addItem problem with offhand and full inventory

[ssomar1607]

Expected behavior

Hello.

The following method when there is a space to add the item should add the item in the inventory:

PlayerInventory inv = p.getInventory();
inv.addItem(new ItemStack(Material.DIAMOND));

On my screenshot, I have space in my offhand, so it should increase to 37, 38, 39 up to 64.

Observed/Actual behavior

But currently, it doesnt work when the space is in the off hand.
The additem method return a non empty Map.

Steps/models to reproduce

Create a small plugin with

PlayerInventory inv = p.getInventory();
inv.addItem(new ItemStack(Material.DIAMOND));

when you do a specific action

put at least one of the item you add in your offhand and fullfill your inventory with another item

Plugin and Datapack List

[21:25:21 INFO]: - AceStore, CustomPiglinsTrades, ExecutableBlocks, ExecutableEvents, ExecutableItems, LuckPerms, MyFurniture, NBTAPI, packetevents, PlaceholderAPI
[21:25:21 INFO]: SCore, Vault, WorldEdit

Paper version

[21:25:29 INFO]: This server is running Paper version 1.21.4-76-main@486e521 (2025-01-05T18:15:54Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.4-53-326c687 (MC: 1.21.4)

Other

No response

[Warriorrrr]

It might make sense if this method followed the same logic as item pickups in vanilla, which is first putting items into the offhand if it contains an item of the same type. If the offhand is empty then it also shouldn't try to put anything in it