How to position a div in the middle of the screen when the page is bigger than the screen

just add position:fixed and it will keep it in view even if you scroll down. see it at http://jsfiddle.net/XEUbc/1/

#mydiv {
    position:fixed;
    top: 50%;
    left: 50%;
    width:30em;
    height:18em;
    margin-top: -9em; /*set to a negative number 1/2 of your height*/
    margin-left: -15em; /*set to a negative number 1/2 of your width*/
    border: 1px solid #ccc;
    background-color: #f3f3f3;
}

I think this is a simple solution:

<div style="
    display: inline-block;
    position: fixed;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    width: 200px;
    height: 100px;
    margin: auto;
    background-color: #f3f3f3;">Full Center ON Page
</div>

Use transform;

<style type="text/css">
    #mydiv {
        position: fixed;
        top: 50%;
        left: 50%;
        transform: translate(-50%, -50%);
    }
</style>

Javascript Solution :

var left = (screen.width / 2) - (530 / 2);
var top = (screen.height / 2) - (500 / 2);
var _url = 'PopupListRepair.aspx';
window.open(_url, self, "width=530px,height=500px,status=yes,resizable=no,toolbar=no,menubar=no,left=" + left + ",top=" + top + ",scrollbars=no");