check out this thread
http://processing.org/discourse/yabb2/YaBB.pl?num=1263445642

ah yes, you have to import it yourself since one of the last processing updates.
the snipped was a bit older.

Hi I've just seen this and it's really helpful indeed. I've been stuck for ages with how to track the mouse out-with the sketch window so good to see it's possible. I've been working on a small evolution of the simple drawing program that tracks your mouse movements and plots onto the sketch in the form of a line and drawing circles when you click - only problem is I can't work out how to track the mouse position outside of the sketch so you can leave it running in the background as opposed to always having it open on screen? 

Good to see that you searched for it... but have you tried anything yet?
It's very simple to implement something like that:

Duplicate the import statements...
Add the code that retrieves the mouse location:

1. Point mouse = MouseInfo.getPointerInfo().getLocation();

I see...
Since the window is not located at (0, 0) on the screen, pmouseX and pmouseY are offset just a little bit, enough to give you that weird result.

You have to either position the window at (0, 0)... or create your own pmouse variables... you could do both...

For putting your window in the top left corner of the screen:

1. //Inside setup()
2. 
   
3. size(screenWidth, screenHeight);
   
4. 
   
5. frame.removeNotify();
6. frame.setUndecorated(true);
7. frame.addNotify();
8. 
   
9. //At the beginning of draw()
10. 
    
11. if(frameCount == 1) frame.setLocation(0, 0);