useReduceMotion

Whether motion is reduced right now: the user's animationSpeed preference, or — while they have expressed none — the operating system's.

The distinction matters for a checkbox. Reading only the combined value leaves a box that is already ticked because of the OS setting and does not untick when pressed, which is a control that appears broken. Reading the raw preference (undefined for "not set") lets the box start in the state the OS asked for and still respond to every press.