fix: WebRTC audio — glare handling (Perfect Negotiation) + autoplay + signalingState guard

frontend/js/classroom-rtc.js

@@ -193,6 +193,18 @@ class ClassroomRTC {
 
   async _handleOffer(fromUid, payload) {
     const peer = this._getOrCreate(fromUid);
+
+    // Perfect Negotiation: resolve offer collision (both peers sending offers simultaneously).
+    // Polite peer (higher uid) yields — rolls back its own offer and handles the incoming one.
+    // Impolite peer (lower uid) ignores the collision offer and waits for the answer to its own.
+    const polite = this._uid > fromUid;
+    const collision = peer.pc.signalingState !== 'stable' || peer.negotiating;
+    if (collision) {
+      if (!polite) return;
+      // Polite: explicit rollback for older browsers; modern Chrome handles it implicitly
+      await peer.pc.setLocalDescription({ type: 'rollback' }).catch(() => {});
+    }
+
     await peer.pc.setRemoteDescription({ type: 'offer', sdp: payload.sdp });
     peer.remoteSet = true;
     await this._flushCandidates(peer);
@@ -277,7 +289,7 @@ class ClassroomRTC {
 
     /* Renegotiation: fires when tracks are added/removed (e.g. screen share) */
     pc.onnegotiationneeded = async () => {
-      if (peer.negotiating || !peer.remoteSet) return;
+      if (peer.negotiating || !peer.remoteSet || pc.signalingState !== 'stable') return;
       peer.negotiating = true;
       try {
         const offer = await pc.createOffer();
@@ -299,6 +311,7 @@ class ClassroomRTC {
           document.body.appendChild(peer.audioEl);
         }
         peer.audioEl.srcObject = stream;
+        peer.audioEl.play().catch(() => {});
         this._startVAD(uid, stream);
       } else if (track.kind === 'video') {
         if (this._onScreen) this._onScreen(stream);