Clicking bottom inventory items while another player's inventory is opened up at the top fires InventoryClickEvent with incorrect items and slots

Expected behavior

InventoryClickEvent should not report incorrect slots and/or items on the bottom inventory when another player's inventory is opened up in the top.

Observed/Actual behavior

When having another player's inventory opened up, attempting to click an item from the bottom inventory (our own inventory) will fire an InventoryClickEvent with an incorrect slot/item:

https://github.com/user-attachments/assets/e8c541ee-25a4-4a4b-ba79-af4f613645a3

I have tested the following versions, all of which have the issue:

• Paper 1.21.1 build 10
• Paper 1.20.6 build 148
• Paper 1.18.2 build 388
• Paper 1.14.4 build 245

The following versions do NOT have the issue:

• Paper 1.13.2 build 657
• Paper 1.12.2 build 1620

Steps/models to reproduce

Example repository: https://github.com/xwmi/paper-bug-poc (check the legacy branch for a version of the plugin that works on older Paper versions using a Diamond instead of netherite hoes)

1. Create an InventoryClickEvent listener that will cancel movement for a specific item (e.g. a netherite hoe):

    @EventHandler
    public void onInventoryClickEvent(InventoryClickEvent event) {
        ItemStack currentItem = event.getCurrentItem();
        int slot = event.getSlot();
        int rawSlot = event.getRawSlot();

        // Broadcast some debug information. We could also just print it to the console.
        String itemType = currentItem != null ? currentItem.getType().name() : "none";
        Bukkit.broadcast(Component.text("The clicked item type is " + itemType));
        Bukkit.broadcast(Component.text("The clicked slot is " + slot));
        Bukkit.broadcast(Component.text("The raw slot is " + rawSlot));

        // Let's not cancel the event if it isn't a Netherite Hoe
        if (currentItem == null || currentItem.getType() != Material.NETHERITE_HOE) {
            return;
        }

        @NotNull HumanEntity entity = event.getWhoClicked();

        // This should, in theory, prevent a Netherite Hoe from being moved
        // around the inventory when the top inventory is from another player
        if (entity.getOpenInventory().getTopInventory().getType() == InventoryType.PLAYER) {
            event.setCancelled(true);
            Bukkit.broadcast(Component.text("Cancelled Netherite Hoe movement " +
                    "because the top inventory is from a player"));
        } else {
            Bukkit.broadcast(Component.text("Not cancelling Netherite Hoe movement " +
                    "because the top inventory is not from a player"));
        }
    }

2. Create a command that will open another player's inventory like this (EssentialsX has an /invsee command that will also work for this):

Player target = ...
player.openInventory(target.getInventory());

3. Attempt to click the Netherite Hoe or some other slots around it. The event will report as if a completely unrelated slot was being clicked:

Plugin and Datapack List

> pl
[21:22:48 INFO]: Server Plugins (1):
[21:22:48 INFO]: Paper Plugins:
[21:22:48 INFO]:  - PaperBugExample
> datapack list
[21:22:51 INFO]: There are 3 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [paper (built-in)]
[21:22:51 INFO]: There are no more data packs available

Paper version

> ver
[21:23:12 INFO]: This server is running Paper version 1.21.1-10-master@098bd39 (2024-08-11T22:37:37Z) (Implementing API version 1.21.1-R0.1-SNAPSHOT)
You are running the latest version

Other

No response

Start with the InventoryClickEvent listener and the player.openInventory(target.getInventory()) reproduction from the linked PaperBugExample repository. Compare the reported slot and item across the listed Paper versions, especially the change between 1.13.2 and 1.14.4. Done means clicks in the bottom inventory report the correct item and slot when another player's inventory is open.