How to access mouse position of parent window in child popup window using javascript

Question

I want to develop a browser extension where if user click on extension icon, it will get a pop up with a input text box and on mouse move it will get current position of mouse pointer(X,Y) of screen, something like that:
X: 100 Y:50

Here i have created a mouse move event handler on my extension popup, something like that

document.addEventListener('mousemove', onMouseMove, false)

function onMouseMove(e) {
    x = e.pageX;
    y = e.pageY;
    document.getElementById("colorcode").value = "X: " + x + " Y: " + y ;
}

But it is showing popup's mouse position instead of user's page mouse position. Please suggest.