[html] Center a 'div' in the middle of the screen, even when the page is scrolled up or down?

I have in my page a button which when clicked displays a div (popup style) in the middle of my screen.

I am using the following CSS to center the div in the middle of the screen:

.PopupPanel
{
    border: solid 1px black;
    position: absolute;
    left: 50%;
    top: 50%;
    background-color: white;
    z-index: 100;

    height: 400px;
    margin-top: -200px;

    width: 600px;
    margin-left: -300px;
}

This CSS works fine as long as the page is not scrolled down.

But, if I place the button at the bottom of my page, when it is clicked, the div is displayed at the top, and the user has to scroll up to view the contents of the div.

I would like to know how to display the div in the middle of the screen, even when the page has been scrolled.

I just found a new trick to center a box in the middle of the screen even if you don't have fixed dimensions. Let's say you would like a box 60% width / 60% height. The way to make it centered is by creating 2 boxes: a "container" box that position left: 50% top :50%, and a "text" box inside with reverse position left: -50%; top :-50%;

It works and it's cross browser compatible.

Check out the code below, you probably get a better explanation:

jQuery('.close a, .bg', '#message').on('click', function() {_x000D_
  jQuery('#message').fadeOut();_x000D_
  return false;_x000D_
});

html, body {_x000D_
  min-height: 100%;_x000D_
}_x000D_
_x000D_
#message {_x000D_
  height: 100%;_x000D_
  left: 0;_x000D_
  position: fixed;_x000D_
  top: 0;_x000D_
  width: 100%;_x000D_
}_x000D_
_x000D_
#message .container {_x000D_
  height: 60%;_x000D_
  left: 50%;_x000D_
  position: absolute;_x000D_
  top: 50%;_x000D_
  z-index: 10;_x000D_
  width: 60%;_x000D_
}_x000D_
_x000D_
#message .container .text {_x000D_
  background: #fff;_x000D_
  height: 100%;_x000D_
  left: -50%;_x000D_
  position: absolute;_x000D_
  top: -50%;_x000D_
  width: 100%;_x000D_
}_x000D_
_x000D_
#message .bg {_x000D_
  background: rgba(0, 0, 0, 0.5);_x000D_
  height: 100%;_x000D_
  left: 0;_x000D_
  position: absolute;_x000D_
  top: 0;_x000D_
  width: 100%;_x000D_
  z-index: 9;_x000D_
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>_x000D_
<div id="message">_x000D_
  <div class="container">_x000D_
    <div class="text">_x000D_
      <h2>Warning</h2>_x000D_
      <p>The message</p>_x000D_
      <p class="close"><a href="#">Close Window</a></p>_x000D_
    </div>_x000D_
  </div>_x000D_
  <div class="bg"></div>_x000D_
</div>

Correct Method is

.PopupPanel
{
    border: solid 1px black;
    position: fixed;
    left: 50%;
    top: 50%;
    background-color: white;
    z-index: 100;
    height: 400px;
    margin-top: -200px;

    width: 600px;
    margin-left: -300px;
}

Quote: I would like to know how to display the div in the middle of the screen, whether user has scrolled up/down.

Change

position: absolute;

To

position: fixed;

W3C specifications for position: absolute and for position: fixed.

Change position:absolute; to position:fixed;