watchparty/docs/architecture.md

# Architecture

## Goals

- Watch the same title, in sync, with other users of the same Emby server.
- Use Emby as the media source (catalog + streaming) without requiring
  changes to Emby itself or its official clients.
- Access gated to people with an Emby account on this server — no anonymous
  use, no separate identity system.

## Non-goals

- Chat/reactions — deliberately out of scope.
- OIDC/SSO — an OIDC provider (LemonLDAP::NG) is available in principle, but
  the explicit choice was Emby-only auth: simpler, no extra infra dependency,
  and everyone already has an Emby account.
- Full library navigation (Shows → Seasons → Episodes) in v1 — search-based
  catalog only, to keep the first version achievable. Revisit if browsing
  turns out to matter more than searching in practice.

## Why this replaced an Emby plugin

The first attempt (still in [`src/WatchParty/`](../src/WatchParty/), kept for
reference) was a real Emby Server plugin: sync via `ISessionManager`'s
existing remote-control commands, a settings page via `IHasWebPages`, and —
after a fair amount of reverse-engineering — a real user-facing page
integrated into Emby's own web client menu (`PluginPageInfo.EnableInUserMenu`,
undocumented but confirmed working; required a `data-controller` AMD module
to satisfy Emby's modern `ViewManager`, found by reading the target server's
actual `viewmanager.js`).

That plugin builds and installs correctly. What it hit a hard wall on: **a
button inside the video player itself**, or on an item's detail page. An
Emby developer confirmed on their own forum that plugins have no API for
this ("It's currently not possible for a plugin to do this"), and nothing
found in research since suggests that's changed. The only way to get a
button into Emby's own player would be patching Emby's shipped web client
files directly inside the Docker container — outside the plugin entirely,
guaranteed to break on every Emby image update, and requiring manual
reapplication each time. That trade-off wasn't worth it.

Building a standalone app sidesteps the question entirely: it's our own
player, so there's no "how do I add a button to someone else's UI" problem
in the first place, and sync can be tighter than nudging Emby's
remote-control commands ever allowed (see below).

Full trace of the plugin investigation (real errors, real fixes, sources
consulted) is in the git history of this file and of `src/WatchParty/` —
worth reading if a future Emby SDK update ever makes in-player integration
possible and this gets revisited.

## Components

```
server/   Node.js + Express + Socket.IO. Owns party/room state and the
          realtime sync channel. Never touches a media byte - see "Media
          never flows through our backend" below.

client/   Static frontend, plain JS + ES modules, no build step (same bias
          as the earlier plugin's control panel). Talks to two different
          things and must not confuse them:
            - Emby directly (embyApi.js): catalog, images, streaming.
            - Our own backend (party.js): create/join/leave a party, and
              the Socket.IO connection for realtime sync.
```

### Auth and Emby access: the server proxies everything

Revised after the first version: the browser used to call Emby directly
(catalog, images, streaming) and only used `server/` for party coordination.
That hit real CORS problems in practice (Emby/its reverse proxy don't
answer cross-origin preflights the way that needed), and separately wasn't
the wanted shape anyway — the browser holding a real Emby access token at
all was more exposure than necessary. Current design:

- The browser sends a username/password **once**, to
  `POST /api/auth/login` on **our own backend** — never to Emby directly.
- Our backend (`server/src/embyAuth.js`) calls Emby's
  `POST /Users/AuthenticateByName` itself, using `EMBY_SERVER_URL` from its
  own environment (not something the user ever types — see `.env.example`).
  On success it mints its own opaque session id, stores
  `{embyToken, userId, userName}` server-side (in-memory, like party state),
  and hands the browser only that session id, as an `httpOnly` cookie
  (`wp_session`). The browser never sees or holds Emby's real access token.
- Every other request — catalog search, item details, images, HLS
  streaming, party actions — goes to **our own origin only**
  (`/api/...`), authenticated by that same cookie. `server/src/embyProxy.js`
  is what actually forwards these to Emby server-side and streams the
  response back, attaching the session's real Emby token itself.
- The `/api/catalog/stream/:itemId/master.m3u8` and `.../raw` routes proxy
  HLS specifically: the master playlist is fetched from Emby server-side,
  and every URI line in it is rewritten to point back through `.../raw`
  (whether Emby wrote it as relative or absolute) — otherwise hls.js would
  follow the playlist's own URLs straight to Emby the moment it tried to
  fetch a segment, defeating the whole point. See `rewritePlaylist` in
  `embyProxy.js`.
- Images (`<img src>`) and video (`hls.js`/`<video>`) still can't carry a
  bearer header, but that's fine now: they're same-origin requests to our
  own `/api/...` paths, so the session cookie rides along automatically —
  no token in any URL anywhere, unlike the first version's `?api_key=...`.

