Material PDF: Drastically reduce time required to stamp complex bookmark trees

chrome/browser/resources/pdf/elements/viewer-toolbar-dropdown/viewer-toolbar-dropdown.css

 /* Copyright 2015 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. */
 
 :host {
   display: inline-block;
   text-align: left;
 }
 
 #container {
@@ skipping 12 matching lines @@
   max-height: 300px;
   overflow-y: auto;
   padding: 6px 0 4px 0;
 }
 
 #icon {
   cursor: pointer;
   display: inline-block;
 }
 
-#icon.open {
+:host([dropdown-open]) #icon {
   background-color: rgb(25, 27, 29);
   border-radius: 4px;
 }
 
 #arrow {
   margin-left: -12px;
   padding-right: 4px;
 }
 
 h1 {
   border-bottom: 1px solid rgb(219, 219, 219);
   color: rgb(33, 33, 33);
   font-size: 77.8%;
   font-weight: 500;
   margin: 0;
   padding: 14px 28px;
 }