choice between progressively loading in /page-a or waiting for /page-a to fully load before showing it, just direct the user to /page-b instead?