This seems to be operating specific. Linux doesn't have this. For windows I don't know.
For mac it does.

mouseX can go to 0
mouseY lowest value is 1

(this is also in processing 2 and probably 1.5.1 but that doesn't start on my mac anymore).

void setup() {
  size(800,  800);  
}

void draw() {
   frame.setTitle(mouseX+" "+mouseY); 
}