Close Preview and remain on detail when back selected

Well, it would mitigate the problem very greatly if the only thing we suppressed when typing in the address bar was the URL that is 'back' from the detail view.
Other ideas:
- Remember the length of the history stack each time current URL changes; suppress URL change only when history stack is getting shorter (and preview active).
- Stick something into the URL when we bring up the preview. (Something like what I did to give the old detail view its own url even though it looked like a dialog.)