When I first programmed the inventory system I was not able to create a drag&drop option. But today I had another idea after I received feedback on this specifically.
You will now be able to drag & drop via mouse leftclick and sort your items in your inventory as you like.
(Locked items cannot be dragged and equipped items neither)
It is a bit clunky on the coding side, because my inventory system was not build for it, but it works.
And I also found a bug, which made it possible to use one potion unlimited times. Fixed it, sorry! :D