Need dialog to extend past vertical viewport

Need dialog to extend past vertical viewport

I'm using a single jQuery UI dialog on a page of mine. The page has numerous buttons that can trigger the opening of that dialog with a click event. Prior to opening the dialog, I want to reposition it so the top of the dialog DIV appears at the same top offset as the button (basically covering the button). 

Here's my problem: let's say that the dialog DIV has a fixed height of 500 pixels, and the "open" button appears right near the bottom of the viewport. When I click the button, there isn't enough real estate at the bottom of the viewport to account for the 500px height of the dialog, so it gets nudged up so it fits in the viewport and subsequently isn't positioned where I want it to be. Ideally i would just want the dialog to expand beyond the viewport so you'd have to scroll to see it all. I just need the dialog to be absolutely positioned with no viewport restraints.

Is there any way to accomplish this?

Here's the gist of the code I'm using:

  2. $(".open_button").click(function(){
  3.       var dOffset = $(this).offset();
  4.       var dScroll = $(window).scrollTop();
  5.       $("#dialog").dialog( "option", "position", [dOffset.left,dOffset.top-dScroll] ).dialog("open");
  6. });