Marker: on "no more to grade", we return to a stale preview
This is pretty rare: we don't finish grading that often, compared to saw #1966 (closed) which I hit all the f'ing time.
I'm fixing #1966 (closed) but can still reproduce this, so filing a new issue to track it. Its been known for a time (TODOs in the code). I'll link those TODOs to this issue at very least.