Each back/forward entry currently represents a navigation + view state. Differing view states cannot create new back/forward entries. View states store the effects of user interaction within a content view, provided that the interaction doesn't create a new navigation.
Saved view state is currently limited to scroll positions of elements nominated by ContentView.prototype.get_scrollableElements. But, some view state that's important to the user (namely, selection ranges in the timeline) are not implemented with DOM scrolling, so that view state is not handled. ContentViews should be able to
.. provide custom save/restore functions. Importantly, these should not save into the BackForwardEntry's cookie, since that is for navigation state only.