sync_replay_project/docs/ARCHITECTURE.md

Architecture

Plain overview first: HOW-IT-WORKS.md. Wire format: PROTOCOL.md. Plans: ROADMAP.md.

Packages

Path	What	Rule
packages/core	protocol messages, sync engine (SyncEngine), constants, config, Range parsing, logging, Jellyfin client	pure Dart, no Flutter
packages/server	Relic HTTP + WebSocket server (SyncServer, ServerHub, Session, media endpoint, relay buffer, Jellyfin relay), the serve() entrypoint, bin/server.dart (dedicated server)	pure Dart, no Flutter
apps/desktop	Flutter app: UI, SyncClient, SessionCoordinator, PlaybackController (media_kit/mpv), ServerIsolate, updater	ships with watch-together-server
tool	harnesses: range_logger, shaper, fake_viewer, e2e_check	dev only
deploy	systemd unit for the dedicated server

Guard: packages/core/test/flutter_free_test.dart fails if anything under core/lib imports Flutter, dart:ui or signals_flutter; CI runs analyze + tests for core and server on every push. Server has no such test but the same rule; CI checks it with dart compile exe of bin/server.dart. The client WebSocket (web_socket_channel) lives in apps/desktop; Relic has no client.

Server lifecycle

One entrypoint: serve(config, events:) in packages/server. config is an AppConfig.toMap() (primitives only, so it crosses isolates). It starts a SyncServer (bound to all IPv4 interfaces) and reports on the events port: ready (with the port and a stop SendPort), error (start failed), and every log record as log. Sending {'t':'stop'} to the stop port closes all sockets and returns, and the isolate exits. {'t':'file', 'path', 'reply'} on the same port makes a local file the hosted one and answers its name and size (ServerIsolate.setFile): the host's in-session File swap then sends a swap with that name and size, which the server matches to the file.

• Hosting from the app: SessionState.host() spawns an isolate running serve (ServerIsolate), waits for ready, then joins it over 127.0.0.1 like any other client. Server logs go to the app's log with a [server] prefix. The isolate runs with errorsAreFatal: false: an uncaught async error is logged and the server keeps going. If the isolate dies anyway, the session shows "server crashed: ..." and leaves; Home shows it on the Host card (during setup the host attempt fails with it). Stop (leave, close, cancel during setup, a failed own join, being kicked) sends stop, waits up to 3 s for exit, then kills the isolate.
• Dedicated server: bin/server.dart compiled with dart compile exe and shipped next to the app as watch-together-server. It parses flags (--port, --secret, --name, --media, --advertise-host, --jellyfin <file>, --config; default port 8080) into the same map and calls serve; SIGINT and SIGTERM stop it. A Flutter binary cannot run without a display, which is why the server is its own build.

SyncServer runs a 1 s heartbeat: stale sweep, closed-socket sweep, silent socket drop, state + feed broadcast. There is one session, id default.

Passwords: the default secret is empty, which means an open server (a warning is logged). Otherwise one shared secret, compared at hello.

Host: when hosting, the app generates a random hostKey per hosting, puts it in the server config and sends it in its own hello. The member whose key matches is the host and cannot be kicked. The address a connection comes from plays no part (so a reverse proxy or tunnel on the same machine does not make everyone the host). The dedicated server has no key, so it has no host. Kick = revoke token, send kicked, close; the person may rejoin at once with a new token.

Sync model

Loosely modeled on Syncplay, with a different readiness design.

