diff --git a/public/app.js b/public/app.js
index bb264625..32a67f69 100644
--- a/public/app.js
+++ b/public/app.js
@@ -1327,6 +1327,40 @@ window.addEventListener('DOMContentLoaded', () => {
         }
       }
       rebuildMoreMenu();
+      // #1396 diagnostic — temporary; remove once root cause known.
+      // Gated on `navdebug=1` appearing anywhere in the URL hash (the
+      // app is hash-routed, so this works with e.g. `#/channels?navdebug=1`).
+      // Operator runs the affected device against staging with that
+      // hash and screenshots the green-on-black banner pinned to the
+      // bottom of the viewport — gives us viewport width, UA, total/
+      // visible/overflow counts, CSS-hidden count, and the actual link
+      // labels in each bucket without needing devtools.
+      try {
+        if (typeof window !== 'undefined' && window.location.hash.indexOf('navdebug=1') !== -1) {
+          var __dbgTotal = allLinks.length;
+          var __dbgVisible = allLinks.filter(function(a){ return !a.classList.contains('is-overflow'); });
+          var __dbgOverflow = allLinks.filter(function(a){ return a.classList.contains('is-overflow'); });
+          var __dbgHiddenByCss = __dbgVisible.filter(function(a){
+            return window.getComputedStyle(a).display === 'none';
+          });
+          var __dbgVisNames = __dbgVisible.map(function(a){ return (a.textContent||'').trim().slice(0,12); }).join(',');
+          var __dbgOvrNames = __dbgOverflow.map(function(a){ return (a.textContent||'').trim().slice(0,12); }).join(',');
+          var __dbgActive = (allLinks.filter(function(a){ return a.classList.contains('active'); })[0] || {}).textContent || '';
+          __dbgActive = (__dbgActive || '').trim().slice(0,20);
+          var __dbgBanner = document.getElementById('__navdebug') || document.createElement('div');
+          __dbgBanner.id = '__navdebug';
+          __dbgBanner.style.cssText = 'position:fixed;bottom:0;left:0;right:0;background:#000;color:#0f0;font:11px ui-monospace,monospace;padding:4px 8px;z-index:99999;white-space:pre-wrap;word-break:break-all;pointer-events:none';
+          __dbgBanner.textContent =
+            '[NAV-DEBUG-1396] vw=' + window.innerWidth + ' total=' + __dbgTotal +
+            ' visible=' + __dbgVisible.length + ' overflow=' + __dbgOverflow.length +
+            ' hidden-by-css=' + __dbgHiddenByCss.length + ' active=' + __dbgActive + '\n' +
+            'visible: [' + __dbgVisNames + ']\n' +
+            'overflow: [' + __dbgOvrNames + ']\n' +
+            'ua: ' + (navigator.userAgent || '').slice(0,80);
+          if (!__dbgBanner.parentNode) document.body.appendChild(__dbgBanner);
+          console.warn('[NAV-DEBUG-1396]', __dbgBanner.textContent);
+        }
+      } catch (e) { /* diagnostic only — never break nav */ }
     }
 
     // Run once on load, again after fonts settle (label widths shift),