Problem with history system and back button when revisited later

[zepheiryan]

Browse an Exhibit. Leave it in a non-vanilla state (state "A") by visiting another site. Come back to the Exhibit and do some more browsing within it, then try to go back to state "A" using the back button. It will fail to go all the way back, stopping at state "A"+1 and not showing state "A" before moving farther back past it to the external site.

[zepheiryan]

This isn't deterministic. There are also problems with subsequently erasing forward history; that is, if one navigates back to an Exhibit from an external site, all forward history will sometimes be eliminated.