Then it should light up when the request is acknowledged, and stay lit up until the elevator arrives.
But wait, there's more: when the elevator arrives until it leaves, the button should flash or change to a more prominent color. Why? Because imagine someone presses up and someone else presses down, and the elevator arrives going up. If the up button switches off at this point, now only the down button is lit which clearly signals the elevator is going down, which is wrong.
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Then it should light up when the request is acknowledged, and stay lit up until the elevator arrives.
But wait, there's more: when the elevator arrives until it leaves, the button should flash or change to a more prominent color. Why? Because imagine someone presses up and someone else presses down, and the elevator arrives going up. If the up button switches off at this point, now only the down button is lit which clearly signals the elevator is going down, which is wrong.