transferSlotTryOffhand

    @ModifyArg(method = "method_30010", index = 1, at = @At(value = "INVOKE", target = "Lnet/minecraft/screen/ScreenHandler;transferSlot(Lnet/minecraft/entity/player/PlayerEntity;I)Lnet/minecraft/item/ItemStack;"))
    private int transferSlotTryOffhand(PlayerEntity playerEntity, int i) {
        ClientPlayerEntity player = MinecraftClient.getInstance().player;
        if (player == null) return i;

        ItemStack offHandStack = player.getOffHandStack();
        if (offHandStack.isEmpty() || offHandStack.getCount() == offHandStack.getMaxCount())
            // no stack or no room in stack in offhand
            return i;


        PlayerInventory playerInventory = player.inventory;
        // index: 9-35, 36-44
        // clickedIndex: 9-35, 0-8
        int clickedIndex = i > 35 ? i - 36 : i;
        ItemStack clickedStack = playerInventory.getStack(clickedIndex);
        if (!ItemStack.areItemsEqual(offHandStack, clickedStack) || !ItemStack.areTagsEqual(offHandStack, clickedStack))
            // stacks can't combine
            return i;

        return 45;
    }