• Transport is playing, held or finished. The server owns the clock: positionAt(now) = position + (playing ? now - serverTime : 0). Reports from clients never move it. Only seek, swap, gate transitions and the end of media write position/serverTime and bump eventId.
• Clock offset: serverTime is on the server's clock. Each client sends a ping with its clock every second and estimates the offset NTP style (ClockSync: offset = s - (t0 + t3) / 2 of the sample with the smallest round trip among the last 16). All extrapolation uses SyncClient.serverNow() (local clock plus offset), so a client whose clock is off still lands on the server position.
• Reports: clients send position, buffered, eventId every 1 s, plus the media duration and its mediaId once the player knows it. The server stores buffered and liveness, and takes the first reported duration for the current media (swap resets it); a report with eventId older than current has its position ignored.
• Ready gate (HELD to PLAYING): the in-set is non-empty, every in-set member is ready, fresh and has buffered >= need, and quorum holds: in-set size >= 2, unless at most one member is fresh. need is 4 s (minBufferSecs), or near the end what is left minus 2 s (endSlackSecs, floor 0), since mpv cannot buffer past the end. So a lone viewer plays alone; with a second viewer present one Space waits for the other. Joining members start with inSet=false, ready=false and never block. Disconnect removes from the set; swap clears everyone.
• Pause: while PLAYING, any in-set member who becomes not ready, under need buffered or stale flips the server to HELD (feed action pause by server). Recovery resumes by itself when the gate passes.
• End: while PLAYING with a known duration, reaching it sets FINISHED (feed finished by server, position = duration). Clients pause where they are; FINISHED never auto-resumes. A seek turns it into HELD at the new position (resuming at once if the gate passes); a swap resets to HELD.
• Seek: allowed from any member in any transport; the server writes the position, bumps eventId and records who sent it (seekBy, seekSeq in state for that event); flags are untouched. Clients diff each state broadcast by eventId (StateDiff): new event means seek to the computed server position, transport change means play/pause. The coordinator applies states one at a time, newest first (a state arriving meanwhile replaces any still waiting): interleaved applies could land an older pause after a newer play. Between events: drift over 5 s for more than 2 s seeks, ahead by more than 1.5 s sets rate 0.95 until level, otherwise nothing.
• User seeks: the player is a SyncPlayer whose public seek is the user's: the session's seek bar (ChapterSeekBar, one seek per pointer move while dragging and one on release, like media_kit's), the arrow keys, skips, chapter jumps and seekTo all become explicit events. The player moves at once (scrub preview); UserSeekGate sends one seek intent with a fresh seq after 300 ms without another request (seekDebounceMs). While the user is seeking, and until the echo of the last sent seq arrives (3 s at most), server seeks never touch the player; the echo of our own seek is not applied either (the player is already there). A server seek that has not landed after 1.5 s is re-applied once (_checkSeekLanded), to the server position recomputed now, and only while its event is still current and the user is not seeking.
• Liveness: WS ping every 5 s; silent over 15 s means stale (out of the gate, stale feed); over 30 s the socket is closed and the member removed. A stale member that was ready goes back into the set with its next frame. Closed sockets are polled with isClosed every second: Relic's server-side onDone does not fire for dead peers, so presence never relies on it.
• Resume: a reconnecting client sends its previous token as hello.resume. If the server still has that member (an unclean drop it has not noticed), the new socket takes it over: same token, member id, ready flag and media URL (the player keeps streaming), no join/leave feed, and the dead socket is closed without removing the member. An unknown or kicked token joins as a new member.
• Client policies: no state broadcast for 5 s means connection lost, clear the server state and reconnect every 1.5 s until cancelled (an epoch counter makes a cancelled attempt stale) or kicked. After a reconnect the ready flag is restored from the server's state. Cancel during setup aborts the join. Closing the window leaves gracefully (2 s timeout).
• Idle (IdleTracker): measured from the later of the last input and the moment playback stopped. While HELD and self ready: warning at 8 min, auto-unready at 10 min, and never less than 2 min after the warning.
• Deviations from Syncplay: clock never follows the slowest watcher; no pause-on-leave; anyone can force ready/unready (all or one); quorum gate; the clock is server time (clients correct for their offset to it).

Additive wire changes only; bump proto and PROTOCOL.md in the same commit.

Media

Three sources, same sync. Any of them can also go through the relay (below).

• Link: the media field holds an http(s) URL. It travels as media.url in swap and state, and as mediaUrl in welcome. Each app plays it directly; the server is only the clock. A pasted link that parseJellyfinLink recognises goes up as a Jellyfin ref instead when the ServerId is known (in the link, else /System/Info/Public), so its api_key is never shared (mediaForLink, host and swap alike).

• Host local file: SyncServer.setLocalFile / mediaPath makes the file the session media. GET /m/<sid>/<token>/<name> serves it with a Range table, gated by the viewer's token. The server builds the URL per viewer (http://<host>:<port>/m/default/<token>/<name>), so tokens never reach other viewers. <host> is the authority the client dialled; advertiseHost (default empty) only overrides it when set explicitly. When a state broadcast has no media.url, the client builds the URL itself from the host it connected to. Bytes stream file to socket and are counted as streamed (bytesServed); nothing is stored. The host plays through loopback like everyone else.

• Jellyfin item: the media is a ref ({serverId, itemId, ...}, wire in PROTOCOL.md "Jellyfin media"); the server never stores or sends a URL or token for it. Each app resolves it with its own account and direct-plays the static stream; guests use the server's own login when it has one ("Jellyfin relay"), else the member who put it up ("Relay").

