CrOS Files.app: Prevent the middle panel from disappearing.

chrome/browser/resources/file_manager/css/file_manager.css

 /* Copyright (c) 2012 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. */
 
 html {
   height: 100%;
 }
 
 html.col-resize * {
   cursor: col-resize !important;
@@ skipping 564 matching lines @@
 body[new-ui] .dialog-body {
   border-top: 1px solid rgb(225, 225, 225);
 }
 
 .main-panel {
   -webkit-box-flex: 1;
   display: -webkit-box;
 }
 
 body[new-ui] .dialog-middlebar-contents {
+  display: -webkit-box;
   max-width: 50%;
   min-width: 45px;
   position: relative;
   width: 180px;
 }
 
 /* Container for the ok/cancel buttons. */
 .dialog-footer {
   -webkit-box-align: center;
   -webkit-box-orient: horizontal;
@@ skipping 1608 matching lines @@
 
 body[new-ui] #iframe-drag-area {
   -webkit-app-region: drag;
   height: 45px;
   left: 64px;
   position: absolute;
   right: 0;
   top: 0;
   z-index: 101;
 }