Background scrolling behind position:fixed; on iOS

$( document ).ready(function() {
  $('.dropdown-toggle').click(function(){
    // Add class to `html` tag instead of `body` tag
    $('html').toggleClass('menu-dropdown-expanded');
  })  
});

body, ul {
  margin: 0;
  padding: 0;
}

.navbar {
  height: 60px;
  background: wheat;
}

.dropdown-window {
  position: fixed;
  top: 60px;
  left: 0;
  width: 100%;
  height: calc(100% - 60px);
  background-color: #e7e7e7;
  display: none;
  -webkit-overflow-scrolling: auto;
  overflow-y: scroll;
}

/* Prevent overscroll/bounce in iOS MobileSafari by CSS */
.menu-dropdown-expanded,
.menu-dropdown-expanded body {
  position: fixed;
  overflow: hidden;
}

.menu-dropdown-expanded .dropdown-window {
  display: block;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.0/jquery.min.js"></script>
<div class="navbar">
  <ul class="main-menu">
    <li>
      <a href="#" class="dropdown-toggle">Dropdown toggle</a>
      <div class="dropdown-window">
        <ul>
          <li class="item1">item 1</li>
          <li class="item2">item 2</li>
          <li class="item3">item 3</li>
          <li class="item4">item 4</li>
          <li class="item5">item 5</li>
          <li class="item6">item 6</li>
          <li class="item7">item 7</li>
          <li class="item8">item 8</li>
          <li class="item9">item 9</li>
          <li class="item10">item 10</li>
          <li class="item11">item 11</li>
          <li class="item12">item 12</li>
          <li class="item13">item 13</li>
          <li class="item14">item 14</li>
          <li class="item15">item 15</li>
          <li class="item16">item 16</li>
          <li class="item17">item 17</li>
          <li class="item18">item 18</li>
          <li class="item19">item 19</li>
          <li class="item20">item 20</li>
          <li class="item21">item 21</li>
          <li class="item22">item 22</li>
          <li class="item23">item 23</li>
          <li class="item24">item 24</li>
          <li class="item25">item 25</li>
          <li class="item26">item 26</li>
          <li class="item27">item 27</li>
          <li class="item28">item 28</li>
          <li class="item29">item 29</li>
          <li class="item30">item 30</li>
          <li class="item31">item 31</li>
          <li class="item32">item 32</li>
          <li class="item33">item 33</li>
          <li class="item34">item 34</li>
          <li class="item35">item 35</li>
          <li class="item36">item 36</li>
          <li class="item37">item 37</li>
          <li class="item38">item 38</li>
          <li class="item39">item 39</li>
          <li class="item40">item 40</li>
          <li class="item41">item 41</li>
          <li class="item42">item 42</li>
          <li class="item43">item 43</li>
          <li class="item44">item 44</li>
          <li class="item45">item 45</li>
          <li class="item46">item 46</li>
          <li class="item47">item 47</li>
          <li class="item48">item 48</li>
          <li class="item49">item 49</li>
          <li class="item50">item 50</li>
          <li class="item51">item 51</li>
          <li class="item52">item 52</li>
          <li class="item53">item 53</li>
          <li class="item54">item 54</li>
          <li class="item55">item 55</li>
          <li class="item56">item 56</li>
          <li class="item57">item 57</li>
          <li class="item58">item 58</li>
          <li class="item59">item 59</li>
          <li class="item60">item 60</li>
          <li class="item61">item 61</li>
          <li class="item62">item 62</li>
          <li class="item63">item 63</li>
          <li class="item64">item 64</li>
          <li class="item65">item 65</li>
          <li class="item66">item 66</li>
          <li class="item67">item 67</li>
          <li class="item68">item 68</li>
          <li class="item69">item 69</li>
          <li class="item70">item 70</li>
          <li class="item71">item 71</li>
          <li class="item72">item 72</li>
          <li class="item73">item 73</li>
          <li class="item74">item 74</li>
          <li class="item75">item 75</li>
          <li class="item76">item 76</li>
          <li class="item77">item 77</li>
          <li class="item78">item 78</li>
          <li class="item79">item 79</li>
          <li class="item80">item 80</li>
          <li class="item81">item 81</li>
          <li class="item82">item 82</li>
          <li class="item83">item 83</li>
          <li class="item84">item 84</li>
          <li class="item85">item 85</li>
          <li class="item86">item 86</li>
          <li class="item87">item 87</li>
          <li class="item88">item 88</li>
          <li class="item89">item 89</li>
          <li class="item90">item 90</li>
          <li class="item91">item 91</li>
          <li class="item92">item 92</li>
          <li class="item93">item 93</li>
          <li class="item94">item 94</li>
          <li class="item95">item 95</li>
          <li class="item96">item 96</li>
          <li class="item97">item 97</li>
          <li class="item98">item 98</li>
          <li class="item99">item 99</li>
          <li class="item100">item 100</li>
        </ul>
      </div>
    </li>
  </ul>
</div>
<div class="my-content">
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</div>