I'd maintain that an important property is for the system to be eventually consistent with regards to history. You don't want a transient network event to potentially result in two users permanently seeing messages in a different order.
During a netsplit, people chatting on opposite sides of the netsplit continue to be able to chat (by design), but will (obviously) see a different history from each other. So when the netsplit heals, you have a dilemma: either you splice the history from the other side in, giving eventual consistency at the cost of changing the history that people have already read, or you keep permanently different histories on servers that were on one side or the other.