Is it possible to use jquery ui's dialog, and span it across the full browser height?
Then, if there is extra page, use the browser default scrollbar to go up and down, freezing the rest of the page behind the overlay?
$(function()
{
$('#category_modal').dialog({
autoOpen: false,
title: 'hello',
modal: true,
height: auto,
width: 500,
resizable: false
});
});