not sure how many people know of this, since afaik it's never stated in the game, and I only managed to stumble onto this myself by mistake. so for the sake of expanding this convenient knowledge further
if you long hold down 'a' when your cursor is over an item while in your inventory menu, you'll grab it and can then move the item to another inventory slot via the directionals. release 'a' when the item's marked over where you want it to go to drop it into that new slot. if dropping into a slot another item is already in, the two items will of course be swapped in inventory placement
if you long hold down 'a' when your cursor is over an item while in your inventory menu, you'll grab it and can then move the item to another inventory slot via the directionals. release 'a' when the item's marked over where you want it to go to drop it into that new slot. if dropping into a slot another item is already in, the two items will of course be swapped in inventory placement