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.
So if I'm WFH and need to be available I can be watching TV or cooking a meal as long as I'm available at the drop of a hat if something comes in. This can be more usefully measured by how quickly I respond or my work output rather than how much my mouse moves.