Some jobs necessarily include idle time when you're waiting for work to come through even if there's nothing to do in that specific moment. The flip side of that is that the employer is able to require that the worker be available instantly. If they're leaving their work area because they're bored then they're not "at work."

My Dad was a career firefighter, and he spent most of his time sitting in the station watching TV, cooking meals, or sleeping. He was paid for every minute of that time because at the drop of a hat he could be called to a wreck, fire, or medical emergency.

The reason he had to be paid is federal law requiring that all workers who are "engaged to wait" are on the clock. If someone is installing mouse-jiggler software so they can leave their workstation and do whatever they want, they're no longer being engaged to wait.