JSFiddle - React, Tailwind, and code Playground

HTML

<div class="widget widget_nav_menu"><h3>MENU</h3><div class="menu-om-os-menu-container"><ul id="menu-om-os-menu" class="menu"><li id="menu-item-272" class="menu-item menu-item-type-post_type menu-item-object-page current-menu-item page_item page-item-229 current_page_item menu-item-272"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/om-os/">om os</a></li>
<li id="menu-item-273" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-273"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/holdet/">Holdet</a></li>
<li id="menu-item-271" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-271"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/vores-support/">Vores support</a></li>
<li id="menu-item-270" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-270"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/social-ansvarlighed/">Social Ansvarlighed</a></li>
<li id="menu-item-269" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-269"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/job-i-spoton/">Job i SpotON</a></li>
<li id="menu-item-268" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-268"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/samarbejdspartnere/">Samarbejdspartnere</a></li>
<li id="menu-item-267" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-267"><a href="http://spotonmarketing.dk.linux11.curanetserver.dk/fa-praktikplads/">Få praktikplads</a></li>
</ul></div></div>
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CSS

.widget.widget_nav_menu{
    background: none repeat scroll 0 0 #2E3337;
    bottom: 0;
    padding: 25px;
    position: fixed;
    right: 2%;
    color: #707480;
    font-size: 0.8em;
    opacity: 1;
    text-transform: uppercase;
}

.widget.widget_nav_menu.fixed_button {
  position: absolute !important;
  top: -180px;
  background: none repeat scroll 0 0 #2E3337;
  padding: 25px;
  right: 2%;
}

JavaScript

$(window).scroll(function(){

       //Cache the Footer Widget
       var $footerMenu = $('.widget.widget_nav_menu');

       //Check if it hits the bottom, toggle Relevant Class.
       if ((window.innerHeight + window.scrollY) >= document.body.offsetHeight)
          $footerMenu.addClass('fixed_button');
       else
          $footerMenu.removeClass('fixed_button');

    });