Swapping items into offhand bypasses inventory conditions

[spannercodes]

I have an inventory condition which I apply to players to prevent them from moving items in their inventory:

public static final InventoryCondition readOnlyInventoryCondition =
    (player, slot, clickType, result) -> result.setCancel(true);

This works, except items in the hotbar can be moved by being put in the offhand (using the keybind), then by moving the held slot, and swapping back.

The swap should first check against the player's inventory conditions and respect the result.

This would affect PlayerDiggingListener#swapItemHand (line 153)

A temporary workaround for anyone else with this issue is to listen for PlayerSwapItemEvent and cancel it.

EDIT: More experimentation shows this bug also applies to armour and the elytra when right-clicked.

[mworzala]

Will be closed by #1992