Run every pass by the drop controller, the sprite-$31 record at $EE58 whose room MOVE_PLAYER keeps equal to the player's. With the player in play, the pick-up key held and the press not yet used (PICKUP_USED clear), it marks the press used, lets the third slot's object fall at the player's feet (DROP_CARRIED), shifts the queue along, clears the first slot and redraws the inventory. When the key is up it clears the used bit, so each press does one thing. So a press with nothing underfoot moves the queue on one place; an object falls out only when pushed past the third slot (measured in the simulator).