Function awaits_seal_history 

fn awaits_seal_history(state: &TransportState, slot: &SessionSlot) -> bool

Whether slot’s copy is missing rows its epoch opened on.

The mark describes the LATEST seal, so between an occupant change and its seal’s mint the value is still the previous seal’s - which is what notices that gap whenever the previous epoch carried rows. A seat whose first epoch is pending, or whose previous seal was empty, has no such mark, and a read in that mint gap can still synthesise a copy.