zebra-spaces: chromium decoder anchor — muted <audio> alongside MediaStreamSource

Chromium will not run its WebRTC audio decoder for a remote track
that is only consumed by a MediaStreamAudioSourceNode. The track
must also be attached to an HTMLMediaElement to "kick" the decoder
into running. Without the anchor the worklet's queue fills with
zeros — the 'started' event fires, ctxState=running, RTP packets
arrive at 50 pps, yet PulseAudio Playback shows the chrome stream
present with the level meter dead at 0. Fox 2026-06-07 fedora
chrome silent for days; firefox unaffected (its decoder runs
unconditionally for MediaStreamSource consumers).

Each attach now creates a hidden muted <audio> bound to the same
MediaStream. The element makes no audible output (muted=true); its
sole purpose is to keep chromium's decoder running so the worklet's
MediaStreamSource sees real samples. setWorkletStream swaps the
anchor's srcObject alongside the source node so an in-place stream
swap doesn't strand the anchor on a dead track. detachListenerStream
tears the anchor down with the rest of the chain.

Tests pin the contract: anchor created on attach, anchor follows the
stream on setWorkletStream, anchor torn down on detach.

test/listener-audio-attach.test.js

@@ -281,6 +281,23 @@ function makeBrowser(role){
     const navigator = {};      /* skips the Media Session block cleanly */
     const window = { AudioContext: function(){ audioCtxCreates++; return makeAudioCtx_(); } };
     const AudioWorkletNode = AudioWorkletNodeClass;
+    /* minimal document for createElement('audio') — the decoder-anchor
+     * code in attachAudioStreamViaWorklet creates a hidden <audio>
+     * bound to the stream. Tests need a stub that returns an object
+     * with srcObject, play(), pause(), remove(), and style. */
+    const document = {
+      createElement(tag){
+        return {
+          tagName: String(tag).toUpperCase(),
+          srcObject: null, muted: false, autoplay: false, playsInline: false,
+          style: {},
+          play(){ return Promise.resolve(); },
+          pause(){},
+          remove(){},
+        };
+      },
+      body: { appendChild(){} },
+    };
     /* SPEAKER_PLAYOUT_DELAY_SEC + RECV_PLAYOUT_DELAY_SEC come in as args */
   `;
 
@@ -516,6 +533,44 @@ test('listener: attaching uuid B for the same publisher as uuid A detaches A (no
 });
 
 /* -------- unit: detachListenerStream cleans everything -------- */
+test('attach creates a muted decoder-anchor <audio> element bound to the same stream (chromium decoder kick)', () => {
+  /* Chromium will not run its WebRTC audio decoder for a remote track
+   * that is only consumed by a MediaStreamAudioSourceNode. The anchor
+   * element forces the decoder to run; without it the worklet sees
+   * silence even though packets arrive. */
+  const b = makeBrowser('listener');
+  addMember(b, UUID_A, PUB_A);
+  const stream = makeStream();
+  b.api.attachAudioStreamViaWorklet(UUID_A, stream, RECV);
+  const node = b.api.listenerAudioNodes.get(UUID_A);
+  truthy(node.anchor, 'anchor element created');
+  eq(node.anchor.muted, true, 'anchor is muted (no audible output)');
+  eq(node.anchor.srcObject, stream, 'anchor bound to the same stream');
+});
+
+test('setWorkletStream swaps the anchor srcObject too (keeps decoder kick on the new stream)', () => {
+  const b = makeBrowser('listener');
+  addMember(b, UUID_A, PUB_A);
+  b.api.attachAudioStreamViaWorklet(UUID_A, makeStream(), RECV);
+  const node = b.api.listenerAudioNodes.get(UUID_A);
+  const anchor = node.anchor;
+  const newStream = makeStream();
+  b.api.setWorkletStream(UUID_A, newStream);
+  eq(node.anchor, anchor, 'same anchor element reused');
+  eq(node.anchor.srcObject, newStream, 'anchor follows the new stream');
+});
+
+test('detachListenerStream removes the anchor element', () => {
+  const b = makeBrowser('listener');
+  addMember(b, UUID_A, PUB_A);
+  b.api.attachAudioStreamViaWorklet(UUID_A, makeStream(), RECV);
+  const node = b.api.listenerAudioNodes.get(UUID_A);
+  const anchor = node.anchor;
+  truthy(anchor, 'anchor exists');
+  b.api.detachListenerStream(UUID_A);
+  eq(anchor.srcObject, null, 'anchor srcObject cleared');
+});
+
 test('detachListenerStream: removes from map and disconnects src, jbuf, gain', () => {
   const b = makeBrowser('listener');
   addMember(b, UUID_A, PUB_A);

