fix: Resolve listening post audio stuttering introduced in v2.15.0

Throttle audio waterfall rendering (50ms→200ms), eliminate per-frame
Array.from() allocation, drain stale pipe buffer before streaming,
increase chunk size to 8192, and remove debug logging from animation
hot paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/listening_post.py

@@ -1456,13 +1456,30 @@ def stream_audio() -> Response:
         if not proc or not proc.stdout:
             return
         try:
-            # First byte timeout to avoid hanging clients forever
+            # Drain stale audio that accumulated in the pipe buffer
+            # between pipeline start and stream connection.  Keep the
+            # first chunk (contains WAV header) and discard the rest
+            # so the browser starts close to real-time.
+            header_chunk = None
+            while True:
+                ready, _, _ = select.select([proc.stdout], [], [], 0)
+                if not ready:
+                    break
+                chunk = proc.stdout.read(8192)
+                if not chunk:
+                    break
+                if header_chunk is None:
+                    header_chunk = chunk
+            if header_chunk:
+                yield header_chunk
+
+            # Stream real-time audio
             first_chunk_deadline = time.time() + 3.0
             while audio_running and proc.poll() is None:
                 # Use select to avoid blocking forever
                 ready, _, _ = select.select([proc.stdout], [], [], 2.0)
                 if ready:
-                    chunk = proc.stdout.read(4096)
+                    chunk = proc.stdout.read(8192)
                     if chunk:
                         yield chunk
                     else:

static/js/modes/listening-post.js

@@ -1742,9 +1742,6 @@ function initSynthesizer() {
     drawSynthesizer();
 }
 
-// Debug: log signal level periodically
-let lastSynthDebugLog = 0;
-
 function drawSynthesizer() {
     if (!synthCtx || !synthCanvas) return;
 
@@ -1760,19 +1757,6 @@ function drawSynthesizer() {
     let activityLevel = 0;
     let signalIntensity = 0;
 
-    // Debug logging every 2 seconds
-    const now = Date.now();
-    if (now - lastSynthDebugLog > 2000) {
-        console.log('[SYNTH] State:', {
-            isScannerRunning,
-            isDirectListening,
-            scannerSignalActive,
-            currentSignalLevel,
-            visualizerAnalyser: !!visualizerAnalyser
-        });
-        lastSynthDebugLog = now;
-    }
-
     if (isScannerRunning && !isScannerPaused) {
         // Use actual signal level data (0-5000 range, normalize to 0-1)
         signalIntensity = Math.min(1, currentSignalLevel / 3000);
@@ -1864,13 +1848,6 @@ function drawSynthesizer() {
     synthCtx.lineTo(width, height / 2);
     synthCtx.stroke();
 
-    // Debug: show signal level value
-    if (isScannerRunning || isDirectListening) {
-        synthCtx.fillStyle = 'rgba(255, 255, 255, 0.5)';
-        synthCtx.font = '9px monospace';
-        synthCtx.fillText(`lvl:${Math.round(currentSignalLevel)}`, 4, 10);
-    }
-
     synthAnimationId = requestAnimationFrame(drawSynthesizer);
 }
 
@@ -3109,7 +3086,7 @@ let waterfallEndFreq = 108;
 let waterfallRowImage = null;
 let waterfallPalette = null;
 let lastWaterfallDraw = 0;
-const WATERFALL_MIN_INTERVAL_MS = 50;
+const WATERFALL_MIN_INTERVAL_MS = 200;
 let waterfallInteractionBound = false;
 let waterfallResizeObserver = null;
 let waterfallMode = 'rf';
@@ -3436,9 +3413,8 @@ function startAudioWaterfall() {
         if (ts - lastAudioWaterfallDraw >= WATERFALL_MIN_INTERVAL_MS) {
             lastAudioWaterfallDraw = ts;
             visualizerAnalyser.getByteFrequencyData(dataArray);
-            const bins = Array.from(dataArray, v => v);
-            drawWaterfallRow(bins);
-            drawSpectrumLine(bins, 0, maxFreqKhz, 'kHz');
+            drawWaterfallRow(dataArray);
+            drawSpectrumLine(dataArray, 0, maxFreqKhz, 'kHz');
         }
         audioWaterfallAnimId = requestAnimationFrame(drawFrame);
     };