This also means the client no longer needs to ask for a server URL at all —
one less field, one less way to get it wrong. `EMBY_SERVER_URL` lives only
in the backend's environment.

### Control model: host drives, everyone else follows

Now that we own the player, the classic watch-party model (Teleparty,
Amazon Watch Party, ...) fits better than the old plugin's "everyone keeps
their own controls, shared reference position" — that design was specifically
a concession to nudging *pre-existing, independently-running* Emby app
sessions we didn't control. Here, there's only one player implementation,
so:

- The party creator is the host. Their player has normal controls.
- Everyone else's `<video>` has controls hidden; a small notice says the
  host is in charge.
- Host → server → everyone else, over Socket.IO
  (`server/src/socketHandler.js`):
  - Event-driven: every `play`/`pause`/`seeked` on the host's video sends
    `{type:"playstate", isPaused, positionTicks}` immediately.
  - Heartbeat: also sent every 3s regardless, so latecomers/drift get
    corrected even without a discrete host action.
  - Followers apply pause/unpause immediately, and only seek if drift
    exceeds 1.5s (`DRIFT_TOLERANCE_SECONDS` in `client/js/views/player.js`)
    — avoids fighting small timing noise with constant re-seeks.
- If the host leaves, the server promotes another member (same
  fallback logic the old plugin used for its "reference" member).

### Party lifecycle

In-memory only on the backend (`server/src/partyManager.js`) — a restart
clears every party, which is fine: a party is meant to last one watch
session, not persist for weeks. `removeIdleParties` sweeps anything with no
activity for 4 hours.

## Realtime transport: Socket.IO, and two real memory leaks

The realtime layer was originally a raw WebSocket (the `ws` package). Two
things prompted a rework — a requirement (use Socket.IO instead) and a
specific worry (memory leaks) — and auditing for the second turned up two
real, concrete leaks, independent of which transport library is used:

1. **Server-side: dead connections never got cleaned up.** A raw `ws`
   server has no idea a client is gone unless it receives a proper close
   frame. A laptop put to sleep, a network drop, a killed tab that never
   gets to send one — none of those fire `ws`'s `'close'` event, so the
   `PartyMember`'s socket reference (and everything it closes over) would
   sit in memory forever, and the party would never get cleaned up by
   anything (`removeIdleParties` only looks at `lastActivityUtc`, which a
   phantom connection doesn't affect). `ws`'s own docs recommend a manual
   ping/pong heartbeat to detect this - Socket.IO has one built in
   (`pingInterval`/`pingTimeout`), which is a real, not just cosmetic,
   reason to prefer it here, not only the requested API ergonomics.
2. **Client-side: leaving the player any way other than its own "Quitter"
   button leaked everything.** `router.js` only ever did
   `currentRoot.innerHTML = ''` between routes — that destroys DOM nodes,
   but nothing told the outgoing view's *JavaScript* to stop. Clicking a
   sidebar link or the browser's back button while watching left the
   player's 3-second heartbeat `setInterval`, its realtime connection, and
   its `hls.js` instance all running against a detached `<video>` element
   indefinitely - for as long as the tab stayed open. Fixed by having
   `route()` handlers optionally return a cleanup function; `router.js`
   calls whatever the previous view returned right before rendering the
   next one, regardless of how navigation happened. `player.js` returns one
   that clears the heartbeat, destroys `hls`, disconnects the socket, and
   removes the one `document`-level click listener it adds (named, not
   inline-anonymous, specifically so it's removable at all - an anonymous
   listener added fresh on every visit would itself have been a slow leak
   even with everything else fixed).

Both are now covered by the same class of test: drive the router through
several "join a party, then navigate away without clicking Quitter" cycles
and check nothing accumulates. Verified directly (see session history) by
instrumenting `document.addEventListener`/`removeEventListener` and
`setInterval`/`clearInterval` around three such cycles against a really-running
local server (Socket.IO connection included, not mocked) - adds and removes
matched exactly (3/3 listeners, 2/2 intervals across two cycles), nothing
left pending. The server-side heartbeat fix is Socket.IO's own
well-established mechanism, not custom code, so it wasn't separately
instrumented the same way.

