fix(client): a seek's first segments are held while it lands, not dropped

They are the new stream, header first. Dropped, a cast relay restarted at the
landing got no ftyp/moov and the receiver gave up; they are now replayed in
order once reinitAt/resumeAt is done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

3 files changed, 106 insertions, 3 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/video-player.js b/packages/meshbay-hub/src/meshbay_hub/static/video-player.js
index 2a4d2ea..4fa652b 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/video-player.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/video-player.js
@@ -438,6 +438,10 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
   const castRestartPendingRef = useRef(false);
   const castDeviceRef = useRef(null);
   const castRestartGenRef = useRef(0);
+  // Segments of the *new* stream that arrive while its seek is still landing
+  // (`reinitAt`/`resumeAt` wait on the SourceBuffer). Null when not holding.
+  const heldRef = useRef(null);
+  const holdGenRef = useRef(0);
   const landingPlayheadRef = useRef(false);
   // The current Screen Wake Lock sentinel, if the browser granted one — see
   // the effect below. Null on any platform/context that does not support it,
@@ -764,6 +768,7 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
           ranges: describeRanges(),
         });
         awaitingInitRef.current = true;
+        heldRef.current = null; holdGenRef.current++;
         // A seek supersedes a resume that had been asked for and not landed:
         // this one empties the buffer, and taking the resume branch on its
         // `stream_init` would leave the film we navigated away from in place.
@@ -798,6 +803,7 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
       });
       resumingRef.current = true;
       awaitingInitRef.current = true;
+      heldRef.current = null; holdGenRef.current++;
       seekTargetRef.current = null;
       outstandingRef.current = STREAM_WINDOW;
       console.log('[resume] request', +target.toFixed(1));
@@ -1064,7 +1070,23 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
             castRestartPendingRef.current = true;
           }
           const land = resuming ? resumeAt : reinitAt;
-          land(msg.start || 0).catch(() => {
+          // What follows this message on the ordered channel is the new
+          // stream, its header first. The landing waits on the SourceBuffer,
+          // and those segments used to arrive in that gap and be dropped as
+          // the old film's: the player kept its header from the start of the
+          // film and never noticed, but a cast relay restarted here got a
+          // stream beginning at a moof — no ftyp, no moov — and the receiver
+          // gave up on it within a second (measured on a phone). So they are
+          // held, and taken in order once the landing is done.
+          const hold = ++holdGenRef.current;
+          heldRef.current = [];
+          land(msg.start || 0).then(async () => {
+            while (holdGenRef.current === hold && heldRef.current && heldRef.current.length) {
+              await consumeSegment(heldRef.current.shift());
+            }
+            if (holdGenRef.current === hold) heldRef.current = null;
+          }).catch(() => {
+            if (holdGenRef.current === hold) heldRef.current = null;
             setError(t('video.err_transport'));
             setPhase('error');
           });
@@ -1178,6 +1200,12 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
         // for credit that cannot come. A race, which is why the same seek
         // worked twice and hung on the third.
         outstandingRef.current = Math.max(0, outstandingRef.current - 1);
+        // The new stream, arriving while its seek lands: kept, not dropped.
+        // Counted above already, so the replay must not count it again.
+        if (heldRef.current) {
+          if (!msg.file_id || msg.file_id === entry.id) heldRef.current.push(msg);
+          return;
+        }
         if (awaitingInitRef.current) {
           console.log('[seek] dropping segment (awaitingInit), outstanding:', outstandingRef.current);
         }
@@ -1190,6 +1218,13 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
         // would otherwise be decrypted against the wrong file — which fails,
         // loudly, in the console, for something that is simply not ours.
         if (msg.file_id && msg.file_id !== entry.id) return;
+        await consumeSegment(msg);
+      };
+
+      // Everything a segment goes through once it is known to belong to the
+      // stream being played: by arrival, or replayed after a landing.
+      const consumeSegment = async (msg) => {
+        if (cancelled) return;
         try {
           const plaintext = await window.MeshBayCrypto.decryptChunkBin(
             gekRef.current, entry.id, msg.segment_index, msg.nonce, msg.ct);
diff --git a/packages/meshbay-hub/tests/harness/window_leak.mjs b/packages/meshbay-hub/tests/harness/window_leak.mjs
index d5185de..f05d8c0 100644
--- a/packages/meshbay-hub/tests/harness/window_leak.mjs
+++ b/packages/meshbay-hub/tests/harness/window_leak.mjs
@@ -41,20 +41,23 @@ let cancelled = false;
 const pump = () => {};
 const flushQueue = () => {};
 const gekRef = { current: null };
+// Not holding: these are the abandoned stream's segments, before any stream_init.
+const heldRef = { current: null };
+const consumeSegment = async () => {};
 const window_ = { MeshBayCrypto: { decryptChunkBin: async () => new Uint8Array(4) } };
 
 const silentConsole = { log() {}, warn() {}, error() {} };
 
