kPhaseChanged is a phase value that show the wheel event has non-zero deltas and is in the middle of a scroll sequence. kPhaseChanged is historically used on Mac and it shows that the wheel event has some scroll delta. On this platform while the user hasn't moved or lifted their fingers from trackpad, we still receive wheel events from the OS. These wheel events will have zero deltas and marked with kPhaseStationary showing that nothing is changed.
 
Now that we use phase information on non-Mac platforms as well, it is better to change kPhaseChanged to something like kPhaseContinuous since kPhaseChanged can falsely imply that the phase is changed.