javascript - problem with positioning the "mask" area of a modal popup on different resolutions

Question

The "mask" area I'm referring to is the dark background area behind the popup itself. Right now it looks perfectly centered up on a 1280X1024 resolution but any other resolution and the "mask" area is off-centered. It shows up pushed to the left of the screen.

Here's the JS, if you need the markup layout let me know.

 $(document).ready(function() {

    //select all the a tag with name equal to modal  
    $('#tagsmodalbtn').click(function(e) {
        //Cancel the link behavior  
        //e.preventDefault();  
        //Get the A tag  
        var id = "#dialog";

        //Get the screen height and width  
        var maskHeight = $(document).height();
        var maskWidth = $(window).width();

        //Set height and width to mask to fill up the whole screen  
        $('#mask').css({ 'width': maskWidth, 'height': maskHeight });

        //transition effect       
        $('#mask').fadeIn(1000);
        $('#mask').fadeTo("slow", 0.8);

        //Get the window height and width
        var winH = $(window).height();  
        var winW = $(window).width();

        //Set the popup window to center  
        $(id).css('top', (winH / 2 - $(id).height() / 2));
        $(id).css('left', (winW / 2 - $(id).width() / 2));

        //transition effect  
        $(id).fadeIn(2000);

    });

    //if close button is clicked  
    $('.window .close').click(function(e) {
        //Cancel the link behavior  
        e.preventDefault();
        $('#mask, .window').hide();
    });

    //if mask is clicked  
    $('#mask').click(function() {
        $(this).hide();
        $('.window').hide();
    });
});