Range table (resolveMediaRequest, tested byte-exact):

Request	Response
no Range	200, Content-Length = size
bytes=a-b	206, Content-Range: bytes a-b/size
bytes=a-	206 to EOF
bytes=-n	206 last n bytes
unsatisfiable	416, Content-Range: bytes */size
host has no file	503 + Retry-After: 2
bad / unknown token, kicked, unknown session	401, 403, 410

Always Accept-Ranges: bytes; Content-Type from the extension (contentTypeForFile). Relic hands the Range over as a typed RangeHeader; the endpoint takes the first range and keeps the unit, so non-bytes units still get 416.

What mpv does against it (recorded with tool/range_logger.dart): mp4 opens once with no Range then streams bytes=0- to the end; mkv opens bytes=0- twice (probe, playback), reads a small tail at size-n (cues, expressed open-ended), then every seek is a fresh bytes=N- connection. A server outage of a few seconds is ridden out from mpv's cache with no error. Longer outages exhaust the cache and rely on reconnect (not chaos-tested yet).

mpv settings (PlaybackController): Player with bufferSize 256 MiB (media_kit's bufferSize sets both demuxer-max-bytes and demuxer-max-back-bytes), libass on; pinned via runtime property: demuxer-readahead-secs=150, cache-secs=150 (about two minutes of read-ahead: rides out short drops, intro skips are instant), demuxer-max-bytes=268435456 (150 s up to ~14 Mbit/s; a 1.4 GB / 24 min file is ~8), demuxer-max-back-bytes=67108864, stream-lavf-o=reconnect_streamed=1,reconnect_on_http_error=1,reconnect_delay_max=4 (comma form; this mpv build rejects colon-joined). The cache is in RAM: media_kit sets cache-on-disk without a cache-dir, and mpv falls back. mpv ignores invalid values silently, so every pin is read back and a mismatch is logged and kept in pinMismatches. Measured against the /m endpoint (readahead_e2e_test): a paused player buffers ~147 s ahead (10 s before). media_kit has no public setProperty/getProperty; the code calls them through a guarded dynamic call on player.platform. Hardware decoding comes from Settings (auto, off, custom) and is passed to the VideoController; env SYNC_RELAY_HWDEC (e.g. no) overrides it. It is read per player, so a change applies on the next join or Reload video.

Shaders (per viewer, local only, no wire change): mpv glsl-shaders, set through the same dynamic setProperty and read back (PlaybackController.applyShaders). The value is the platform path list (: joined, ; on Windows). One saved choice in AppSettings (shaderPreset, shaderQuality, customShaders): applied on every player creation (join, Reload video) and live when changed from the session (the control bar's shader menu, Ctrl+0..6, both also save it). The Anime4K v4.0.1 files live in apps/desktop/assets/shaders/anime4k/ (only the 14 the presets use, plus its MIT LICENSE); mpv needs real paths, so extractAnime4k copies them once per version to <app support>/shaders/anime4k-<version>/ (a .complete marker is written last). anime4kFiles maps each mode to the file list of Anime4K's mpv templates: HQ = the high-end GPU set (VL + M networks), Fast = the low-end set (M + S). A missing file, a path containing the list separator, a read-back mismatch, or an mpv libmpv_render error while shaders are on turns shaders off (saved) with a toast.

Render mode: media_kit's Linux/Windows video output creates mpv's render context (OpenGL, else its software renderer) when the VideoController is created, and nothing in its API says which. The player therefore requests mpv logs at v: OpenGL init logs GL_VERSION under libmpv_render (RenderMode.gpu; "suspected software renderer" means llvmpipe, softwareGl); none within 1.5 s of the controller being ready means the software path (software, where shaders do nothing; the shader toast, the menu and Settings say so). After startup the v level adds no steady traffic (measured: 0 messages over 5 s of playback). The hardware path is fragile: media_kit borrows whatever EGL context GDK left current on the main thread (upstream media-kit#1404), so an X11 session or a Flutter/GDK change can silently fall back to software.

Player lifecycle: the Player and its VideoController live for one session: created on join, disposed on leave, replaced by recreate() for "Reload video" (button or Ctrl+R; volume kept). openUrl opens with play: false; play/pause/seek/rate come only from StateDiff actions, the play gate (localPlayAllowed: self in-set and ready) or the user's own seeks (see Sync model). A server seek within 0.5 s of the current position is skipped. leave() stops the player (otherwise audio keeps playing). On media change (mediaId bump) the coordinator opens the new URL and StateDiff seeks to 0, holds and resets rate.

