Sorry that this took so long. -99 means nothing... slot_origin is used to snap the item back to its original slot (that it was dragged from) when you try to drop it somewhere invalid, it is not updated during the dragging process, only when the item is successfully placed somewhere. -99 is just something that I put there to remind myself that its value is updated during game play; there will never be an item with an actual -99 value in it, look at the function that adds an item to the inventory, it updates slot_origin right away.