modal window css

I'm using these styles for my modal window.  they work great as long as i don't have to scroll down the browser. when i scroll down and click to open up a modal window, the modal window still loads in that absolute position. any ideas how to get the actual #dialog style to load relative to where i am in my browser window?

#mask {
  position:absolute;
  left:0;
  top:0;
  z-index:9000;
  background-color:#000;
  display:none;
}
  
#boxes .window {
  position:absolute;
   left:0;
  top:0;
  display:none;
  z-index:9999;
  padding:20px;
  font:14px Verdana, Arial, Helvetica, sans-serif;
}
#boxes .window label{
font:12px Verdana, Arial, Helvetica, sans-serif;
}

#boxes #dialog {
  padding:10px;
  background-color:#ffffff;
  border:4px solid #CCCCCC;
}

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