perf: event-driven idle + lazy audio device for near-zero idle CPU

raylib's frame limiter partial-busy-waits ~5% of every frame interval, so
capping the FPS couldn't get idle CPU below ~5% of a core, and the audio
output device's mixing thread spun continuously even when nothing played.

- Idle render: go fully event-driven when idle. While active (focused +
  something animating, or within a 0.5s grace window) run at ACTIVE_FPS;
  otherwise EnableEventWaiting() so EndDrawing's PollInputEvents blocks on
  glfwWaitEvents until the next input/window event. Unfocused windows skip
  drawing entirely and block. Headless render is exempt (its hidden window
  gets no events). Replaces the earlier fixed-FPS idle throttle.
- ACTIVE_FPS lowered 60 -> 30: ample for a non-game UI, halves active cost.
- Lazy audio device: open it on demand at first playback (EnsureAudioDevice)
  and release it once idle and not playing (ReleaseAudioDevice), reopened on
  the next play. Never closes mid-playback (gated on !isPlaying), so audio
  always finishes first. Shutdown is guarded with IsAudioDeviceReady().

Idle/backgrounded window now drops to ~0% CPU (no render busy-wait, no audio
thread); snaps back instantly on input.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/audio.c

@@ -216,12 +216,28 @@ static float* BuildSelectionAudio(int* outNumSamples)
     return regionSamples;
 }
 
+void EnsureAudioDevice(void)
+{
+    if (!IsAudioDeviceReady()) InitAudioDevice();
+}
+
+void ReleaseAudioDevice(void)
+{
+    if (!IsAudioDeviceReady()) return;
+    if (AudioPlaybackSound.frameCount != 0) {
+        UnloadSound(AudioPlaybackSound);
+        AudioPlaybackSound = (Sound){ 0 };
+    }
+    CloseAudioDevice();
+}
+
 void PlaySelectedRegion(void)
 {
     int numSamples = 0;
     float* regionSamples = BuildSelectionAudio(&numSamples);
     if (!regionSamples) return;
 
+    EnsureAudioDevice();  // opened on demand; released again once playback ends
     if (AudioPlaybackSound.frameCount != 0) UnloadSound(AudioPlaybackSound);
 
     Wave wave = { .data = regionSamples, .frameCount = (unsigned int)numSamples,

src/audio.h

@@ -10,8 +10,16 @@ bool LoadWavFile(const char* filepath, AudioSignal* signal);
 void FreeSignal(AudioSignal* signal);
 
 // Play the current time/frequency selection (applies an FFT bandpass filter).
+// Opens the audio output device on demand (see EnsureAudioDevice).
 void PlaySelectedRegion(void);
 
+// Audio output device lifecycle. The device is opened lazily (only when
+// something actually plays) and released while idle, so a backgrounded window
+// isn't holding the output device open — keeping it open costs steady CPU in
+// miniaudio's mixing thread.
+void EnsureAudioDevice(void);   // InitAudioDevice() if not already initialized
+void ReleaseAudioDevice(void);  // unload the playback sound + CloseAudioDevice()
+
 // Save the current selection (band-limited, time-cropped — same processing as
 // playback) as a mono WAV in dirPath. Reports the result via app.exportMessage.
 void ExportSelectionWAV(const char* dirPath);

src/spectrogram.c

@@ -70,12 +70,11 @@ static bool IsUserInteracting(void)
     return false;
 }
 
-// Idle FPS throttle. raylib re-renders the whole scene every frame regardless
-// of whether anything changed, so an idle window still pins the GPU (and, on a
-// software GL stack, the CPU) at 60 fps. We drop to IDLE_FPS whenever nothing
-// needs animating, and snap back to ACTIVE_FPS the moment activity resumes.
-#define ACTIVE_FPS         60
-#define IDLE_FPS           10
+// Idle power management. raylib re-renders the whole scene every frame, so an
+// idle window otherwise pins the GPU (and, on software GL, the CPU) at the
+// target rate. We run at ACTIVE_FPS while something needs animating, then go
+// fully event-driven (block until input) when idle — see the loop below.
+#define ACTIVE_FPS         30   // plenty for a non-game UI; halves active-frame cost
 #define IDLE_GRACE_SECONDS 0.5  // stay at full rate briefly after the last activity
 
 /**
@@ -621,9 +620,10 @@ int main(int argc, char* argv[])
     }
 #endif
     InitWindow(headless ? reqW : 1280, headless ? reqH : 800, "Spectrogram Viewer");
-    SetTargetFPS(60);
+    SetTargetFPS(ACTIVE_FPS);
     SetTraceLogLevel(LOG_WARNING);  // Suppress INFO texture logs
-    InitAudioDevice();
+    // Audio device is opened lazily on first playback (see EnsureAudioDevice)
+    // and released while idle, so an idle/backgrounded window holds no device.
     SetExitKey(KEY_NULL);  // ESC should not close the window
 
