The problem

Marc is a wedding photographer. He keeps one folder on his laptop and the same folder on his phone: client notes, contracts, and video clips up to 2 GB.

On the train he edits notes.md on his phone with no signal. That evening, at home, he edits the same file on the laptop before the phone has synced.

When both devices come back online, he expects to lose neither edit, and he does not want 2 GB resent over mobile data when he trims the end of a clip.