That depends on the DM. Some people roll their damage dice with their attack roll to save time. That said, RAW, it would be before damage is rolled, but I don't think it is unfair if it happens afterward.
Personally, I think it behooves the DM to be aware of what interrupts the PCs have, and if not actually suggest them, then to make sure they leave an appropriate amount of time for the the player to respond.