Open Radar

 
http://developer.apple.com/mac/library/documentation/cocoa/Reference/ApplicationKit/Classes/NSEvent_Class/Reference/Reference.html#//apple_ref/occ/clm/NSEvent/mouseLocation

There is a note attached to the class method +[NSEvent mouseLocation]  that says:

"The y coordinate in the returned point starts from a base of 1, not 0."

I couldn't figure out what this meant at first, as it implies that all of the returned y coordinates are offset by 1. 

A quick test showed this to be incorrect.  I then realised that what it really means is:

"The y coordinate is never smaller than 1"  

As the mouse pointer is stopped at the y=1 position.

An update of this note in the documentation would help clarify what is going on with mouse coordinates.

This has been updated by Apple in the documentation.