diff --git a/scripts/build.mjs b/scripts/build.mjs
index e215d7e..b1d7bfd 100644
--- a/scripts/build.mjs
+++ b/scripts/build.mjs
@@ -43,6 +43,8 @@ const stylesheet = new CleanCSS({level:2, rebase:false}).minify(stylesheetSource
 if (stylesheet.errors.length) throw new Error(stylesheet.errors.join('\n'));
 const logo = await readFile(new URL('../src/logo.webp', import.meta.url));
 const logoPath = `/assets/logo.${createHash('sha256').update(logo).digest('hex').slice(0,12)}.webp`;
+const nav = (await minify(await readFile(new URL('../src/nav.js', import.meta.url), 'utf8'), {module:true})).code;
+const navPath = `/assets/nav.${createHash('sha256').update(nav).digest('hex').slice(0,12)}.js`;
 const aiClient = (await minify(await readFile(new URL('../src/ai-client.js', import.meta.url), 'utf8'), {module:true})).code;
 const aiClientPath = `/assets/ai-client.${createHash('sha256').update(aiClient).digest('hex').slice(0,12)}.js`;
 const aiPanelSource = (await readFile(new URL('../src/ai-panel.js', import.meta.url), 'utf8')).replace("'./ai-client.js'", JSON.stringify(aiClientPath));
@@ -175,18 +177,20 @@ function document({ title, description, path, content, current, noindex = false
   <link rel="preload" href="${interfaceFontPath}" as="font" type="font/woff2" crossorigin>
   <style>${stylesheet.styles}</style>
   <script type="module" src="${analyticsPath}"></script>
+  <script type="module" src="${navPath}"></script>
   ${current ? `<script type="module" src="${aiPanelPath}"></script>` : ''}
 </head>
 <body data-analytics-page="${escapeHTML(path)}" data-analytics-concept="${escapeHTML(current?.slug || (noindex ? '404' : 'home'))}">
   <a class="skip-link" href="#main-content">Skip to content</a>
-  <aside class="sidebar">
+  <aside class="sidebar" id="concept-sidebar">
     <a class="brand" href="/"><img class="brand-mark" src="${logoPath}" width="32" height="32" alt=""><span>JavaScript <strong>in 30 words</strong></span></a>
     <p class="brand-tagline">A refresher on JavaScript concepts in less than 30 words</p>
     <div class="sidebar-heading"><span>Concepts</span><span>${definitions.length}</span></div>
     <nav class="concept-nav" aria-label="Concepts">${navigation(current)}</nav>
   </aside>
+  <div class="nav-backdrop" hidden></div>
   <div class="page">
-    <header class="mobile-header"><a class="brand" href="/"><img class="brand-mark" src="${logoPath}" width="32" height="32" alt=""><span>JavaScript <strong>in 30 words</strong></span></a><details class="mobile-navigation"><summary>Browse all ${definitions.length} concepts</summary><nav aria-label="Concepts">${navigation(current)}</nav></details></header>
+    <header class="mobile-header"><a class="brand" href="/"><img class="brand-mark" src="${logoPath}" width="32" height="32" alt=""><span>JavaScript <strong>in 30 words</strong></span></a><button class="nav-toggle" type="button" aria-expanded="false" aria-controls="concept-sidebar">Browse all ${definitions.length} concepts</button></header>
     <div class="content-layout">
       ${current ? `<nav class="breadcrumb" aria-label="Breadcrumb"><a href="/">All concepts</a><span aria-hidden="true"> / </span><span>${escapeHTML(labelFor(current))}</span></nav>` : ''}
       <div class="scroll-region"><main id="main-content" tabindex="-1">${content}</main></div>
@@ -249,6 +253,7 @@ await writeFile(new URL('assets/Manrope-OFL.txt', output), await readFile(new UR
 await writeFile(new URL(aiClientPath.slice(1), output), aiClient);
 await writeFile(new URL(aiPanelPath.slice(1), output), aiPanel);
 await writeFile(new URL(analyticsPath.slice(1), output), analytics);
+await writeFile(new URL(navPath.slice(1), output), nav);
 await writeFile(new URL('index.html', output), home());
 for (const [index, concept] of definitions.entries()) {
   const directory = new URL(`${concept.slug}/`, output);
diff --git a/src/main.css b/src/main.css
index 4d87fa6..ac2ff87 100644
--- a/src/main.css
+++ b/src/main.css
@@ -998,8 +998,22 @@ code,samp {
   body {
     padding:8px
   }
+  /* Above .site-footer (z-index:50), below .skip-link (100) and the
+     consent banner (150), which must stay reachable over the drawer. */
   .sidebar {
-    display:none
+    z-index:91;
+    inset:8px auto 8px 8px;
+    display:none;
+    width:min(300px, 84vw)
+  }
+  body[data-nav-open] .sidebar {
+    display:flex
+  }
+  .nav-backdrop {
+    position:fixed;
+    z-index:90;
+    inset:0;
+    background:rgba(20,33,50,.38)
   }
   .page {
     --page-gutter:1.15rem;
@@ -1022,12 +1036,11 @@ code,samp {
     width:27px;
     height:27px
   }
-  .mobile-navigation {
-    margin-top:.85rem
-  }
-  .mobile-navigation>summary {
+  .nav-toggle {
     display:flex;
+    width:100%;
     min-height:38px;
+    margin-top:.85rem;
     align-items:center;
     padding:.45rem .7rem;
     color:#4f5b6d;
@@ -1035,18 +1048,11 @@ code,samp {
     border:1px solid var(--line);
     border-radius:9px;
     cursor:pointer;
+    font:inherit;
     font-size:.84rem;
-    list-style:none
-  }
-  .mobile-navigation>summary::-webkit-details-marker {
-    display:none
-  }
-  .mobile-navigation>nav {
-    max-height:58svh;
-    padding-top:.55rem;
-    overflow-y:auto
+    text-align:left
   }
-  .mobile-navigation .nav-group ul {
+  .sidebar .nav-group ul {
     margin-left:1rem
   }
   .content-layout {
diff --git a/src/nav.js b/src/nav.js
new file mode 100644
index 0000000..9c61604
--- /dev/null
+++ b/src/nav.js
@@ -0,0 +1,36 @@
+// The concept list is rendered once, in the sidebar. Below the mobile
+// breakpoint that sidebar becomes an off-canvas drawer this toggle opens,
+// so the 35 links never need a second copy in the markup.
+const toggle = document.querySelector('.nav-toggle');
+const sidebar = document.getElementById('concept-sidebar');
+const backdrop = document.querySelector('.nav-backdrop');
+
+if (toggle && sidebar) {
+  const isOpen = () => document.body.hasAttribute('data-nav-open');
+
+  const setOpen = (open) => {
+    document.body.toggleAttribute('data-nav-open', open);
+    toggle.setAttribute('aria-expanded', String(open));
+    if (backdrop) backdrop.hidden = !open;
+  };
+
+  toggle.addEventListener('click', () => {
+    const opening = !isOpen();
+    setOpen(opening);
+    if (opening) sidebar.querySelector('a, summary')?.focus();
+  });
+
+  if (backdrop) backdrop.addEventListener('click', () => setOpen(false));
+
+  document.addEventListener('keydown', (event) => {
+    if (event.key !== 'Escape' || !isOpen()) return;
+    setOpen(false);
+    toggle.focus();
+  });
+
+  // Following a link inside the drawer navigates away, but closing keeps the
+  // state clean for same-page anchors and back-forward cache restores.
+  sidebar.addEventListener('click', (event) => {
+    if (event.target.closest('a')) setOpen(false);
+  });
+}