Every new team member we add gets bitten in Differential by the fact that they can reply inline to code review comments and miss the subtle "not yet submitted" text next to every comment. It is not obvious to them that if they have no overall comments they still need to scroll to the bottom and press submit.
Some solution to make that more obvious or auto submit would be a good thing.
Would it be possible to have a popup that says "You have unsubmitted comments. Do you really want to leave the page?" on exit?