position: absolute;
   top: 0;
   bottom: 0;
-  left: 0;
+  left: -200px;
   width: 200px;
   -moz-transition-property: left;
   -moz-transition-duration: 200ms;
   -webkit-transition-duration: 200ms;
   -webkit-transition-timing-function: ease;
 }
-#sidebarContainer.hidden {
-  left: -200px;
+#outerContainer.sideBarOpen > #sidebarContainer {
+  left: 0px;
 }
 
 #mainContainer {
   -webkit-transition-duration: 200ms;
   -webkit-transition-timing-function: ease;
 }
-#mainContainer.sideBarOpen {
+#outerContainer.sideBarOpen > #mainContainer {
   left: 200px;
 }
 
 
   <body>
     <div id="outerContainer">
 
-      <div id="sidebarContainer" class="hidden">
+      <div id="sidebarContainer">
         <div id="toolbarSidebar">
           <div class="splitToolbarButton toggled">
             <button id="viewThumbnail" class="toolbarButton toggled" title="Show Thumbnails" onclick="PDFView.switchSidebarView('thumbs')"></button>
 
   document.getElementById('sidebarToggle').addEventListener('click',
     function() {
       this.classList.toggle('toggled');
-      document.getElementById('sidebarContainer').classList.toggle('hidden');
-      mainContainer.classList.toggle('sideBarOpen');
+      console.log('toggling');
+      document.getElementById('outerContainer').classList.toggle('sidebarOpen');
       updateThumbViewArea();
     });