When the inventory is full, you can't pick up items in the off hand.

What I expected:
Item would stack with the items in the off hand.

What happened:
The item didn't stack and want picked up.

How to reproduce:
Fill your inventory with item A, then put item B in the off hand.
When you try to pick up item B from the floor, it won't be picked up unless you clear any other inventory slot, in that case, it'll stack with the other items in the off hand.