When my inventory has no empty spaces (not necessarily full stacks, just no empty spaces) and I have a stackable item in my off hand, such as torches, ONLY in my off hand and nowhere else in my inventory, the game will refuse to pick up any dropped items of that type, despite having room in the stack. As soon as I click another item in my inventory, and make an empty space, the dropped item on the ground will proceed to stack with the other items in my off hand.