Michael Natterer authored Jan 24, 2012 and Matthias Clasen committed Mar 01, 2012

nsevent scrollingDeltaX/Y (available on OSX >= Lion) is used to
provide the smooth scrolling values. In any case, old fashioned
events are still sent, setting _gdk_event_set_pointer_emulated()
if the event contains both smooth and non-smooth values.