diff --git a/static/css/components.css b/static/css/components.css
index a9b05dc..7c77f6a 100644
--- a/static/css/components.css
+++ b/static/css/components.css
@@ -291,10 +291,7 @@ nav.site-nav {
 
 @media (max-width: 540px) {
     .nav-logo {
-        position: static;
-        height: 2rem;
-        width: 2rem;
-        flex-shrink: 0;
+        display: none;
     }
 
     .nav-row-primary {
@@ -345,6 +342,59 @@ nav.site-nav {
     .nav-portals a {
         padding: 0.3rem 0.55rem;
     }
+
+    /* Extra bottom padding so the last content line isn't hidden
+       behind the fixed mobile TOC bar. */
+    body {
+        padding-bottom: 2.5rem;
+    }
+
+    /* Mobile TOC bar — fixed bottom strip: progress line + current
+       section label. toc.js sets --toc-progress and syncs the label. */
+    #toc-mobile-bar {
+        display: flex;
+        align-items: center;
+        position: fixed;
+        bottom: 0; left: 0; right: 0;
+        height: 2.25rem;
+        background: var(--bg-nav);
+        border-top: 1px solid var(--border);
+        padding: 0 1rem;
+        z-index: 90;
+        overflow: hidden;
+    }
+
+    /* Progress track (faint full-width line at top of bar) */
+    #toc-mobile-bar::before {
+        content: '';
+        position: absolute;
+        top: 0; left: 0; right: 0;
+        height: 2px;
+        background: var(--border);
+    }
+
+    /* Progress fill — grows with scroll via --toc-progress */
+    #toc-mobile-bar::after {
+        content: '';
+        position: absolute;
+        top: 0; left: 0;
+        height: 2px;
+        width: calc(var(--toc-progress, 0) * 100%);
+        background: var(--text-muted);
+        transition: width 0.12s ease;
+    }
+
+    .toc-mobile-label {
+        font-family: var(--font-sans);
+        font-size: 0.68rem;
+        font-weight: 600;
+        font-variant-caps: all-small-caps;
+        letter-spacing: 0.07em;
+        color: var(--text-faint);
+        white-space: nowrap;
+        overflow: hidden;
+        text-overflow: ellipsis;
+    }
 }
 
 /* Row 2: portal links — hidden until nav.js adds .is-open */
@@ -386,6 +436,12 @@ nav.site-nav {
    and the --toc-progress custom property for the progress bar.
    ============================================================ */
 
+/* Mobile TOC bar — hidden on all viewports by default; the ≤540px
+   media query below makes it visible on mobile only. */
+#toc-mobile-bar {
+    display: none;
+}
+
 #toc {
     font-family: var(--font-sans);
     font-size: 0.85rem;
diff --git a/static/css/sidenotes.css b/static/css/sidenotes.css
index b5e014e..5f1fafd 100644
--- a/static/css/sidenotes.css
+++ b/static/css/sidenotes.css
@@ -155,3 +155,64 @@ a.footnote-ref {
 a.footnote-ref:hover {
     color: var(--text-muted);
 }
+
+
+/* ============================================================
+   MOBILE SIDENOTE POPUP
+   Bottom sheet shown when tapping a sidenote ref on narrow
+   viewports (where .sidenote is hidden).  The overlay is
+   display:none by default; sidenotes.js adds .is-open to show it.
+   No media query needed — JS only opens the popup when the
+   sidenote span is hidden, so it never fires on wide viewports.
+   ============================================================ */
+
+.sidenote-popup-overlay {
+    display: none;
+    position: fixed;
+    inset: 0;
+    z-index: 500;
+    background: rgba(0, 0, 0, 0.45);
+    align-items: flex-end;
+}
+
+.sidenote-popup-overlay.is-open {
+    display: flex;
+}
+
+.sidenote-popup {
+    background: var(--bg);
+    border-top: 1px solid var(--border);
+    border-radius: 0.75rem 0.75rem 0 0;
+    padding: 1.25rem 1.25rem 2.75rem;
+    width: 100%;
+    max-height: 65vh;
+    overflow-y: auto;
+    position: relative;
+    font-family: var(--font-serif);
+    font-size: 0.9rem;
+    line-height: 1.6;
+    color: var(--text);
+    outline: none;
+}
+
+.sidenote-popup-close {
+    position: absolute;
+    top: 0.7rem;
+    right: 0.9rem;
+    background: none;
+    border: none;
+    font-size: 1.5rem;
+    line-height: 1;
+    color: var(--text-faint);
+    cursor: pointer;
+    padding: 0.1rem 0.35rem;
+}
+
+.sidenote-popup-close:hover {
+    color: var(--text);
+}
+
+/* Preserve sidenote-num badge style inside the popup */
+.sidenote-popup .sidenote-num {
+    display: inline-block;
+}
diff --git a/static/js/sidenotes.js b/static/js/sidenotes.js
index 656afe2..d2e52df 100644
--- a/static/js/sidenotes.js
+++ b/static/js/sidenotes.js
@@ -97,12 +97,16 @@
         sn.addEventListener('mouseenter',  function () { hovering = true;  update(); });
         sn.addEventListener('mouseleave',  function () { hovering = false; update(); });
 
