[Client] Detect main-thread event loop stalls (#7155)

* [Client] Detect main-thread event loop stalls

LagMonitor ticks the GUI event loop every 500 ms and records gaps
beyond 2 s as stalls, warning with their duration and keeping a
bounded ring of recent records for diagnostics. Measurement uses a
monotonic QElapsedTimer so wall-clock steps and suspend do not
fabricate stalls. Recorded timestamps stay in wall time for
correlating with user reports.

Took 1 minute

Took 13 minutes


Took 2 minutes

* [Client] Rename LagMonitor constants to SCREAMING_SNAKE_CASE

Took 15 minutes

* [Client] Discard suspend-spanning gaps in LagMonitor

Windows counts sleep time in its monotonic clock, so a suspend would
fabricate one bogus stall per resume. Reset the clock on application
state changes and drop implausibly huge gaps; extract recordGap() for
testability.

Took 3 minutes

* [Client] Unit test LagMonitor stall recording

Drives recordGap() directly to cover the threshold, plausibility cap,
trim, and clear behavior without timing-dependent waits.

Took 36 seconds

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/client/lag_monitor.h

@@ -0,0 +1,87 @@
+/**
+ * @file lag_monitor.h
+ * @ingroup Client
+ */
+
+#ifndef LAG_MONITOR_H
+#define LAG_MONITOR_H
+
+#include <QDateTime>
+#include <QElapsedTimer>
+#include <QList>
+#include <QLoggingCategory>
+#include <QObject>
+
+inline Q_LOGGING_CATEGORY(LagMonitorLog, "lag_monitor");
+
+class QEvent;
+class QTimer;
+
+/**
+ * @brief Detects main-thread event loop stalls ("UI freezes") from the inside.
+ *
+ * A timer is expected to fire every TICK_INTERVAL_MS of wall time. When the
+ * observed gap greatly exceeds that interval, some other task blocked the
+ * event loop for roughly the overshooting duration. This is what separates
+ * "my client froze" from "the network is lagging" in user reports.
+ *
+ * Gaps that span an application state change (suspend, minimize, focus
+ * loss) are discarded, and implausibly huge gaps are dropped, so operating
+ * system power events do not fabricate stalls. This handling is load-bearing
+ * on Windows, where the monotonic clock used by Qt counts sleep time.
+ *
+ * Healthy operation costs one timer wakeup per tick and two integer
+ * comparisons. Allocations happen only when a stall is actually recorded.
+ */
+class LagMonitor : public QObject
+{
+    Q_OBJECT
+
+public:
+    struct StallRecord
+    {
+        qint64 timestampMsSinceEpoch = 0; ///< when the stalled period ended
+        qint64 durationMs = 0; ///< approximate length of the freeze; measured tick to tick, so it can exceed the true
+                               ///< stall by up to TICK_INTERVAL_MS
+    };
+
+    static constexpr int TICK_INTERVAL_MS = 500;
+    static constexpr int STALL_THRESHOLD_MS = 2000;
+    static constexpr int MAX_RECORDED_STALLS = 32;
+
+    /// Gaps beyond this are treated as suspend artifacts rather than stalls.
+    static constexpr qint64 MAX_PLAUSIBLE_STALL_MS = 600000;
+
+    explicit LagMonitor(QObject *parent = nullptr);
+
+    /**
+     * @brief Stalls recorded during this session, oldest first.
+     *
+     * Intended consumers are log output and the diagnostics export. The list
+     * holds at most MAX_RECORDED_STALLS entries.
+     */
+    QList<StallRecord> recentStalls() const;
+
+    void clearStalls();
+
+    /**
+     * @brief Feeds a measured tick-to-tick gap through the detection logic.
+     *
+     * Split out of checkTick so threshold, plausibility, and trim behavior
+     * stay unit-testable without real timing.
+     */
+    void recordGap(qint64 gapMs);
+
+protected:
+    bool eventFilter(QObject *obj, QEvent *event) override;
+
+private slots:
+    void checkTick();
+
+private:
+    QTimer *timer;
+    QElapsedTimer tickClock; ///< monotonic clock, so wall clock steps do not fabricate stalls
+    QList<StallRecord> stalls;
+};
+
+#endif