For Centering a Div On Screen and accounting for the scrolling and resizing

help.js

// Define your target div / popup container here.
var popup_target = '#checkout_open';

// Call the function like this
// jQuery("#checkout_open").center();


// Attach a function to jQuery
jQuery.fn.center = function () {
    this.css("position","absolute");
    this.css("top", ( jQuery(window).height() - this.height() ) / 2+jQuery(window).scrollTop() + "px");
    this.css("left", ( jQuery(window).width() - this.width() ) / 2+jQuery(window).scrollLeft() + "px");
    return this;
}

//  Bind a function for onresize
window.onresize= function () {
    jQuery(popup_target).center();
};

// Bind a function for window scroll
$(window).scroll(function() { 
	jQuery(popup_target).center(); 
});