diff --git a/book/theme/index.hbs b/book/theme/index.hbs
index 0a0bc5501..0a46ac649 100644
--- a/book/theme/index.hbs
+++ b/book/theme/index.hbs
@@ -1,5 +1,5 @@
 <!DOCTYPE HTML>
-<html lang="{{ language }}" class="{{ default_theme }}" dir="{{ text_direction }}">
+<html lang="{{ language }}" class="{{ default_theme }} sidebar-visible" dir="{{ text_direction }}">
     <head>
         <!-- Book generated using mdBook -->
         <meta charset="UTF-8">
@@ -52,15 +52,17 @@
         <!-- MathJax -->
         <script async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
         {{/if}}
-    </head>
-    <body class="sidebar-visible no-js">
-    <div id="body-container">
+
         <!-- Provide site root to javascript -->
         <script>
             var path_to_root = "{{ path_to_root }}";
             var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "{{ preferred_dark_theme }}" : "{{ default_theme }}";
         </script>
-
+        <!-- Start loading toc.js asap -->
+        <script src="{{ path_to_root }}toc.js"></script>
+    </head>
+    <body>
+    <div id="body-container">
         <!-- Work around some values being stored in localStorage wrapped in quotes -->
         <script>
             try {
@@ -82,19 +84,16 @@
             var theme;
             try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
             if (theme === null || theme === undefined) { theme = default_theme; }
-            var html = document.querySelector('html');
+            const html = document.documentElement;
             html.classList.remove('{{ default_theme }}')
             html.classList.add(theme);
-            var body = document.querySelector('body');
-            body.classList.remove('no-js')
-            body.classList.add('js');
+            html.classList.add("js");
         </script>
 
         <input type="checkbox" id="sidebar-toggle-anchor" class="hidden">
 
         <!-- Hide / unhide sidebar before it is displayed -->
         <script>
-            var body = document.querySelector('body');
             var sidebar = null;
             var sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
             if (document.body.clientWidth >= 1080) {
@@ -104,39 +103,21 @@
                 sidebar = 'hidden';
             }
             sidebar_toggle.checked = sidebar === 'visible';
-            body.classList.remove('sidebar-visible');
-            body.classList.add("sidebar-" + sidebar);
+            html.classList.remove('sidebar-visible');
+            html.classList.add("sidebar-" + sidebar);
         </script>
 
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
-            <div class="sidebar-scrollbox">
-                {{#toc}}{{/toc}}
+            <!-- populated by js -->
+            <mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox>
+            <noscript>
+                <iframe class="sidebar-iframe-outer" src="{{ path_to_root }}toc.html"></iframe>
+            </noscript>
+            <div id="sidebar-resize-handle" class="sidebar-resize-handle">
+                <div class="sidebar-resize-indicator"></div>
             </div>
-            <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
         </nav>
 
-        <!-- Track and set sidebar scroll position -->
-        <script>
-            var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
-            sidebarScrollbox.addEventListener('click', function(e) {
-                if (e.target.tagName === 'A') {
-                    sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
-                }
-            }, { passive: true });
-            var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
-            sessionStorage.removeItem('sidebar-scroll');
-            if (sidebarScrollTop) {
-                // preserve sidebar scroll position when navigating via links within sidebar
-                sidebarScrollbox.scrollTop = sidebarScrollTop;
-            } else {
-                // scroll sidebar to current active section when navigating via "next/previous chapter" buttons
-                var activeSection = document.querySelector('#sidebar .active');
-                if (activeSection) {
-                    activeSection.scrollIntoView({ block: 'center' });
-                }
-            }
-        </script>
-
         <div id="page-wrapper" class="page-wrapper">
 
             <div class="page">
