In first post said that want make a puzzle. @ AFRLme said make inventory for it.
My question is : How can I drag drop items on empty slots? Similar to inventory in "The Dig" game
Never played the dig, but i guess you need to fake the empty slot but having placeholder items with no graphic. Then you can swap items out via lua. When clicking on an item, it gets dragged. therefore you can iterate through all inventory items via a lua loop and when the item in your hand is the same as on the field, set this field to another item with "nothing" as graphic.
When having an item in hour hand and clicking on an "empty" slot, swap them out again.