It's a server authoritative system, so the final state is determined by the server. There's a single global order of changes (basically the order the changes were processed by the server), so any conflicts are basically resolved by the server, but are applied optimistically on the clients when online or offline.
I can't tell exactly from your question if you're asking about the client going offline before or after making the change, or before or after recieving the server's version of the change, but either way the servers version wins and the client will have to catch up after it's back online.
You nailed it! In SQLSync, conflict handling happens in a WebAssembly module provided by the developer (for now). This module is free to handle conflicts in any way you see fit. You could even embed CRDT state into a table cell if you wanted to do rich collaborative text editing for example. My goal is to solve replication/durability and leave the specifics of conflicts up to the developer.
I can't tell exactly from your question if you're asking about the client going offline before or after making the change, or before or after recieving the server's version of the change, but either way the servers version wins and the client will have to catch up after it's back online.