relative position with scrollspy

HTML

<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<body data-spy="scroll" data-target="#navbar">
    <div id="header" class="box">
        <h1>Header</h1>
    </div>
    <div id="content">
        <div id="post1" class="box">
            <h1>Post 1</h1>
            <p> Scroll Down↓</p>
        </div>
        <div id="post2" class="box"><h1>Post 2</h1></div>
        <div id="post3" class="box"><h1>Post 3</h1></div>
        
        <div id="navbar">
            <ul class="nav">
                <li><a href="#post1">Post 1</a></li>
                <li><a href="#post2">Post 2</a></li>
                <li><a href="#post3">Post 3</a></li>
            </ul>
        </div>
    </div>
</body>

CSS

#header { height: 200px !important;}
#content{ position:relative;}

.box{
    margin: 20px; padding: 15px;
    background: #eee;
    height: auto;
}
#navbar{
    position: fixed;
    bottom: 0; left: 20px;
    width: 100%;
    background: #fff;
}
.nav li a{
    float: left;
    width: 80px;
    padding: 15px 0;
}
.nav li a:hover{
    color: #f33 !important;
    background: none;
}
.nav li.active a{
    color: #f55;
    text-decoration: underline;
}

JavaScript

/* =============================================================
 * bootstrap-scrollspy.js v2.0.4
 * http://twitter.github.com/bootstrap/javascript.html#scrollspy
 * =============================================================
 * Copyright 2012 Twitter, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * ============================================================== */


!function ($) {

  "use strict"; // jshint ;_;


  /* SCROLLSPY CLASS DEFINITION
   * ========================== */

  function ScrollSpy( element, options) {
    var process = $.proxy(this.process, this)
      , $element = $(element).is('body') ? $(window) : $(element)
      , href
    this.options = $.extend({}, $.fn.scrollspy.defaults, options)
    this.$scrollElement = $element.on('scroll.scroll.data-api', process)
    this.selector = (this.options.target
      || ((href = $(element).attr('href')) && href.replace(/.*(?=#[^\s]+$)/, '')) //strip for ie7
      || '') + ' .nav li > a'
    this.$body = $('body')
    this.refresh()
    this.process()
  }

  ScrollSpy.prototype = {

      constructor: ScrollSpy

    , refresh: function () {
        var self = this
          , $targets

        this.offsets = $([])
        this.targets = $([])

        $targets = this.$body
          .find(this.selector)
          .map(function () {
            var $el = $(this)
              , href = $el.data('target') || $el.attr('href')
              , $href = /^#\w/.test(href) && $(href)
            return (...