TransferItem.cs

// Transfer the item in the inventory slot we are currently hovering over
private void TransferItem()
{
	// Do not transfer item if this item is currently being animated
	if (CurrentlyAnimating)
		return;

	// There is no item here thus no item to transfer
	if (InventoryItem == null)
		return;

	// Get the 'other inventory'
	var targetInv = GetOtherInventory();

	// No 'other inventory' is open, lets use the player inventory so the
	// item gets transfered to the same inventory instead of doing nothing
	if (targetInv == null)
		targetInv = Player.Inventory;

	// Try to find a empty slot in the target inventory
	var emptySlot = targetInv.TryGetEmptyOrSameTypeSlot(InventoryItem.Item.Type);

	// A empty slot was found!
	if (emptySlot != -1)
	{
		// Store temporary reference to the item in this inventory slot
		var itemRef = InventoryItem.Item;

		// Get a copy of the item sprite that is being transfered
		// This is purely for visuals and does not effect the item logic
		var graphic = InventoryItem.GenerateGraphic();
		graphic.GlobalPosition = Position;

		// Get the other slot this item is being transfered to
		var otherInvSlot = targetInv.InventorySlots[emptySlot];

		//if (otherInvSlot.CurrentlyAnimating)
		//	return;

		// Remove the item before it gets transfered
		this.RemoveItem();

		var otherInvSlotItem = otherInvSlot.InventoryItem;

		var hide = false;

		// If the other inventory slot has no item in it, then hide the item that
		// gets transfered over. So it does not look like there is a duplicate
		// when the graphic sprite is animated over
		if (otherInvSlot.InventoryItem == null)
			hide = true;

		// Set the other inventory slot item to the item that is being transfered over
		if (otherInvSlotItem == null)
			otherInvSlot.SetItem(itemRef);
		else
		// If the item transfered has more than one than add that
		{
			itemRef.Count += otherInvSlotItem.Item.Count;
			otherInvSlot.SetItem(itemRef);
		}

		// Hide the other item
		if (hide)
			otherInvSlot.InventoryItem.Hide();

		// Start animation
		CurrentlyAnimating = true;
		otherInvSlot.CurrentlyAnimating = true;

		// Add graphic to the world
		Main.AddToCanvasLayer(graphic);

		var tween = Panel.GetTree().CreateTween();
		tween.TweenProperty(graphic, "global_position", otherInvSlot.Position, 1)
			.SetEase(Tween.EaseType.Out)
			.SetTrans(Tween.TransitionType.Cubic);

		tween.TweenCallback(Callable.From(() =>
		{
			// Sprite graphic reached its destination, lets show the other inventory slot item now
			otherInvSlot.InventoryItem.Show();
			CurrentlyAnimating = false;
			otherInvSlot.CurrentlyAnimating = false;
			graphic.QueueFree();
		}));
	}
}