`server/src/socketHandler.js` mirrors the old `wsHandler.js` protocol
closely (see its own top comment for the exact event list) - the one naming
difference is the error event, sent as `"errorMessage"` rather than
`"error"`, since Socket.IO reserves the latter for its own internal error
handling.

## Live lobby, host handoff, no join codes

Three related changes, all in service of "the home page should reflect
reality without a refresh":

- **The home page's party list is a live subscription, not a fetch.**
  `client/js/party.js`'s `connectLobby` joins a `"lobby"` Socket.IO room
  (`subscribeLobby` → `server/src/socketHandler.js`) and gets a fresh
  snapshot on subscribe, then again every time `broadcastLobby(io)` runs -
  which is now wired into every place party membership changes: creating a
  party, joining one (both in `index.js`'s REST handlers), and leaving
  (`applyLeave`, used by both the socket `leave`/`disconnect` handlers and
  the REST leave endpoint, so this - and the host-handoff notification below
  - only has to be right in one place). `home.js` returns this subscription
  as part of its router cleanup function, same as the player does for its
  own socket - an open lobby subscription left running after navigating away
  would have been exactly the same class of leak just fixed above.
- **Host and elapsed time are shown per party**, per the request. Host name
  and member count come from the same snapshot. Elapsed time doesn't: it
  ticks client-side from a `createdUtc` timestamp the server includes once
  (`home.js`'s `setInterval` re-renders every `.elapsed` label every
  second) - pushing a recomputed duration from the server every second for
  every viewer would be pure waste when the client can derive it from one
  number. Verified live (see session history): faking a `parties` push with
  a `createdUtc` two minutes in the past and waiting rendered "2:06", not a
  static value.
- **No join codes.** `PartyManager` no longer generates or tracks one - see
  its class comment. The home page's live list is now the only way to find
  a party; a code that has to be copied and typed out of band doesn't add
  anything on top of a list that's already always current. `POST
  /api/parties/:partyId/join` replaces the old `:joinCode` route.
- **Host handoff is live, not just correct in the data model.**
  `PartyManager.leave` already promoted another member to host when the
  host left and destroyed the party once empty - that part predates this
  change. What was missing: a player already open as a follower had no way
  to find out it became host mid-session. `applyLeave` now emits a personal
  `"hostChanged"` (to that one socket, not the room) when this happens;
  `player.js`'s `becomeHost()` - the same function `init()` calls when you
  already are the host - unlocks the controls live, no reload needed.
  `becomeHost` is written to tolerate being called more than once safely:
  `addEventListener` with the same function reference is a no-op if already
  attached, and the heartbeat `setInterval` is explicitly guarded
  (`if (!heartbeat)`) since `setInterval` has no such built-in idempotence -
  calling it twice would leak the first interval's id, the exact class of
  bug the rest of this pass was about fixing, not reintroducing.

One incidental bug found and fixed while touching this code: the home
page's "Rejoindre" button on a live party card used to `navigate()` straight
to the player **without ever calling the join endpoint first** - it worked
well enough to load the video and read basic party info (`GET
/api/parties/:id` never checked membership), but the realtime `"join"`
would be rejected server-side ("Tu n'es pas membre de cette party.") since
`socketHandler.js`'s join handler does check membership - meaning sync
silently never worked for anyone who joined this way. Fixed by awaiting
`joinParty()` before navigating, same as the library grid's tiles already
did correctly.

## Player: no page scroll

`.playerWrap` was `width:100%; height:100vh` sitting inside `.mainContent`,
which has its own padding - the two stacked (100vh *plus* padding) made the
whole shell taller than the viewport, so the page grew a scrollbar even
though the video itself was already full-height. Changed to
`position:fixed; inset:0` so it's sized against the real viewport
regardless of any ancestor's padding, and (like most players' theater mode)
covers the sidebar while watching - confirmed via computed style
(`position: fixed`, all four insets `0px`) rather than just asserted.

## Presence overlay and "request pause"

Requested as a pair: a semi-transparent overlay showing who's in the party,
and a way for a non-host to ask the host to pause without a chat feature
(deliberately out of scope — see Non-goals). Revised once after the first
pass, based on concrete feedback: the overlay moved from bottom-left to the
top of the player, and the "someone requested a pause" indicator became a
specific orange border + pause-icon badge rather than a generic pulse
animation.

