AnvilUpdateEvent is called only when both inputs are estabilished. I understand the safety implications, and it does make sense to check if the left item exists before doing anything, but the Anvil can be used even with just one item and requiring the right slot to also be filled cuts the AnvilMenu method in a weird spot where a user doesn't have full control over the block's behavior.
I'm working on 1.18.2, so i don't know if 1.19.x is any different, but since anvils have worked like this for the longest time it's probably the same as well. Feel free to correct me though.
Example: i'm making an anvil customization mod. Amongst other things, i planned for a feature that allows users to change the level cost of renaming an item. As of now, this feature would only work as intended if two items are in the anvil, while renaming with only one item in the ui would run vanilla behavior no matter what.
Edit: i wasn't sure if this counts as a suggestion or an issue so i had to choose one. Please do tell me if this topic should go somewhere else.
