Bukkit.createInventory() is weird

[Shesper]

Expected behavior

After creating an anvil inventory with the Bukkit.createInventory() method, PrepareAnvilEvent must be called when controlled after it is opened to the player.

Observed/Actual behavior

However, PrepareAnvilEvent is not actually called and the inventory behaves as if the InventoryClickEvent was cancelled, and the item disappears on SHIFT+LEFT click control.

Steps/models to reproduce

The code is like this:

Plugin and Datapack List

Paper version

Other

No response

[electronicboy]

This is not a bug as much as it's long missing functionality from bukkit, not all createInventory method calls will end up creating a "real" inventory of a specific type, right now, you have an inventory that looks like an anvil, but has literally 0 of its functionality, the only reason you see anything happening otherwise is because the client has its own logic which "predicts" the outcome

[Sytm]

You probably want to use this "family" of methods instead because it opens real inventories

[Lori3f6]

I have searched some posts and found the problem has been persisted for a long time, as discussed in this SpigotMC post.
The anvil inventory instance created by server.createInventory is a instance of CraftInventoryCustom, which as this post states, it is not a real AnvilInventory and can not cast as well. Additionally, it does not invoke PrepareAnvilEvent.
The biggest missed function of this is fake anvil inventory is the plugin unable to fetch the content inside its text bar.
It would be nice if paper add a implementation for it someway. For instance, maybe opening the UI from a real anvil created internally, not present in the world?

[Machine-Maker]

It would be nice if paper add a implementation for it someway. For instance, maybe opening the UI from a real anvil created internally, not present in the world?

There is a way, the openAnvil method that was mentioned above your post. You can customize the title too with the InventoryOpenEvent title override.

[Warriorrrr]

Closing as a duplicate of #5825, use the openAnvil method instead as a workaround