[fix] correct video latency measurement and smooth connection RTT

apps/desktop/src/gui/runtime/connection_latency.h

@@ -0,0 +1,59 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-09-26
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _CONNECTION_LATENCY_H_
+#define _CONNECTION_LATENCY_H_
+
+#include <chrono>
+#include <cmath>
+#include <mutex>
+#include <optional>
+
+namespace crossdesk::gui_detail {
+
+// Presentation-only smoothing. The transport's raw RTT remains available for
+// recovery, congestion control and diagnostics. A missing one-second report
+// does not flicker, but a disconnected/stalled peer cannot leave a stale value.
+class ConnectionLatencyStats {
+ public:
+  using Clock = std::chrono::steady_clock;
+
+  void Update(double rtt_ms, Clock::time_point now = Clock::now()) {
+    if (!std::isfinite(rtt_ms) || rtt_ms < 0 || rtt_ms > 2000) return;
+    std::lock_guard lock(mutex_);
+    if (average_ms_ && now < updated_) return;
+    if (!Fresh(now)) {
+      average_ms_ = rtt_ms;
+    } else {
+      *average_ms_ += 0.25 * (rtt_ms - *average_ms_);
+    }
+    updated_ = now;
+  }
+
+  std::optional<double> Get(Clock::time_point now = Clock::now()) const {
+    std::lock_guard lock(mutex_);
+    return Fresh(now) ? average_ms_ : std::nullopt;
+  }
+
+  void Reset() {
+    std::lock_guard lock(mutex_);
+    average_ms_.reset();
+  }
+
+ private:
+  bool Fresh(Clock::time_point now) const {
+    // The UI may snapshot `now` just before the report thread updates us.
+    // A newer valid report must not cause a one-frame unavailable flicker.
+    return average_ms_ && now - updated_ < std::chrono::seconds(3);
+  }
+  mutable std::mutex mutex_;
+  std::optional<double> average_ms_;
+  Clock::time_point updated_{};
+};
+
+}  // namespace crossdesk::gui_detail
+
+#endif