perf(mining): run the mine-when-idle check at 1 Hz, probe the GPU sparingly

checkIdleMining ran every frame: an X11 idle query and, with GPU-aware
idle detection, an nvidia-smi launch on the UI thread per frame.

- The check runs once a second, since its decisions work on a seconds
  scale (the idle delay is tens of seconds or more). A miner it started
  or scaled up still yields on the first frame that sees the user back:
  that path skips the gate, because frames can be seconds apart while
  the app idles.
- The GPU is probed only when input is idle and the result can change
  a decision, at most every 5 s, with hysteresis (busy at 10%+, clear
  below 5%) so a GPU hovering near 10% can't keep restarting the miner.
- Thread scaling returns early when no miner is running or a toggle is
  in flight.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app_security.cpp

@@ -752,6 +752,17 @@ void App::checkIdleMining() {
     // Skip idle mining adjustments while thread benchmark is running
     if (ui::IsMiningBenchmarkActive()) return;
 
+    // The decisions below work on a seconds scale (the idle delay is tens of seconds or more), so check
+    // at 1 Hz. This used to run every frame: an X11 idle query and, with GPU-aware idle, an nvidia-smi
+    // launch on the UI thread per frame. Steady clock rather than ImGui::GetTime(), whose clock stops
+    // while minimized (update() still runs then).
+    const auto now = std::chrono::steady_clock::now();
+    // ...except while a miner this feature started (or scaled up) is running: it must yield on the first
+    // frame that sees the user back, and frames can be seconds apart when the app idles, so a gated frame
+    // may be the only one carrying that input. The idle query is cheap; the GPU probe has its own cache.
+    if (!(idle_mining_active_ || idle_scaled_to_idle_) && now < idle_mining_next_check_) return;
+    idle_mining_next_check_ = now + std::chrono::seconds(1);
+
     int idleSec = util::Platform::getSystemIdleSeconds();
     int delay = settings_->getMineIdleDelay();
     bool isPool = settings_->getPoolMode() || !supportsSoloMining();
@@ -759,26 +770,40 @@ void App::checkIdleMining() {
     bool threadScaling = settings_->getIdleThreadScaling();
     int maxThreads = std::max(1, (int)std::thread::hardware_concurrency());
 
-    // GPU-aware idle detection: if enabled, treat GPU utilization >= 10%
-    // as "user active" (e.g. watching a video).  Disabled = unrestricted
-    // mode that only looks at keyboard/mouse input.
-    bool gpuBusy = false;
-    if (settings_->getIdleGpuAware()) {
-        int gpuUtil = util::Platform::getGpuUtilization();
-        gpuBusy = (gpuUtil >= 10);
-    }
-    bool systemIdle = (idleSec >= delay) && !gpuBusy;
-
     // Check if mining is already running (manually started by user)
     bool miningActive = isPool
         ? (xmrig_manager_ && xmrig_manager_->isRunning())
         : state_.mining.generate;
 
+    // Thread scaling only adjusts a miner the user started: with none running there is nothing to
+    // decide, or to probe for.
+    if (threadScaling && (!miningActive || mining_toggle_in_progress_.load())) return;
+
+    // GPU-aware idle detection: if enabled, treat GPU utilization >= 10% as "user active" (e.g. watching
+    // a video). Disabled = unrestricted mode that only looks at keyboard/mouse input. The probe can only
+    // matter once input is idle and a decision can change (start/stop mode leaves a miner the user
+    // started alone), so it runs only then, at most every 5 s, with hysteresis (busy at >= 10%, clear
+    // below 5%) so a GPU hovering near 10% can't keep restarting the miner. Unknown (-1) reads as not busy.
+    const bool inputIdle = idleSec >= delay;
+    const bool decisionPossible = threadScaling || !miningActive || idle_mining_active_;
+    bool gpuBusy = false;
+    if (inputIdle && decisionPossible && settings_->getIdleGpuAware()) {
+        if (now >= idle_gpu_next_sample_) {
+            const int gpuUtil = util::Platform::getGpuUtilization();
+            if (gpuUtil >= 10) idle_gpu_busy_ = true;
+            else if (gpuUtil < 5) idle_gpu_busy_ = false;
+            idle_gpu_next_sample_ = now + std::chrono::seconds(5);
+        }
+        gpuBusy = idle_gpu_busy_;
+    } else if (!inputIdle) {
+        idle_gpu_busy_ = false;          // input activity decides now; sample afresh once idle again
+        idle_gpu_next_sample_ = {};
+    }
+    bool systemIdle = inputIdle && !gpuBusy;
+
     if (threadScaling) {
         // --- Thread scaling mode ---
-        // Mining must already be running (started by user). We just adjust threads.
-        if (!miningActive || mining_toggle_in_progress_.load()) return;
-
+        // Mining is already running (started by user; checked above). We just adjust threads.
         int activeThreads = settings_->getIdleThreadsActive();
         int idleThreads   = settings_->getIdleThreadsIdle();
         // Resolve auto values: active defaults to half, idle defaults to all