Bramus Van Damme replies to Lea Verou with the case a two-state control can’t cover: an OS that switches themes by time of day. Pick light during the day, and a toggle that maps one of its values back to system goes dark at night, against an explicit choice. Two states work only if the automatic value is labelled, or if real values are stored, and that needs a reset option, which is tri-state again. His own poll ran 43 to 7 for three states. Declan Chidlow’s pair of buttons, with neither highlighted while the OS is in charge, hides all three behind two.
The case for tri-state dark mode toggles
