title: Prevent state change on back button with UI-Router
body: <p>There is a page editor in my app, and I want it to prompt a user to save if they try to navigate away when there are unsaved changes. I managed to cover my bases with regards to window reload and links, but I can't seem to prevent UI-Router from engaging in the state change with a back/forward button click. </p>
  
  <p>I am trying to do it from the <code>$stateChangeStart</code> event and am a little unsure of how to halt the state change. I have also tried <code>$window.onpopstate</code> but it seems to be one step behind the router.</p>