Frozen picture: FrameWatchdog samples about once a second. It hashes a small capture of the compositor's centre (ShownFrameProbe) and suspects a freeze when position advances while that hash stays constant for 3 s. Two differing mpv screenshot-raw frames show the decoder is still producing pictures, which confirms the freeze (a still scene is dismissed). A confirmed freeze reloads the video with a toast. It fires once until the picture moves again for 10 s, so it cannot loop. Seeks, pause, buffering and a hidden window disarm it.

Chapters, segments, tracks (PlaybackController signals, values from playback/media_marks.dart, no widgets; any controls UI reads them):

• chapters: mpv chapter-list (observed through the same dynamic observeProperty and read as JSON): MediaChapter index, title, start; end = next start, the last one's = duration.
• segments: the Jellyfin item's MediaSegments (GET /MediaSegments/{id}, 10.10+: Intro, Outro, Recap, Preview, Commercial), fetched by the coordinator with OwnPlay only; relay and blocked viewers cannot call Jellyfin and use chapters alone. Empty without a segment provider plugin (the case on the owner's 10.11 server).
• tracks: audio and subtitle MpvTracks from mpv track-list (id, language, title, codec, channels, default / forced / hearing-impaired / external, selected), re-read after every selection.
• position (250 ms steps), duration, buffered (end of the cached range: mpv demuxer-cache-time, media_kit's buffer, 250 ms steps); computed opening, ending (SkipRange) and currentChapter.

Skip rules (pure, media_marks.dart):

• Opening = first Intro segment, else the first chapter whose title has a whole word OP / Opening / Intro (any case, digits allowed: "OP1"). Ending = first Outro segment, else ED / Ending / Credits / Outro.
• skipOfferAt (the coordinator's skipOffer, null at finished): inside the opening (up to its last second) "Skip OP" to its end; inside the ending "Skip ED" to its end only (no guessing about previews: the viewer decides). Without any opening and with media over 20 min, "Skip 1:30" during the first 6 min.
• +1:30 (skipForward: the pill, the controls' "1:30" button, Shift+Right) is a nudge, so repeated presses add up.
• Chapter steps (PgUp / PgDn, chapterStepTarget): forward to the next start; back to the current start, or the previous one within 5 s of it (mpv's chapter-seek-threshold).
• Every skip and chapter jump is a user seek (debounced, everyone follows).

Never unmount GPU-textured widgets (avatar images) on a transport-gated branch: teardown races mpv's frame upload and the window goes black. Keep them mounted and skip painting (Visibility with maintainState).

Jellyfin client

packages/core/lib/src/jellyfin/, pure Dart over dart:io HttpClient behind a JellyfinHttp interface (tests inject a fake). Plans: ROADMAP.md "Epic: Jellyfin".

• JellyfinClient: one server + one user. Auth (Quick Connect, password, /Users/Me check, logout), browsing (views, Resume, NextUp, Latest, paged /Items, search, seasons, episodes), image URLs (tag + maxWidth, no token needed), PlaybackInfo, stream URLs, MediaSegments (a 404 = none), Playing / Progress / Stopped reports, played / unplayed / resume position.
• normalizeServerUrl, parseJellyfinLink: pasted addresses and item links -> (baseUrl, itemId); accounts are keyed by ServerId (/System/Info/Public).
• episodeAfter: next episode is the successor in /Shows/{id}/Episodes (crosses seasons, skips other copies of the same episode); NextUp means first unwatched, not next-after-X.
• matchTrack / matchTracks: carry audio/subtitle choice to the next episode (language required, then title words, forced/HI flags, codec, channels).

Gotchas (Jellyfin 10.11):

• Every request sends Authorization: MediaBrowser Client, Device, DeviceId, Version[, Token] (values percent-encoded). Unauthenticated calls missing a field get 400 with no body.
• A POST must carry Content-Length even when empty: behind nginx a body-less POST without it gets 400 (this was the Quick Connect Initiate failure).
• Jellyfin overwrites a token's stored device name and app version with the header's. Only use tokens this app obtained itself; a relay using a viewer's token must send that viewer's client fields (the server relay sends exactly its credential's fields).
• Static streams (/Videos/{id}/stream?static=true) serve the original file with byte ranges. The share URL has no credentials; each side adds its own api_key (authorizeUrl).
• Ticks: 10,000,000 per second.

