feat: audio playback issue

Task #60 completed by ClaudeQueue

ClaudeQueue

frontend/app.js

@@ -407,6 +407,7 @@ const Player = {
   master: els.video,
   secondary: null,     // synced audio element in dual mode
   driftTimer: null,
+  bufferGraceTimer: null, // pending "pause audio after a stall that outlasts the grace window" timer
   _wantsPlaying: false, // tracks user/app *intent* to be playing, independent
                          // of what the underlying <audio>/<video> elements
                          // report — iOS Safari (esp. PWA/standalone on iPhone)
@@ -675,6 +676,7 @@ const Player = {
   },
   pause() {
     this._wantsPlaying = false;
+    this.clearBufferGrace();
     this.master.pause();
     if (this.secondary) this.secondary.pause();
   },
@@ -698,6 +700,47 @@ const Player = {
     if (this.secondary) this.secondary.playbackRate = r;
   },
 
+  // Drift-correction thresholds for dual mode (muted video + synced audio).
+  // A hard `currentTime` jump on a *playing* <audio> element causes an
+  // audible pop/glitch — every earlier version of the watchdog snapped audio
+  // back into sync any time drift exceeded 0.3s, which is exactly what read
+  // as "audio stutter" (video stayed smooth because it was never touched).
+  // We now use a tiered response instead of one hard threshold:
+  //   < SOFT_SYNC_THRESHOLD  — inaudible, ignore (avoids constant micro-corrections)
+  //   < HARD_SYNC_THRESHOLD  — gently slew playbackRate so the audio eases back
+  //                            into sync over ~1s with no audible artifact
+  //   >= HARD_SYNC_THRESHOLD — snap (only reached from seeks, buffer flushes,
+  //                            or background-resume, where a single correction
+  //                            is unavoidable and rare)
+  SOFT_SYNC_THRESHOLD: 0.08,
+  HARD_SYNC_THRESHOLD: 0.3,
+  SLEW_RATE: 0.04, // playbackRate offset applied while easing back into sync
+
+  // Shared by the 1s watchdog tick and the master 'playing' handler so both
+  // paths get the same no-pop behavior.
+  correctDrift() {
+    if (this.mode !== 'dual' || !this.secondary || this.master.paused) return;
+    const drift = this.secondary.currentTime - this.master.currentTime;
+    const abs = Math.abs(drift);
+    if (abs >= this.HARD_SYNC_THRESHOLD) {
+      this.secondary.currentTime = this.master.currentTime;
+      this._resyncSpeed();
+    } else if (abs >= this.SOFT_SYNC_THRESHOLD) {
+      // Audio ahead of video -> slow audio down; audio behind -> speed it up.
+      const base = parseFloat(els.speed.value) || 1;
+      this.secondary.playbackRate = drift > 0 ? base - this.SLEW_RATE : base + this.SLEW_RATE;
+    } else {
+      this._resyncSpeed();
+    }
+  },
+  // Restore the secondary's playbackRate to the user-selected speed once
+  // drift is within tolerance (or after a hard snap) so a slew correction
+  // never lingers and overshoots.
+  _resyncSpeed() {
+    const base = parseFloat(els.speed.value) || 1;
+    if (this.secondary && this.secondary.playbackRate !== base) this.secondary.playbackRate = base;
+  },
+
   // Runs for every mode (not just 'dual') as a watchdog: iOS Safari — most
   // visibly in PWA/standalone mode on iPhone — can silently pause the element
   // actually producing sound (the <audio> tag in 'dual'/'audio' mode) while
@@ -710,11 +753,9 @@ const Player = {
     this.stopDrift();
     this.driftTimer = setInterval(() => {
       if (!this._wantsPlaying) return;
-      // Dual-mode: keep the secondary audio track in sync with the master.
-      if (this.mode === 'dual' && !this.master.paused) {
-        const drift = Math.abs(this.secondary.currentTime - this.master.currentTime);
-        if (drift > 0.3) this.secondary.currentTime = this.master.currentTime;
-      }
+      // Dual-mode: keep the secondary audio track in sync with the master,
+      // via the tiered corrector above (no hard snap for small, normal drift).
+      this.correctDrift();
       // The element that's actually producing sound in the current mode.
       const sounder = this.soundEl;
       if (sounder && sounder.paused) {
@@ -726,6 +767,17 @@ const Player = {
   stopDrift() {
     if (this.driftTimer) clearInterval(this.driftTimer);
     this.driftTimer = null;
+    this.clearBufferGrace();
+  },
+  // Cancel a pending "pause synced audio after a stall" timer — used once the
+  // video recovers (playing/canplay), on an explicit pause, and when tearing
+  // down the current element (attach()/stopDrift()) so a stale timer never
+  // fires against a track that's already moved on.
+  clearBufferGrace() {
+    if (this.bufferGraceTimer) {
+      clearTimeout(this.bufferGraceTimer);
+      this.bufferGraceTimer = null;
+    }
   },
   // Called when the page/tab regains foreground focus (screen unlock, app
   // switch back). This is the single most common trigger for iOS silently
@@ -800,10 +852,28 @@ function wirePlayerEvents() {
 
   function bind(el) {
     el.addEventListener('play', () => { if (masterIs(el) && Player.secondary && Player.secondary.paused) { Player.secondary.currentTime = el.currentTime; Player.secondary.play().catch(() => {}); } updatePlayBtn(); });
-    el.addEventListener('pause', () => { if (masterIs(el) && Player.secondary) Player.secondary.pause(); updatePlayBtn(); });
+    el.addEventListener('pause', () => { if (masterIs(el) && Player.secondary) { Player.clearBufferGrace(); Player.secondary.pause(); } updatePlayBtn(); });
     el.addEventListener('seeking', () => { if (masterIs(el) && Player.secondary) Player.secondary.currentTime = el.currentTime; });
-    el.addEventListener('waiting', () => { if (masterIs(el)) { showSpinner(true); if (Player.secondary) Player.secondary.pause(); } });
-    el.addEventListener('playing', () => { if (masterIs(el)) { showSpinner(false); if (Player.secondary && !el.paused) { Player.secondary.currentTime = el.currentTime; Player.secondary.play().catch(() => {}); } } });
+    // A brief 'waiting' (video re-buffering under CPU/memory pressure) used to
+    // pause the synced audio immediately — every few-hundred-ms video stall
+    // cut audio output, which is heard as a stutter even though the audio
+    // pipeline itself was fine. Give the video a short grace window to
+    // recover on its own before touching the still-playing audio track; only
+    // pause it if the stall actually outlasts that window.
+    el.addEventListener('waiting', () => {
+      if (!masterIs(el)) return;
+      showSpinner(true);
+      if (Player.secondary && !Player.secondary.paused) {
+        Player.clearBufferGrace();
+        Player.bufferGraceTimer = setTimeout(() => {
+          Player.bufferGraceTimer = null;
+          if (Player.master.paused || Player.master.readyState < 3) {
+            if (Player.secondary) Player.secondary.pause();
+          }
+        }, 250);
+      }
+    });
+    el.addEventListener('playing', () => { if (masterIs(el)) { showSpinner(false); Player.clearBufferGrace(); if (Player.secondary && !el.paused) { Player.secondary.currentTime = el.currentTime; Player.secondary.play().catch(() => {}); } } });
     el.addEventListener('canplay', () => { if (masterIs(el)) showSpinner(false); });
     el.addEventListener('timeupdate', () => {
       if (masterIs(el)) {