Plain Code

// comment scroll
var $form = $('#new_comment');
if ( $form.length ) {
    var offset = $form.offset(),
        offsetTop = offset.top, offsetLeft = offset.left,
        fixedPos = false, timeout;
    $(window).bind('resize', function(event) {
        if ( timeout ) clearTimeout(timeout);
        timeout = setTimeout(resize, 50);
        function resize() {
            $form.css({
                position: 'relative',
                top: 0,
                left: 0,
                width: ''
            });
            offsetLeft = $form.offset().left;
            fixedPos = false;
            $(window).scroll();
        }
    });
    $(window).bind('scroll', function(event) {
        var scrollTop = $(this).scrollTop(),
            diff = scrollTop - offsetTop;
        if ( diff > -20 && !fixedPos ) {
            $form.css({
                position: 'fixed',
                top: 20,
                width: $form.width(),
                left: offsetLeft
            });
            fixedPos = true;
        } else if ( diff < -20 && fixedPos ) {
            $form.css({
                position: 'relative',
                top: 0,
                left: 0,
                width: ''
            });
            fixedPos = false;
        }
    }).scroll();
}