When picking up a stackable item in your off-hand, the item does not stack in the off hand.

What I expected to happen was...:
The item to stack in my off hand.

What actually happened was...:
The item started its own stack in my inventory, or joined an existing stack there.

Steps to Reproduce:
1. Have an item (such as a torch or cobblestone) in the off hand, but not a full stack.
2. Drop or break that item so it is a drop on the ground (break the torch or mine the stone).
3. Pick the item up. It will create a new stack for that item in your inventory instead of stacking in your off hand.