- **Layout**: the player's top gradient bar is now `.topOverlay`, a column
  of two stacked rows — `.actionBar` (title/members-text/next/settings/leave,
  what used to be the whole bar) and, directly below it, `#presenceOverlay`.
  Stacking them in normal flow inside one column means the avatar row always
  lands right after the action bar regardless of how tall that bar renders
  (it wraps to two lines on narrow screens) — deliberately not a second
  absolutely-positioned block at a guessed pixel offset, which would either
  overlap the action bar when it wraps or leave an inconsistent gap when it
  doesn't.
- **Presence**: `client/js/views/player.js`'s `renderMembers()` — already
  responsible for the text member list — now also rebuilds
  `#presenceOverlay`, one circle per member. Each circle shows the first two
  characters of `userName`, uppercased (`initialsFor`), on a background color
  picked deterministically from a small fixed palette by hashing the name
  (`colorForName`) — same person, same color, every render, with no need for
  the server to assign or track one. `opacity: .75` on `.avatar` is the
  "semi transparence" itself, applied to the whole circle (initials included)
  rather than just its background, since a chat-bubble-style translucent chip
  reads clearly enough at that opacity and avoids converting the hash color
  to an rgba string just to keep text fully opaque.
- **Which circle is "yours"** is decided client-side by comparing
  `member.userName === auth.userName` (the same `auth` singleton
  `embyApi.js` already populates at login/session-restore) rather than
  by `userId` — consistent with the rest of the client already treating
  `userId` as a server/protocol-internal detail (see `hostChanged`'s empty
  payload) and not something to key UI logic on directly.
- **Request pause**: your own circle — only when you're *not* the host, the
  host already has direct controls — gets `.avatarClickable` and a click
  handler that calls the party connection's `requestPause()`
  (`client/js/party.js` → `"requestPause"` socket event). The server
  (`server/src/socketHandler.js`) broadcasts `"pauseRequested"
  {userId, userName}` to **everyone** in the party, not just the host, so
  the whole group sees why playback is about to pause, and throttles it
  server-side per member (5s, `member.lastPauseRequestUtc`) independently of
  the client's own identical cooldown (`PAUSE_REQUEST_COOLDOWN_MS` in
  `player.js`) — the server-side one is the real guard (anyone poking the
  socket directly bypasses a client-only check); the client one just avoids
  a pointless click doing nothing.
- Receiving `"pauseRequested"` shows a short banner (`#pauseRequestBanner`,
  "`<name>` demande une pause") and flags the matching person's circle
  (`.avatarPauseRequested`: an orange `#ff9800` border plus a small pause-icon
  badge in its corner — `.avatarPauseBadge`, a plain inline two-bar SVG
  rather than a Unicode glyph, since pause/play characters default to emoji
  presentation on several platforms and look muddy at 10px) — both matched
  by the event's `userId` against each circle's `data-user-id`, both clearing
  automatically after `PAUSE_REQUEST_HIGHLIGHT_MS` (4s), for every member,
  including the requester and the host — deliberately not host-only, so a
  request landing while the host isn't looking at the tab is still visible
  to everyone else, and the requester gets confirmation their own click
  registered. This replaced an earlier plain box-shadow pulse animation and
  a separate immediate-local-feedback ring on just the requester's own
  circle (`.avatarSent`) — collapsed into one unambiguous visual language
  driven purely by the server broadcast (which the requester also receives,
  so there's no need for a separate client-only echo).
- `.followerNotice` picked up a `margin: 0` it was missing: it's a `<p>`,
  and for an absolutely positioned element `bottom` places the *margin*
  edge, so its default ~1em user-agent margin was quietly rendering it 13px
  higher than its declared `bottom: 24px` ever suggested. Harmless on its
  own until `.pauseRequestBanner` needed to sit a specific gap above it —
  found by measuring both rects and seeing the gap read as 2px instead of
  the ~15px the two `bottom` values alone implied.
- Nothing about this feature actually pauses anything automatically — "request"
  is literal, the host still has to act on it. Adding an actual forced-pause
  would be a real design decision (can any member override the host's
  playback state? that cuts against "host drives, everyone follows") and
  wasn't asked for.

Verified end to end with mocked `members`/`pauseRequested` Socket.IO events
against the real running local server (see session history): the presence
row renders directly under the action bar; initials/colors render correctly
for a two-member party; the host's circle never gets `.avatarClickable`
(neither when viewed by themselves nor by others); a `pauseRequested` event
gives the right circle `rgb(255, 152, 0)` as its computed border color and
`display: flex` on its badge (confirmed via computed styles, not just class
names) while leaving every other circle untouched; rapid repeat clicks on
your own circle within the cooldown collapse to exactly one `requestPause`
emission (three synchronous clicks in one script → the emitted-event count
increased by exactly one — an earlier two-separate-clicks check had looked
like a cooldown failure at first, but that was real wall-clock time between
two separate tool calls exceeding the 5s window, not a code bug); and the
banner/`.followerNotice` gap measured 15px after the margin fix, versus 2px
before it. The host-only controls (`becomeHost`, settings menu,
next-episode button) were re-checked after this change and still work.

