zebra-spaces: rebuild <video> on MSID supplant — autoplay grant resets

Live-room defect (2026-06-03): Will refreshed his browser; the
supplant fired a SECOND ontrack for kind=camera pub=Will at +16s.
The page swapped srcObject on the existing <video>, called play(),
got 'fetching process for the media resource was aborted by the
user agent at the user's request' — browser refused to start a new
playback session on the same element after its autoplay grant had
already been consumed. Tile sat black on the moderator's screen.

Fix: swapFreshVideoElement() — on supplant, replace the <video>
with a freshly-built one carrying the same attrs. A brand-new
<video> is eligible for muted-autoplay even when the prior one had
its play() rejected, so the supplant lands cleanly without needing
a tap. Applied to both the thumbnail and the spotlight tile.

Mesh test harness extracts the helper so renderVideoTile still
runs end-to-end under the sandbox.

test/multi-peer-mesh.test.js

@@ -51,6 +51,7 @@ function extractConst(name){
 
 const watchSrc          = extractFn(/function watchVideoTrackForRemoval\(/);
 const firstFrameSrc     = extractFn(/function watchFirstFrame\(/);
+const swapVideoSrc      = extractFn(/function swapFreshVideoElement\(/);
 const renderTileSrc     = extractFn(/function renderVideoTile\(/);
 const removeTileSrc     = extractFn(/function removeVideoTile\(/);
 const renderScreenShim  = extractFn(/function renderScreenTile\(/);
@@ -231,6 +232,7 @@ function makeBrowser(name, pubKeyHex){
     tileKindsSrc + '\n' +
     watchSrc + '\n' +
     firstFrameSrc + '\n' +
+    swapVideoSrc + '\n' +
     renderTileSrc + '\n' +
     removeTileSrc + '\n' +
     renderScreenShim + '\n' +

web/zebra-spaces.html

@@ -1869,6 +1869,26 @@ function updateContainerVisibility(){
   $('tiles-h2').classList.toggle('hidden', !anyThumb);
 }
 
+/* Replace a <video> with a freshly-built one carrying the same
+ * autoplay/playsinline/muted attributes, in the same DOM slot. Returns
+ * the new element so the caller can update its reference. Browsers
+ * treat a brand-new <video> as eligible for muted-autoplay even when
+ * the old element had its play() rejected — so use this on any
+ * supplant where srcObject would otherwise be swapped mid-life. */
+function swapFreshVideoElement(oldVideo){
+  if (!oldVideo) return oldVideo;
+  const fresh = document.createElement('video');
+  fresh.setAttribute('autoplay', '');
+  fresh.setAttribute('playsinline', '');
+  fresh.setAttribute('muted', '');
+  fresh.autoplay = true; fresh.playsInline = true; fresh.muted = true;
+  if (oldVideo.parentNode){
+    try { oldVideo.srcObject = null; } catch(_){}
+    oldVideo.parentNode.replaceChild(fresh, oldVideo);
+  }
+  return fresh;
+}
+
 /* build a tile DOM. opts.thumb = true for thumb-style (no fullscreen
  * button, gets .tile-thumb class). Returns { tile, video, fsBtn }. */
 function buildTile(kind, pubHex, label, opts){
@@ -2036,6 +2056,19 @@ function renderVideoTile(kind, pubHex, stream, opts){
     $(k.thumbContainer).appendChild(built.tile);
     entry = { tile: built.tile, video: built.video, label };
     k.store.set(pubHex, entry);
+  } else {
+    /* MSID-supplant retarget: when the same pubHex+kind republishes
+     * (publisher refreshed / sfu PC rebuilt), reassigning srcObject on
+     * the existing <video> hits the browser's autoplay policy as a
+     * mid-life srcObject swap and a fresh play() rejects with
+     * "fetching process for the media resource was aborted by the
+     * user agent at the user's request" — the tile stays paused on a
+     * black frame. Building a fresh <video> element resets that
+     * decision: muted-video autoplay applies again because the
+     * element has no prior gesture-consumed play() to defend against.
+     * Fox 2026-06-03 ("still black share camera from will" after a
+     * 16s supplant). */
+    entry.video = swapFreshVideoElement(entry.video);
   }
   entry.video.srcObject = stream;
   try {
@@ -2051,9 +2084,15 @@ function renderVideoTile(kind, pubHex, stream, opts){
     logLine('err','play threw: '+e.message);
     entry.tile.classList.add('needs-tap');
   }
-  /* if the same tile is currently spotlit, mirror the stream into the big tile */
+  /* if the same tile is currently spotlit, mirror the stream into the big
+   * tile — same supplant-vs-autoplay trap, same fix. */
   if (spotlight && spotlight.kind === kind && spotlight.pubHex === pubHex && spotlight.video){
+    spotlight.video = swapFreshVideoElement(spotlight.video);
     spotlight.video.srcObject = stream;
+    try {
+      const p = spotlight.video.play();
+      if (p && p.catch) p.catch(()=>{ spotlight.tile.classList.add('needs-tap'); });
+    } catch(_){}
   }
   /* spotlight promotion rules:
    *   - no spotlight yet → first tile auto-promotes (anyone)
@@ -4207,8 +4246,8 @@ logLine('', 'ready — pick a handle, type a rendezvous code, enter the space');
 
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