-        /* Click on the superscript link: sticky focus on wide viewports,
-           normal anchor scroll on narrow viewports (sidenote hidden). */
+        /* Click on the superscript link: sticky focus on wide viewports;
+           mobile popup on narrow viewports (sidenote hidden). */
         const link = ref.querySelector('a');
         if (link) {
             link.addEventListener('click', function (e) {
                 e.preventDefault();
+                if (getComputedStyle(sn).display === 'none') {
+                    openMobilePopup(sn);
+                    return;
+                }
                 toggleFocus();
             });
 
@@ -126,6 +130,65 @@
         }
     }
 
+    /* ------------------------------------------------------------------ */
+    /*  Mobile popup — bottom sheet for narrow viewports where .sidenote  */
+    /*  is display:none.  Created lazily on first use.                    */
+    /* ------------------------------------------------------------------ */
+
+    var mobileOverlay = null;
+
+    function ensureMobileOverlay() {
+        if (mobileOverlay) return;
+
+        var overlay = document.createElement('div');
+        overlay.className = 'sidenote-popup-overlay';
+        overlay.setAttribute('role', 'dialog');
+        overlay.setAttribute('aria-modal', 'true');
+        overlay.setAttribute('aria-label', 'Note');
+
+        var sheet = document.createElement('div');
+        sheet.className = 'sidenote-popup';
+        sheet.setAttribute('tabindex', '-1');
+
+        var closeBtn = document.createElement('button');
+        closeBtn.className = 'sidenote-popup-close';
+        closeBtn.setAttribute('aria-label', 'Close note');
+        closeBtn.textContent = '\u00d7'; /* × */
+
+        var body = document.createElement('div');
+        body.className = 'sidenote-popup-body';
+
+        sheet.appendChild(closeBtn);
+        sheet.appendChild(body);
+        overlay.appendChild(sheet);
+        document.body.appendChild(overlay);
+
+        overlay.addEventListener('click', function (e) {
+            if (!sheet.contains(e.target)) closeMobilePopup();
+        });
+        closeBtn.addEventListener('click', closeMobilePopup);
+
+        document.addEventListener('keydown', function (e) {
+            if (e.key === 'Escape' && mobileOverlay &&
+                    mobileOverlay.classList.contains('is-open')) {
+                closeMobilePopup();
+            }
+        });
+
+        mobileOverlay = overlay;
+    }
+
+    function openMobilePopup(sn) {
+        ensureMobileOverlay();
+        mobileOverlay.querySelector('.sidenote-popup-body').innerHTML = sn.innerHTML;
+        mobileOverlay.classList.add('is-open');
+        mobileOverlay.querySelector('.sidenote-popup').focus();
+    }
+
+    function closeMobilePopup() {
+        if (mobileOverlay) mobileOverlay.classList.remove('is-open');
+    }
+
     /* Click anywhere outside a focused pair dismisses it. */
     document.addEventListener('click', function (e) {
         if (focusedPair &&
diff --git a/static/js/toc.js b/static/js/toc.js
index 91ae54f..217787e 100644
--- a/static/js/toc.js
+++ b/static/js/toc.js
@@ -20,8 +20,10 @@
 
         if (!headings.length) return;
 
-        const label     = toc.querySelector('.toc-active-label');
-        const toggleBtn = toc.querySelector('.toc-toggle');
+        const label       = toc.querySelector('.toc-active-label');
+        const toggleBtn   = toc.querySelector('.toc-toggle');
+        const mobileBar   = document.getElementById('toc-mobile-bar');
+        const mobileLabel = mobileBar && mobileBar.querySelector('.toc-mobile-label');
 
         const pageTitleEl = document.querySelector('#markdownBody .page-title');
         const pageTitle   = pageTitleEl ? pageTitleEl.textContent.trim() : 'Contents';
@@ -29,14 +31,15 @@
         function activateTitle() {
             links.forEach(a => a.classList.remove('is-active'));
             if (label) label.textContent = pageTitle;
+            if (mobileLabel) mobileLabel.textContent = pageTitle;
         }
 
         function activate(id) {
             links.forEach(a => a.classList.toggle('is-active', a.dataset.target === id));
-            if (label) {
-                const activeLink = linkMap.get(id);
-                label.textContent = activeLink ? activeLink.textContent : pageTitle;
-            }
+            const activeLink = linkMap.get(id);
+            const text = activeLink ? activeLink.textContent : pageTitle;
+            if (label) label.textContent = text;
+            if (mobileLabel) mobileLabel.textContent = text;
         }
 
         // Collapse / expand. The collapsed state is hidden from
@@ -70,11 +73,13 @@
         }
 
 
-        // Progress indicator — drives the horizontal bar under .toc-header.
+        // Progress indicator — drives the horizontal bar under .toc-header
+        // and the mobile bar's progress line.
         function updateProgress() {
             const scrollable = document.documentElement.scrollHeight - window.innerHeight;
             const progress = scrollable > 0 ? Math.min(1, window.scrollY / scrollable) : 0;
             toc.style.setProperty('--toc-progress', progress);
+            if (mobileBar) mobileBar.style.setProperty('--toc-progress', progress);
         }
         window.addEventListener('scroll', updateProgress, { passive: true });
         updateProgress();
diff --git a/templates/essay.html b/templates/essay.html
index 8d19a84..2d112ae 100644
--- a/templates/essay.html
+++ b/templates/essay.html
@@ -14,4 +14,7 @@
         $body$
     </main>
 </div>
+<div id="toc-mobile-bar" aria-hidden="true">
+    <span class="toc-mobile-label">Contents</span>
+</div>
 $partial("templates/partials/page-footer.html")$