Opening another page now ends the edit instead of letting Save write its
draft onto a copied page's lineups. The edit records only the ends that
were dragged, so an undo made while it is open is not reversed on Save.
A drag adds up its deltas: the first update reports where the pointer went
down, which put the end one step ahead of the pointer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>