disable background using css when popup appear

Question

I am using the below css code to block the entire page when the popup appeared. But it\'s working(The visible page only covered not all). I have a problem, if my page having

Answer 1

use this css

.parentDisable{
    position: fixed;
    top: 0;
    left: 0;
    background: #000;
    opacity: 0.8;
    z-index: 998;
    height: 100%;
    width: 100%;}

#popup{
    position: absolute;
    z-index: 999;}

and this html

<div class="parentDisable"></div>
<div id="popup">My pop up</div>