Hi there,
I was testing the latest 1.4.0 upgrade I performed today. Looks great by the way, nice new features! 
One thing I noticed though, and I don’t think it’s expected behaviour, is that I was able to remove a product line item on a saved invoice that was in “read only” mode. The little red bin icon on the far right of every product line item shows even on a “read only” invoice, and I am able to click it and it removes the line item completely. I can constantly reproduce this behaviour.
I think that defeats the purpose of the “read only” setting on the invoice, but perhaps I’m missing something?