Jellyfin relay

For viewers without an account on the item's Jellyfin (packages/server/lib/src/jellyfin_relay.dart). The server may hold one JellyfinCredential (config key jellyfin: baseUrl, serverId, token, client, device, deviceId, version; primitives, so it crosses the isolate boundary in the serve() map). The hosting app fills it with its own app-obtained token and that token's device fields; the dedicated server reads it from --jellyfin <file> (YAML/JSON with those keys, keeps the token off the command line) or a jellyfin: map in --config. An empty serverId is looked up at start (/System/Info/Public); if that fails the relay stays off. The token is never logged (toString and config parse errors leave it out).

• Available (state.relay) only while the current media is a ref whose serverId equals the credential's. Only that item is relayed, only to a valid, unkicked viewer token: no open proxy.
• Each request opens /Videos/{id}/stream?static=true&mediaSourceId= with Authorization: MediaBrowser built from exactly the configured fields (Jellyfin rewrites a token's device fields from the header), forwards Range untouched and Accept-Encoding: identity, and returns Jellyfin's status and range headers. Bytes stream through (no buffering) and count toward bytesServed as sent. A viewer disconnect cancels the upstream response, which closes that connection (tested: HttpClientRequest.abort does nothing once the response has arrived; cancelling the response subscription does).
• No progress reporting in the server: watched state is per client.

Relay

A member's media through the server's memory, for viewers who cannot reach it themselves (packages/core/lib/src/relay.dart for the wire and the source, packages/server/lib/src/relay_buffer.dart, relay_routes.dart; wire in PROTOCOL.md "Relay").

What gets relayed (SessionCoordinator.swapPicked):

Put up	Others get it
File, by the host	/m from the hosting app (no relay)
File, by anyone else	relayed from that member's computer
Link, "Everyone fetches it" (default)	each app fetches it
Link, "Server fetches it"	the server reads it once into the buffer; the link is never broadcast; refused on an open server (it would fetch any address it can reach for anyone)
Link, "Through me"	relayed from that member's computer; for links only they can open
Jellyfin item (browser pick or pasted link)	own account first, then the server's login (/j), then relayed from the member who put it up (with their sign-in, or the pasted link's key, which stays on their machine)

Relayed links must be one file the relay can range-read: a text/* response (YouTube and other pages, which only the player resolves) is refused before the swap.

How bytes move:

• The source dials /r/.../source (outbound, so NAT/CGNAT are fine) and answers the server's pulls from the file or the URL (RelaySource, FileRelayBytes, HttpRelayBytes; a Jellyfin source sends its own Authorization header from this machine). Viewers only ever talk HTTP to the server, so the source never sees mpv's request pattern.
• The server keeps one RelayBuffer per relayed media: 1 MiB slices in memory, filled by one sequential pull at a time. Kept: the first and last 2 MiB (container indexes; mpv reads both on open and on seeks), 64 MiB behind the slowest viewer and 256 MiB past the furthest, the same as each viewer's mpv buffer (PlaybackController), so a skip inside the viewers' buffer window is served from memory. Evicted by distance from the viewers, never LRU. With 2+ viewers the source sends each byte about once.
• Filling: a viewer blocked on a slice first (oldest wait first, handing the source over at slice boundaries), then the pinned slices, then read-ahead from the slowest viewer. Nothing is fetched while nobody watches through the relay (a Jellyfin source with only account holders uploads nothing).
• mpv leaves superseded requests open but unread after a seek: only a viewer's newest request moves its position or asks the source for anything; older ones still get bytes that are already buffered.
• Flow control on the source (RelayWindow): one in-flight budget per connection, 2 x delivery rate x min RTT (rate max-filtered over 2 s), clamped 256 KiB to 8 MiB; the server acks bytes as they arrive. A cancelled pull's unacked bytes are written off, and a cancel that lands while a frame is being read stops it before it is sent.
• Cleanup: the buffer is memory only. Any swap disposes it (its URLs then 404, the source is closed with 4001 and stops), and so does server stop. A crash frees it with the process.
• Source away: viewers keep playing from the buffer; a request waits up to 60 s for missing bytes. The source reconnects with backoff (same token after a resume); a different file size drops the buffer. state.media.relay.live follows the channel, and viewers show "waiting for to pass the video on" while it is down.

