1. Start game as engineer (has both vest and backpack)
2. Open inventory.
3. Remove all items from the vest and backpack.
4. Drag vest to the ground slot.
5. Drag vest on top of backpack slot.

Expected:
Backpack slot should be "red" indicating vest cannot be dropped. Dropping the vest should cancel to drag action.

Actual:
Backpack slot is highlighted as an valid drop target. Dropping the vest on the backpack slot results in backpack being dropped on ground still attached to the units body (i.e. character appearance is broken).

If vest contains a single item the drop action is prevented as expected. Backpack does not have to be empty, just contain enough room for the vest to fit.

It appears the reverse, i.e. dropping the backpack on the vest slot, does not cause this bug to appear.

If the steps are repeated several times, it eventually causes the game to crash.