 const handler = new Function(
   'msg', 'cancelled', 'awaitingInitRef', 'outstandingRef', 'queueRef',
-  'entry', 'gekRef', 'pump', 'flushQueue', 'console', 'window',
+  'entry', 'gekRef', 'pump', 'flushQueue', 'console', 'window', 'heldRef', 'consumeSegment',
   `return (async () => {${source}})();`);
 
 const deliver = (n) => Promise.all(
   Array.from({ length: n }, (_, i) => handler(
     { file_id: entry.id, segment_index: i, nonce: 'n', ct: 'c' },
     cancelled, awaitingInitRef, outstandingRef, queueRef, entry, gekRef,
-    pump, flushQueue, silentConsole, window_)));
+    pump, flushQueue, silentConsole, window_, heldRef, consumeSegment)));
 
 const run = async () => {
   // The whole window arrives while reinitAt is still awaiting its updateends.
diff --git a/packages/meshbay-hub/tests/test_video_seek.py b/packages/meshbay-hub/tests/test_video_seek.py
index 3289eda..3bcdb71 100644
--- a/packages/meshbay-hub/tests/test_video_seek.py
+++ b/packages/meshbay-hub/tests/test_video_seek.py
@@ -320,3 +320,68 @@ def test_the_resume_strings_exist_everywhere(locale):
     text = (STATIC / "locales" / f"{locale}.js").read_text(encoding="utf-8")
     for key in ("video.resumed_at", "video.from_start"):
         assert key in text, f"{locale} is missing {key}"
+
+
+def test_a_seeks_first_segments_are_held_while_it_lands_not_dropped(tmp_path):
+    """What follows a `stream_init` is the new stream, its header first.
+
+    The landing (`reinitAt`/`resumeAt`) waits on the SourceBuffer, and those
+    segments used to arrive in that gap and be dropped as the old film's. The
+    player never noticed — its SourceBuffer kept the header from the start of
+    the film — but a cast relay restarted at that landing was handed a stream
+    beginning at a moof, and the receiver gave up within a second. Measured on
+    a phone: two segments dropped one millisecond after `stream_init`, a relay
+    header of 0 bytes; with them held, a 2 KB header and the film on the TV.
+
+    The real handler and the real drain run here: held while landing, counted
+    once, another file's segment refused, and replayed in arrival order.
+    """
+    import json
+    import subprocess
+
+    if shutil.which("node") is None:
+        pytest.skip("node is not available")
+    app = APP.read_text(encoding="utf-8")
+    start = app.index("transport.onStreamData = async (msg) => {")
+    handler = app[app.index("{", start) + 1:app.index("\n      };", start)]
+    drain_at = app.index("land(msg.start || 0).then(async () => {")
+    drain = app[app.index("{", drain_at) + 1:app.index("}).catch(", drain_at)]
+
+    script = tmp_path / "hold.mjs"
+    script.write_text(
+        f"const handlerSrc = {json.dumps(handler)}, drainSrc = {json.dumps(drain)};\n" + """
+const consumed = [];
+const env = {
+  cancelled: false, entry: { id: 'film' },
+  outstandingRef: { current: 8 }, awaitingInitRef: { current: true },
+  heldRef: { current: [] }, holdGenRef: { current: 1 }, hold: 1,
+  consumeSegment: async (m) => { consumed.push(m.segment_index); },
+  console: { log() {} },
+};
+const names = Object.keys(env);
+const handler = new Function(...names, 'msg', `return (async () => {${handlerSrc}})();`);
+const drain = new Function(...names, `return (async () => {${drainSrc}})();`);
+const call = (fn, msg) => fn(...names.map((k) => env[k]), msg);
+
+// Landing: the new stream arrives, plus one stray segment from another file.
+for (const [i, file] of [[0, 'film'], [1, 'film'], [2, 'other'], [3, 'film']]) {
+  await call(handler, { file_id: file, segment_index: i });
+}
+const held = env.heldRef.current.map((m) => m.segment_index);
+const consumedWhileLanding = consumed.length;
+const outstanding = env.outstandingRef.current;
+// The landing completes.
+env.awaitingInitRef.current = false;
+await call(drain);
+console.log(JSON.stringify({ held, consumedWhileLanding, outstanding, consumed,
+                             heldAfter: env.heldRef.current }));
+""", encoding="utf-8")
+    proc = subprocess.run(["node", str(script)], capture_output=True, text=True,
+                          encoding="utf-8", timeout=60)
+    assert proc.returncode == 0, proc.stderr
+    out = json.loads(proc.stdout)
+    assert out["held"] == [0, 1, 3], "the new stream's segments were dropped or mixed"
+    assert out["consumedWhileLanding"] == 0, "a segment was taken before the landing finished"
+    assert out["outstanding"] == 4, "each arrival is counted once, held or not"
+    assert out["consumed"] == [0, 1, 3], "the replay must keep arrival order, header first"
+    assert out["heldAfter"] is None, "holding must stop once the landing has drained"