Measured (mpv 0.41 viewers, real server and source, a shaped link between source and server; tool/relay_e2e.dart, 2-minute 1080p test file averaging 10 Mbit/s with a 15 Mbit/s busy section):

Source uplink	Open	Seeks (incl. +90 s skip)	Stalls in 50 s
400 Mbit/s, 20 ms	0.8 s	0.01 to 0.03 s	0
40 Mbit/s, 40 ms	2.1 s	1.1 s first, then 0.03 s	0
20 Mbit/s, 150 ms	5.4 s	1.1 to 5.3 s	3
12 Mbit/s, 40 ms	5.4 s	0.5 to 2.9 s	3

3 viewers skipping together on 40 Mbit/s: no stalls, the source sent 101 MiB for 287 MiB served. A late joiner opened in 0.04 s from the buffer. A 5 s source outage: no stall; 30 s: one stall once the buffer ran out, then it recovered. Freezes of 0.3 to 0.8 s in 30% of seconds: no stall. So the source's upload must beat the media's average bitrate (peaks are covered by read-ahead once it has some lead), and comfortably so for quick seeks. Rejected along the way: a fixed per-request window (seeks of 14 s on slow links, data for abandoned positions clogging the pipe), one shared window across requests (deadlocked on mpv's unread requests), Dart http2 (connection window stuck at 64 KiB).

Jellyfin in a session

Client side (lib/src/jellyfin/session_media.dart, progress.dart, track_prefs.dart, wired in SessionCoordinator):

• Resolve: a state whose media is a ref is resolved per viewer (resolveJellyfinRef): an account for that ServerId that is not expired -> PlaybackInfo with its own user (PlaySessionId, media source; /Items/{id} alongside for series and episode numbers), then mpv plays the static stream with the token in http-header-fields (Authorization: MediaBrowser with this install's fields), not in the URL: OwnPlay, report.jf {serverId: true}. Else, when state.relay, welcome.relayUrl or /j/default/<token>/<itemId> on the dialled host: RelayPlay, jf: false. Else, when media.relay names a source, the /r/ URL: RelayPlay. Else BlockedPlay: the session shows "No Jellyfin account here for , and nobody is passing it on" with a button that opens Integrations over the session; linking (or reconnecting) the account retries by itself. A 401 marks the account expired. The member list shows "via host" for members whose jf is false for the current item. A reconnect to the same item with OwnPlay keeps playing as is.
• Progress (JellyfinProgressReporter, OwnPlay only): Playing once the file is open and mpv has picked its tracks; Progress every 10 s and at once on pause, resume, a seek (position more than 3 s from where playback should be) or a track change; Stopped on swap, leave, kick, window close (bounded to 1.5 s inside the graceful leave) and at finished (position = duration, so Jellyfin marks it played); seeking back from the end sends Playing again. Reports carry PositionTicks, IsPaused and mpv's aid/sid as Jellyfin stream indices (jellyfinIndexForMpvTrack, SubtitleStreamIndex -1 = off). They go out in order on one chain; failures are logged, one quiet toast per session, playback is never touched; a 401 marks the account expired and ends reporting.
• Episodes: with OwnPlay on an episode, adjacentEpisodes (one /Shows/{id}/Episodes call; episodeBefore / episodeAfter) gives the previous and next episode. The controls show previous / next buttons (also < >, Shift+P / Shift+N); from nextPromptStart (the ending's start when one is known, see "Skip rules", else the last 2 min of the runtime) a floating "Next episode" pill shows outside the auto-hiding controls. Auto-advance still waits for finished. Any of them swaps to that episode's ref with the clicker's account. Guests (relay, blocked) see none of it.
• Auto-advance (Settings, on by default): at finished with a known next episode, every client with the setting on shows a 10 s countdown card (Cancel / Play now; without the setting, a "Next" card with Play next). Exactly one client swaps: autoAdvanceSwapper, the lowest member id among non-stale members whose jf for that server is true. Only what every client sees in state decides it (who put the item up is not in state; a late joiner never saw that swap), so clients agree without a wire change. Cancel is local: it stops the countdown on that screen, and the swapper's Cancel stops the swap; a seek by anyone ends finished and with it every countdown.
• Tracks: the controls' Audio and Subtitles buttons list PlaybackController.tracks. A pick is applied, reported, and saved as a TrackChoice (language, title, codec, channels, forced/HI: from the Jellyfin streams with OwnPlay, else from mpv's track list) per series and as the global fallback in the track_prefs Hive box. On the next Jellyfin item it is matched with matchTrack against the source's streams (mapped to mpv ids with mpvTrackId) or mpv's track list, and selected locally. Nothing goes on the wire.
• Swap panel: "Change video" in the side panel opens the shared MediaPicker over the video (screens/session/swap_panel.dart); Link goes through mediaForLink, a Jellyfin pick as a ref, File (host only) through ServerIsolate.setFile and a matching swap.

