fix(landing): reset demo step index when the preview auto-cycle restarts

- reactive isDesktop can re-run the demo effect on a 1024px resize; reset demoIndexRef to 0 so applyDemoStep(step 0) and scheduleNextStep read the same step, instead of replaying from a stale index and skipping beats
- clarify why the chat reveal's previous-value ref is safe here (timer-driven, no concurrent-interruption boundary)

Author: waleedlatif1

apps/sim/app/(landing)/components/landing-preview/components/landing-preview-chat/landing-preview-chat.tsx

@@ -45,8 +45,11 @@ export function LandingPreviewChat({ chat, chatName, animationKey }: LandingPrev
   const [phase, setPhase] = useState<RevealPhase>('hidden')
   const revealRef = useRef<{ key: number; chat: PreviewChat | null }>({ key: animationKey, chat })
 
-  // Restart the reveal synchronously when the timeline (or staged chat) changes,
-  // so the pane never flashes the previous step's reply before the effect reruns.
+  // Restart the reveal synchronously when the timeline (or staged chat) changes, so
+  // the pane never flashes the previous step's reply before the effect reruns. The
+  // previous key/chat are tracked in a ref: these updates come from the parent's
+  // timer-driven state (never a transition/Suspense boundary), so the render can't be
+  // discarded between the ref write and commit - keeping the ref safe here.
   if (revealRef.current.key !== animationKey || revealRef.current.chat !== chat) {
     revealRef.current = { key: animationKey, chat }
     setPhase('hidden')

apps/sim/app/(landing)/components/landing-preview/landing-preview.tsx

@@ -183,6 +183,9 @@ export function LandingPreview({
     // the auto-demo on the cycle still being active - a resize after the visitor
     // has interacted must not restart the sequence from the first step.
     if (!isDesktop || !autoplay || !autoCycleActiveRef.current) return
+    // Reset the step index so the restart shows step 0 AND `scheduleNextStep` reads
+    // step 0's delay - otherwise a resize would replay from a stale index and skip beats.
+    demoIndexRef.current = 0
     applyDemoStep(DESKTOP_STEPS[0])
     scheduleNextStep()
     return clearDemoTimer