Browser history is hard. Even github can't get it right (I found a bug writing this: Click "Raw" and then go Back in Chrome).
AB test single-page-checkout vs. 3 step checkout with minimal code duplication for fewer bugs and chances to pollute the test. 3-step checkout is already in place and refactored using a combination of emerson views and traits.
<div data-view='checkout' data-checkout-path='checkout/customer'> ... </div>