components/scenario-form.jsx

@@ -237,16 +237,9 @@ export default function ScenarioForm() {
 
   const branchScoped = useBranchScopedFixture();
   // Toggle to control when the experiment view is visible vs production
-  const [showExperimentView, setShowExperimentView] = useState(
-    typeof window !== "undefined" ? (() => {
-      const saved = sessionStorage?.getItem("ce-show-experiment");
-      // Only default to true if user explicitly toggled ON (saved === "true")
-      // or has an active investigation context (session exists with situationGraph)
-      if (saved === "true") return true;
-      const session = getSession();
-      return Boolean(session?.situationGraph);
-    })() : false,
-  );
+  // Always start false for SSR-safe deterministic first render.
+  // sessionStorage reads are deferred to useEffect (after mount).
+  const [showExperimentView, setShowExperimentView] = useState(false);
 
   // Use fixture branches as the authoritative source when available
   const BRANCHES = useMemo(() => branchScoped.getBranches(), [branchScoped]);
@@ -309,6 +302,20 @@ export default function ScenarioForm() {
     setStatus("success");
   }, []);
 
+  /* Restore experiment view preference from storage (after hydration) ─ */
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+    const savedExp = sessionStorage?.getItem("ce-show-experiment");
+    if (savedExp === "true") {
+      setShowExperimentView(true);
+      return;
+    }
+    const session = getSession();
+    if (session?.situationGraph) {
+      setShowExperimentView(true);
+    }
+  }, []);
+
   /* Restore facilitator dismiss preference (Experiment 05) ─── */
   useEffect(() => {
     if (typeof window === "undefined") return;