Create & Center Popup Windows (Also on dual monitor)

JavaScript - Center Popup

function PopupCenter(url, title, w, h) {
	// Fixes dual-screen position                         Most browsers      Firefox
	var dualScreenLeft = window.screenLeft != undefined ? window.screenLeft : screen.left;
	var dualScreenTop = window.screenTop != undefined ? window.screenTop : screen.top;
	
	width = window.innerWidth ? window.innerWidth : document.documentElement.clientWidth ? document.documentElement.clientWidth : screen.width;
	height = window.innerHeight ? window.innerHeight : document.documentElement.clientHeight ? document.documentElement.clientHeight : screen.height;
	
	var left = ((width / 2) - (w / 2)) + dualScreenLeft;
	var top = ((height / 2) - (h / 2)) + dualScreenTop;
	var newWindow = window.open(url, title, 'scrollbars=no, width=' + w + ', height=' + h + ', top=' + top + ', left=' + left);
	
	// Puts focus on the newWindow
	if (window.focus) {
	    newWindow.focus();
	}
}

// Fixes dual-screen position                         Most browsers      Firefox
var dualScreenLeft = window.screenLeft != undefined ? window.screenLeft : screen.left;

This can't be right. window.screenLeft returns the X coordinate of the window, while screen.left returns the X coordinate of the leftmost screen