Desktop UI

One HomeScreen (screens/home/) with a single top bar (top_bar.dart): profile chip, Join and Host tabs, the update indicator, Settings / Integrations / About buttons, and the window drag area and buttons. Only the selected view is mounted; form state lives in HomeShell. Join (join_tab.dart): one compact address/port/password row, then up to 6 Recent server tiles in a grid that adds columns as the window widens. While the Join tab is visible each tile polls the server's GET /health every 10 s (health_probe.dart) for online, name, viewer count and a "needs update" badge when proto differs from ours. Host (host_tab.dart): the media picker (media_picker.dart: Jellyfin, Link, File; a reusable widget driven by a MediaPickerController whose selection is a sealed PickedMedia: LinkMedia, FileMedia or JellyfinMedia, see picked_media.dart) fills the main area; port, password and Host sit in a 340 px column on the right, below the picker under 860 px. With the Jellyfin source the browser fills the height and can be enlarged, which turns the column into a one-row bar under it. Text fields have fixed or capped widths (720 px max) so they stay compact on ultrawide screens. Submit state (busy, cancel, per-tab errors, server crash) is ConnectFlow. Settings (profile, hardware decoding, experimental mpv-style controls, Jellyfin auto-advance, updates, video shaders, shortcuts), Integrations (linked Jellyfin servers) and About open in the same body. Settings persist in two Hive boxes: connect_form (last form and recents; saved even when a join fails) and settings. Link media names are derived from the URL path and never contain a query string.

Jellyfin on the desktop (lib/src/jellyfin/, over the core client):

• accounts.dart: JellyfinAccounts (singleton instance), one JellyfinAccount per ServerId (address list, user, token) in the jellyfin Hive box with a random per-install DeviceId; Client = app name, Version = app version, Device = hostname. Only tokens obtained by sign_in.dart (Quick Connect or password) are stored. Tokens are checked on start (/Users/Me, each known address in turn): online, offline, or expired (kept, shown as Reconnect; never dropped silently). A 401 while browsing also marks it expired. clientFor(serverId) gives the shared signed-in client; cached() memoises browse calls for 3 min.
• sign_in.dart + sign_in_panel.dart: address -> /System/Info/Public -> Quick Connect (code, poll, cancel) or password; reconnect must hit the same ServerId. accounts_panel.dart is the Integrations card.
• browser.dart: back stack of BrowseRoutes (Home rows, library grid, series, detail) kept alive under Visibility(maintainState) so back keeps scroll and loaded pages; search (300 ms debounce) overlays them. Cards (cards.dart) fill the width with whole columns (fitColumns); rows and grids are lazy builders; library grids page 100 at a time with skeletons up to the total count. Images (images.dart) are Image.network at width steps (160..2560) with the image tag in the URL, so the image cache key changes only when the image does. An inspector pane follows hover/focus from 1400 px browser width.
• Picking an item gives a JellyfinMedia (serverId, baseUrl, itemId, mediaSourceId?, title, subtitle, runtime; no credentials). The Host tab remembers the last pick in the jellyfin box and hosts it as a ref (hostJellyfinMedia, ConnectFlow.submit(media:)); the hosted server gets the relay credential (relayCredentialFor: the account of the item's server, else the default one, not expired; its token with exactly JellyfinAccounts.clientInfo's device fields).
• Browser location: every navigation saves the route stack per server (browse:<serverId> in the jellyfin box, encodeBrowseStack: ids, names and the fields pages start from), so the Host tab, the swap panel and the next start open where the browser was. A BrowseStart overrides it: the swap panel opens on the series of the episode playing, its season selected and the episode highlighted.

Session controls: media_kit's MaterialDesktopVideoControls with our bars (screens/session_screen.dart, pieces in screens/session/player_controls.dart). Its seek bar is off; ours (ChapterSeekBar: chapter diamonds, hover time with the chapter title) sits in the bottom bar, which is taller by the bar's height (the top bar keeps its row at the top). A drag sends no hover events, so the bar keeps the controls up through a guarded dynamic onHover() on media_kit's private controls state (keepVideoControlsAwake). Pills (Skip OP / ED / 1:30, Next episode) and cards live outside the auto-hiding controls, so they show in fullscreen (whose route renders only the video and controls).

