> Yes a client may be out of date and therefore show something different, but once it becomes up to date it should be showing the same state even if that means amending history. Why? Because the humans reading it will be confused otherwise! An app getting more data is something we intuitively understand, but if my client shows something and yours shows something else, we will conclude different meanings from it.

That's interesting because I have the complete opposite take and would hard disagree with this. I intuitively understand that if we both write messages at the same time, we will see them in different order. Snail mail has worked this way for centuries, and I very much prefer this to an app silently altering the content as time goes. It is confusing when it happens under my eyes (something moved at the top of the screen while I was reading the bottom, what was it?) and easily leads to missed messages especially in group conversations (my buddy sent a message with a poor connection at 11am, it is retried and sent at 2pm and appears before the lengthy discussion others had at noon).