Use modes to switch cvox panel between collapsed, menus, focused, and tutorial

chrome/browser/resources/chromeos/chromevox/cvox2/background/panel.css

 /* Copyright 2016 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 body {
   border: 0;
   height: 35px;
   margin: 0;
   overflow: hidden;
   padding: 0;
@@ skipping 293 matching lines @@
   border-spacing: 0;
 }
 
 table#braille-table * {
   color: #fff;
 }
 
 table#braille-table tr:first-child {
   font-size: 85%;
 }
+
+/*
+ * The html hidden attribute doesn't make elements not focusable by default.
+ * This rule ensures that elements that are hidden truly don't show up.
+ */
+*[hidden] {
+  display: none !important;
+}