fix(node): a seek left the audio a GOP behind the picture

-ss before -i cannot trim copied video, which must begin on a keyframe, but
accurate_seek did trim the re-encoded audio to the exact request. Every seek
on a copied stream therefore opened with a GOP-wide hole in the audio and ran
a GOP out of sync afterwards — 9.979s on a real film with a 10s keyframe
interval. Accurate seeking is now off wherever video is copied, and stays on
where it is re-encoded, which is the only path that could already begin where
it was asked to.

Every timestamp was correct throughout, which is why nothing caught it; the
tests assert on decoded audio and on frames compared against the source.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/MESHBAY_NODE_PROTOCOL.md b/docs/MESHBAY_NODE_PROTOCOL.md
index 19cced9..d4b9552 100644
--- a/docs/MESHBAY_NODE_PROTOCOL.md
+++ b/docs/MESHBAY_NODE_PROTOCOL.md
@@ -1636,7 +1636,9 @@ array while MediaSource consumes it a segment at a time.
   |                                                    | ffprobe: codec, duration, the
   |                                                    |   audio tracks
   |                                                    | spawn ffmpeg
-  |                                                    |   -ss before -i (index seek)
+  |                                                    |   -ss before -i (index seek),
+  |                                                    |     -noaccurate_seek when the
+  |                                                    |     video is copied
   |                                                    |   video: copy, or libx264 when
   |                                                    |     the browser cannot decode
   |                                                    |   audio: always AAC, 2 ch,
@@ -1667,6 +1669,7 @@ array while MediaSource consumes it a segment at a time.
 | `n == 0` | keepalive: a viewer buffered 90 s ahead grants nothing and must still be able to say it is there |
 | Concurrent transcodes | 8 node-wide, semaphore on the transport context |
 | Seeking | a new `stream_req` with `start`; the previous stream is retired first, ffmpeg respawned with `-ss` |
+| Accurate seek | **off when the video is copied, on when it is re-encoded.** Copied video has to begin on a keyframe and cannot be trimmed to the request; re-encoded audio can, and is. Leaving both at the default put a whole GOP of silence at the head of every seek and left sound and picture a GOP apart — with correct timestamps throughout, so nothing downstream could detect it |
 | `start` in `stream_init` | the value actually used — seeking lands on the keyframe at or before the request, and the client adds it back as `SourceBuffer.timestampOffset` |
 | `audio_tracks` in `stream_init` | every audio track: `i` (the **audio ordinal**, what `-map 0:a:<n>` takes, never the container stream index), `lang`, `title`, `codec`, `ch`. Empty for a file with no audio |
 | `audio_track` in `stream_req` | which ordinal to map. Absent, out of range or malformed is the first track |