Experimental mpv-style controls (controls/uosc/, after mpv's uosc theme; Settings toggle AppSettings.mpvControls, default off): the Video's controls builder wraps both sets in SessionControlsSwitch, which follows the setting live, in the window and in fullscreen. UoscControls is our own frame (no media_kit controls state): mouse movement shows it, 3 s without any hides it and the cursor; hold keeps it up during a timeline drag and while a menu route is open, double-click toggles fullscreen. UoscTimeline runs the full width at the bottom: played part filled, cacheRect (position to buffered) hatched (hatchLines), chapter diamonds on the top edge, current time and duration in boxes inside it, a hover tip with the chapter title over the hover time; hidden, it shrinks to a 3 px line with chapter ticks. Above it a row of flat buttons: menu (≡: change video, people, reload, mark everyone ready, fullscreen, leave), subtitles and audio with track counts, a shader badge ("A HQ", opening the shared showShaderMenu), ready, mark everyone ready, reload; 1:30, chapters, previous / next episode, change video, people, volume, fullscreen. Menus (showUoscMenu: a popup route above the button, the current row highlighted) list tracks and chapters with start times. All actions are the coordinator's (seeks through player.seek, the user path; selectTrack, skipForward, seekTo); keys and the floating pills are the same in both sets.

Window chrome is frameless (window_manager): every screen brings its own drag area and min/max/close buttons (Home's top bar; the session uses widgets/window_chrome_bar.dart; both use window_buttons.dart); the session drag layer is window_drag_layer.dart.

Health reporting

Every failure must show in the UI with a cause and a next step; nothing stalls silently. The app has a HealthStatus signal (severity + message) for connecting, reconnecting, wrong password, version mismatch, kicked, host offline, and starvation ("waiting for X - N.N s buffered"). Errors are logged with a stack trace at the boundary, then turned into a displayable state. The server's GET /health returns JSON (ok, name, proto, viewers, bytesServed, uptimeSecs); it is unauthenticated, so it carries nothing sensitive. Home's recent-server tiles poll it for live status.

Updater

apps/desktop/lib/src/update/updater.dart (no Flutter imports, unit tested). Reads a manifest (version, protocol, url, optional sha256, notes) from updateManifestUrl, compares versions, downloads the tarball, checks sha256 when present, then extracts into a staging dir inside the install folder (same filesystem, so moves are renames; the folder must be user-writable). Each top-level entry is moved aside and the new one moved in, with rollback on failure (writing the running executable fails with ETXTBSY; rename works). A packaged install is detected by data/ and lib/ next to the binary; anything else exits with "update manually". A manifest whose protocol is newer than the local one is a required update. Windows apply renames in-use files to *.old (swept on next start); CI tests it against a running exe on NTFS. The manifest's assets maps linux-x64 / windows-x64 to archive URLs; top-level url (Linux) is for builds <= 0.2.1. The version is stamped with --dart-define=APP_VERSION (dev runs report 0.0.0-dev and skip checks). watch_together --upgrade (or the shipped update.sh) runs it from the CLI; UpdateController runs the same code for Home's update indicator, which opens the update card, and restarts the app ("Update and restart"). Settings has Check now and an auto-check toggle. The manifest is updates/manifest.json in this repo, written by tool/release.sh.

Decisions

• Desktop + mpv only, no web clients: exists because Jellyfin SyncPlay is bad.
• No transcoding: bandwidth is plentiful and the Jellyfin box should not be loaded.
• No roles, friends trust model: simplest thing that fits the use.
• Blank password = open server: the owner chooses between open and private.
• The host cannot be kicked: the server dies with the host anyway.
• Server runs inside the app on an isolate, and ships as a separate compiled server: a Flutter binary cannot run headless, so a dedicated server needs its own build of packages/server. An isolate keeps the UI responsive and shares the process (sockets, file paths) without a second executable.
• Host by key, not by address: loopback detection broke behind a same-machine reverse proxy or tunnel.
• Server owns the clock; reports never move it: avoids ping-pong between clients.
• Quorum gate: a lone viewer must be able to watch; two or more must start together.
• Relic for the server, web_socket_channel for the client; packages are chosen on merit, not locked.
• Plain ws://, no TLS: friends use LAN, VPN or a forwarded port; TLS (e.g. a reverse proxy) is a documentation item.
• One session per server, id default: the map is keyed for later but unused.
• No server storage: config and logs only; media never touches the server disk. The relay buffer is memory only and sized like a viewer's buffer (about 320 MiB at most), so a small VPS can carry it.
• The relay is the source's own connection through the server, not a shared token: a member's Jellyfin sign-in or a link's key never leaves their machine. Prior art (Plex Relay, Tailscale DERP) relays as a bandwidth-capped fallback on someone else's servers; here the server is the owner's, so it carries full quality.
• Avatars ride in the 1 Hz state broadcast (cap 256 KiB base64 per avatar): simple, costs host uplink; hash-based serving is on the roadmap.