Yeah I could have been a bit more explicit, but was typing on the phone. I’ve actually wanted this feature myself. Thing is though, assuming we were in sync when Syncthing was shut down we will still be in sync when Syncthing starts up. We must then connect to other devices in order to realize we are possibly not in sync. But not all other devices might be online or reachable. Do we wait forever or just a while? What if one device disconnects and another connects during that time? Do we wait for other devices to become in sync or just ourselves? What if they can’t? etc.
(rsync or unison, on the other hand, are great at just starting up and doing a one shot sync before exiting)