     // Save original working directory so command-line args resolve correctly
@@ -782,26 +782,42 @@ int main(int argc, char* argv[])
         // Track the browser viewport (fill + reflow on resize, like desktop).
         SyncCanvasToWindow();
 #else
-        // When the window isn't focused there's nothing for the user to see, so
-        // skip the entire frame: pump window events (so focus regain and the
-        // close button still register) and idle, with zero drawing. The
-        // headless render uses a hidden window that never reports focus, so it
-        // is exempt — otherwise it would never draw its single capture frame.
-        if (!headless && !IsWindowFocused()) {
-            PollInputEvents();
-            WaitTime(0.1);  // ~10 Hz event poll, no rendering
-            continue;
-        }
-
-        // Idle FPS throttle (desktop only; the browser drives RAF/vsync itself).
-        // Keep full rate while active or within a short grace window after the
-        // last activity, then fall back to a low idle rate.
-        {
+        // Desktop power management. raylib's frame limiter partial-busy-waits
+        // ~5% of every frame interval, so capping the FPS can't get idle CPU
+        // below ~5% of a core. Instead we go fully event-driven when idle:
+        // block in PollInputEvents() (glfwWaitEvents) until an input/window
+        // event arrives, so an idle/backgrounded window costs ~0% CPU and only
+        // redraws on demand. Headless render (hidden window, single frame) opts
+        // out — its window never receives events, so waiting would deadlock.
+        if (!headless) {
             static double lastActive = -1000.0;
-            static int    curFps = ACTIVE_FPS;
-            if (IsAppActive()) lastActive = GetTime();
-            int wantFps = (GetTime() - lastActive < IDLE_GRACE_SECONDS) ? ACTIVE_FPS : IDLE_FPS;
-            if (wantFps != curFps) { SetTargetFPS(wantFps); curFps = wantFps; }
+            static int waiting = -1;  // -1 unset, 0 = active (poll), 1 = idle (event-wait)
+
+            bool focused = IsWindowFocused();
+            if (focused && IsAppActive()) lastActive = GetTime();
+            // Active = focused AND something needs animating (or just did, within
+            // the grace window). Anything else is a static frame we can sleep on.
+            bool active = focused && (GetTime() - lastActive < IDLE_GRACE_SECONDS);
+
+            if (active) {
+                if (waiting != 0) { DisableEventWaiting(); SetTargetFPS(ACTIVE_FPS); waiting = 0; }
+            } else {
+                // Idle: no busy-wait limiter; EndDrawing's PollInputEvents blocks.
+                if (waiting != 1) { SetTargetFPS(0); EnableEventWaiting(); waiting = 1; }
+                // Release the output device while idle — but never mid-playback
+                // (isPlaying gates it, so audio always finishes first). Reopened
+                // on the next play. Safe on the unfocused path too: only closes
+                // when nothing is playing.
+                if (!app.isPlaying && IsAudioDeviceReady()) ReleaseAudioDevice();
+                if (!focused) {
+                    // Unfocused: nothing to show. Block on events (refocus/close)
+                    // without drawing at all.
+                    PollInputEvents();
+                    continue;
+                }
+                // Focused but idle: fall through to draw this one static frame,
+                // then EndDrawing blocks until the next event.
+            }
         }
 #endif
 
@@ -1682,7 +1698,7 @@ int main(int argc, char* argv[])
 
     TraceLog(LOG_INFO, "Shutting down...");
     if (mainFont.texture.id != 0) UnloadFont(mainFont);
-    if (AudioPlaybackSound.frameCount != 0) UnloadSound(AudioPlaybackSound);
+    if (IsAudioDeviceReady() && AudioPlaybackSound.frameCount != 0) UnloadSound(AudioPlaybackSound);
     if (app.stftComputed) { FreeSTFT(&app.stft); UnloadImage(app.spectrogramImage); UnloadTexture(app.spectrogramTexture); }
     if (app.visibleTexture.id != 0) UnloadTexture(app.visibleTexture);
     app.visibleTexture = (Texture2D){ 0 };
@@ -1693,7 +1709,7 @@ int main(int argc, char* argv[])
     free(app.reassignBuffer);
     FreeMlnl(&app.annotations);
     FreeSignal(&app.signal);
-    CloseAudioDevice();
+    if (IsAudioDeviceReady()) CloseAudioDevice();
     CloseWindow();
     return headlessRc;
 }