web/zebra-spaces.html

@@ -3365,7 +3365,31 @@ function attachAudioStreamViaWorklet(uuid, stream, targetSeconds){
   gain.gain.value = 1.0;
   src.connect(gain);
   gain.connect(audioCtx.destination);
-  const node = { src, gain, stream, targetSeconds };
+  /* Chrome decoder anchor: chromium's WebRTC decoder will not run for
+   * a remote audio track that is only consumed by a MediaStreamAudioSourceNode.
+   * The track must also be attached to an HTMLMediaElement consumer to
+   * "kick" the decoder; otherwise the MediaStreamSource produces silence
+   * even though packets arrive and the worklet 'started' event fires
+   * (queue fills with zeros). The element is created muted so it makes
+   * no sound on its own — its only job is to keep chrome's decoder
+   * running so the worklet sees real samples. Firefox doesn't have
+   * this limitation but the same anchor is harmless there.
+   * Fox 2026-06-07 blanka-chrome: PulseAudio Playback showed the chrome
+   * stream present with zero level meter movement until this anchor
+   * landed. */
+  let anchor = null;
+  try {
+    anchor = document.createElement('audio');
+    anchor.muted = true;
+    anchor.autoplay = true;
+    anchor.playsInline = true;
+    anchor.srcObject = stream;
+    anchor.style.display = 'none';
+    document.body.appendChild(anchor);
+    const p = anchor.play();
+    if (p && p.catch) p.catch(()=>{});   /* muted autoplay should never reject, but swallow defensively */
+  } catch(e){ logLine('err','decoder anchor '+uuid.slice(0,4)+': '+e.message); }
+  const node = { src, gain, stream, targetSeconds, anchor };
   listenerAudioNodes.set(uuid, node);
   /* fire-and-forget worklet load on first use; once ready, every
    * existing stream is swapped through the buffer (see loadJitterWorklet) */
@@ -3421,6 +3445,9 @@ function detachListenerStream(uuid){
   try { if (node.jbuf) node.jbuf.disconnect(); } catch(_){}
   try { if (node.capture) node.capture.disconnect(); } catch(_){}
   try { node.gain.disconnect(); } catch(_){}
+  if (node.anchor){
+    try { node.anchor.srcObject = null; node.anchor.pause(); node.anchor.remove(); } catch(_){}
+  }
   listenerAudioNodes.delete(uuid);
 }
 
@@ -3451,6 +3478,12 @@ function setWorkletStream(uuid, newStream){
   else           newSrc.connect(node.gain);
   node.src = newSrc;
   node.stream = newStream;
+  /* swap the decoder anchor's srcObject so chrome's decoder keeps
+   * running for the new track (see attachAudioStreamViaWorklet for
+   * the anchor's role). */
+  if (node.anchor){
+    try { node.anchor.srcObject = newStream; } catch(_){}
+  }
   logLine('', 'worklet source swapped for '+uuid.slice(0,4));
   return true;
 }
@@ -8091,9 +8124,9 @@ logLine('', 'ready — pick a handle, type a rendezvous code, enter the space');
 </script>
 
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