Repository: rust-lang/mdBook (language: rust)

Issue:
After upgrading to 0.4.41 navigation menu folding not working.
### Problem

I used `brew update && brew upgrade` command.  After upgrading  to  v0.4.41 navigation menu folding and unfolding button stopped to  react on click events.

My book have many sections and subsection with many pages inside. SUMMARY.md have a tree-like structure.  All was working well until upgrade to newer version of MdBook. 

Remedy: changed line `enable true` to ` enable false` in [output.html.fold] section of book.toml file. 

### Steps

_No response_

### Possible Solution(s)

_No response_

### Notes

_No response_

### Version

mdbook v0.4.41



Discussion / hints:

Fix patch:
diff --git a/src/theme/book.js b/src/theme/book.js
index 02b1908ddc..178f1e902d 100644
--- a/src/theme/book.js
+++ b/src/theme/book.js
@@ -463,17 +463,6 @@ function playground_text(playground, hidden = true) {
         try { localStorage.setItem('mdbook-sidebar', 'visible'); } catch (e) { }
     }
 
-
-    var sidebarAnchorToggles = document.querySelectorAll('#sidebar a.toggle');
-
-    function toggleSection(ev) {
-        ev.currentTarget.parentElement.classList.toggle('expanded');
-    }
-
-    Array.from(sidebarAnchorToggles).forEach(function (el) {
-        el.addEventListener('click', toggleSection);
-    });
-
     function hideSidebar() {
         body.classList.remove('sidebar-visible')
         body.classList.add('sidebar-hidden');
diff --git a/src/theme/index.hbs b/src/theme/index.hbs
index b62b29e8f3..7775f262d6 100644
--- a/src/theme/index.hbs
+++ b/src/theme/index.hbs
@@ -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>
-    <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 {
@@ -107,7 +109,7 @@
 
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
             <!-- populated by js -->
-            <div class="sidebar-scrollbox"></div>
+            <mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox>
             <noscript>
                 <iframe class="sidebar-iframe-outer" src="{{ path_to_root }}toc.html"></iframe>
             </noscript>
@@ -116,8 +118,6 @@
             </div>
         </nav>
 
-        <script async src="{{ path_to_root }}toc.js"></script>
-
         <div id="page-wrapper" class="page-wrapper">
 
             <div class="page">
diff --git a/src/theme/toc.js.hbs b/src/theme/toc.js.hbs
index eb48c8ba59..7adf0c2789 100644
--- a/src/theme/toc.js.hbs
+++ b/src/theme/toc.js.hbs
@@ -3,52 +3,68 @@
 // This is a script, and not included directly in the page, to control the total size of the book.
 // The TOC contains an entry for each page, so if each page includes a copy of the TOC,
 // the total size of the page becomes O(n**2).
-var sidebarScrollbox = document.querySelector("#sidebar .sidebar-scrollbox");
-sidebarScrollbox.innerHTML = '{{#toc}}{{/toc}}';
-(function() {
-    let current_page = document.location.href.toString();
-    if (current_page.endsWith("/")) {
-        current_page += "index.html";
+class MDBookSidebarScrollbox extends HTMLElement {
+    constructor() {
+        super();
     }
-    var links = sidebarScrollbox.querySelectorAll("a");
-    var l = links.length;
-    for (var i = 0; i < l; ++i) {
-        var link = links[i];
-        var href = link.getAttribute("href");
-        if (href && !href.startsWith("#") && !/^(?:[a-z+]+:)?\/\//.test(href)) {
-            link.href = path_to_root + href;
+    connectedCallback() {
+        this.innerHTML = '{{#toc}}{{/toc}}';
+        // Set the current, active page, and reveal it if it's hidden
+        let current_page = document.location.href.toString();
+        if (current_page.endsWith("/")) {
+            current_page += "index.html";
         }
-        // The "index" page is supposed to alias the first chapter in the book.
-        if (link.href === current_page || (i === 0 && path_to_root === "" && current_page.endsWith("/index.html"))) {
-            link.classList.add("active");
-            var parent = link.parentElement;
-            while (parent) {
-                if (parent.tagName === "LI" && parent.previousElementSibling) {
-                    if (parent.previousElementSibling.classList.contains("chapter-item")) {
-                        parent.previousElementSibling.classList.add("expanded");
+        var links = Array.prototype.slice.call(this.querySelectorAll("a"));
+        var l = links.length;
+        for (var i = 0; i < l; ++i) {
+            var link = links[i];
+            var href = link.getAttribute("href");
+            if (href && !href.startsWith("#") && !/^(?:[a-z+]+:)?\/\//.test(href)) {
+                link.href = path_to_root + href;
+            }
+            // The "index" page is supposed to alias the first chapter in the book.
+            if (link.href === current_page || (i === 0 && path_to_root === "" && current_page.endsWith("/index.html"))) {
+                link.classList.add("active");
+                var parent = link.parentElement;
+                if (parent && parent.classList.contains("chapter-item")) {
+                    parent.classList.add("expanded");
+                }
+                while (parent) {
+                    if (parent.tagName === "LI" && parent.previousElementSibling) {
+                        if (parent.previousElementSibling.classList.contains("chapter-item")) {
+                            parent.previousElementSibling.classList.add("expanded");
+                        }
                     }
+                    parent = parent.parentElement;
                 }
-                parent = parent.parentElement;
             }
         }
-    }
-})();
-
-// Track and set sidebar scroll position
-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' });
+        // Track and set sidebar scroll position
+        this.addEventListener('click', function(e) {
+            if (e.target.tagName === 'A') {
+                sessionStorage.setItem('sidebar-scroll', this.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
+            this.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' });
+            }
+        }
+        // Toggle buttons
+        var sidebarAnchorToggles = document.querySelectorAll('#sidebar a.toggle');
+        function toggleSection(ev) {
+            ev.currentTarget.parentElement.classList.toggle('expanded');
+        }
+        Array.from(sidebarAnchorToggles).forEach(function (el) {
+            el.addEventListener('click', toggleSection);
+        });
     }
 }
+window.customElements.define("mdbook-sidebar-scrollbox", MDBookSidebarScrollbox);


Test patch:
diff --git a/tests/rendered_output.rs b/tests/rendered_output.rs
index f5078a70c3..707b997db6 100644
--- a/tests/rendered_output.rs
+++ b/tests/rendered_output.rs
@@ -243,7 +243,7 @@ fn toc_js_html() -> Result<Document> {
     let toc_path = temp.path().join("book").join("toc.js");
     let html = fs::read_to_string(toc_path).with_context(|| "Unable to read index.html")?;
     for line in html.lines() {
-        if let Some(left) = line.strip_prefix("sidebarScrollbox.innerHTML = '") {
+        if let Some(left) = line.strip_prefix("        this.innerHTML = '") {
             if let Some(html) = left.strip_suffix("';") {
                 return Ok(Document::from(html));
             }