diff --git a/simulator/static/style.css b/simulator/static/style.css
index 69cfe18..f64d528 100644
--- a/simulator/static/style.css
+++ b/simulator/static/style.css
@@ -80,6 +80,9 @@ input[type=range], select { width: 100%; }
 .dev-switch input:focus-visible + .dev-switch-track { outline: 2px solid #9af; outline-offset: 1px; }
 .dev-switch-label { color: #9af; font-weight: 600; letter-spacing: 0.3px; }
 .dev-panel { display: flex; flex-direction: column; gap: 0.8rem; }
+/* Beat the generic `.hidden` (defined earlier, equal specificity): two classes
+   win, so the panel truly collapses when Dev Mode is off. */
+.dev-panel.hidden { display: none; }
 .dev-panel legend { color: #4e9; }
 .dev-btn { margin-top: 0.5rem; width: 100%; padding: 0.4rem; cursor: pointer;
            background: #182028; color: #cde; border: 1px solid #2c3c5c; border-radius: 4px; }