## Player track selection: forcing our pick onto Emby's own negotiation

First real-world bug report once someone actually watched something end to
end: picking a language or subtitle track in the settings menu had no
effect on playback. Streaming itself working at all (this is the first
confirmation of that - see below) narrowed it down: `PlaybackInfo` and its
`TranscodingUrl` are reachable and produce a playable HLS stream, but the
specific track requested wasn't the one that came out the other end.

Likely cause, reasoned from how Emby/Jellyfin's own web client actually
negotiates this (not verified against this exact server - no way to, from
here): that client sends `AudioStreamIndex`/`SubtitleStreamIndex` as part
of a `POST /Items/{id}/PlaybackInfo` body alongside a `DeviceProfile`, not
as bare query-string params on a `GET` - which is all this app has ever
sent. Emby's `TranscodingUrl` reflects whatever `PlaybackInfo` actually
negotiated, so if the plain `GET` doesn't apply those params, the returned
URL just encodes Emby's own default track choice regardless of what we
asked for - and the old code used that URL as-is. Separately, even a
correctly-selected subtitle track has a second way to silently not show up:
Emby can deliver text subtitles either burned into the video
(`SubtitleMethod=Encode`) or as their own selectable HLS rendition
(`SubtitleMethod=Hls`) - this player's hls.js integration never wires up
`subtitleTrack`/`subtitleTracks` to pick a rendition, so a `Hls`-method
subtitle would be present in the manifest but never actually rendered.

Fixed defensively rather than by trying to exactly replicate Emby's own
client (`server/src/index.js`'s `/master.m3u8` route + its new
`applyPlaybackOverrides` helper): whichever URL we end up with - Emby's own
`TranscodingUrl`, or the hand-built fallback (which, before this fix,
dropped the audio/subtitle/quality query params entirely - a second,
independent way the same symptom could happen if a title ever took that
path) - our own `AudioStreamIndex`/`SubtitleStreamIndex`/
`MaxStreamingBitrate` are forced onto its query string afterward,
overwriting whatever Emby put there. Picking a subtitle also forces
`SubtitleMethod=Encode`; picking "Aucun" removes both
`SubtitleStreamIndex` and `SubtitleMethod` rather than just not adding one,
so it also cancels a subtitle Emby's own negotiation had defaulted to. This
means the negotiation call itself no longer has to be trusted at all - it
only needs to return *a* playable `MediaSource`, since every parameter this
app's UI actually exposes gets forced afterward regardless.

Verified as far as possible without real Emby access: `applyPlaybackOverrides`
is pure `URLSearchParams` manipulation, unit-tested in isolation (adding a
subtitle Emby didn't pick, removing one Emby had defaulted to, re-sending an
unchanged selection across an audio-only change, quality override, and the
no-selection-yet initial-load case) - all five cases produce the expected
final query string. Separately confirmed client-side, by mocking `Hls` to
capture what URL `loadSource` was actually called with: the initial load
already sends the default-selected track's index, and each dropdown change
re-sends a full reload with exactly the accumulated params expected
(switching audio → `AudioStreamIndex` changes; adding a subtitle → both
params present; back to "Aucun" → `SubtitleStreamIndex` dropped again).
What's **not** verified is the actual Emby round trip - whether
`applyPlaybackOverrides`'s forced params, once they reach the real server,
produce a transcode that really carries the requested track. That needs a
real retest.

**Round 2, after a retest: subtitles specifically still didn't show.**
Found a real gap in the fix above: `applyPlaybackOverrides` was only ever
applied to the *final* stream URL, not to the `PlaybackInfo` **negotiation**
call itself - so `SubtitleMethod=Encode` never reached Emby until after it
had already decided, during negotiation, whether this playback session
would be a full re-encode or a plain remux. Burning a subtitle into the
video is only possible with a real re-encode - appending
`SubtitleMethod=Encode` to a URL for a session Emby had already set up as a
remux plausibly does nothing, regardless of how correct the URL looks.
Fixed by applying `applyPlaybackOverrides` to the negotiation's own params
too (`server/src/index.js`), so Emby is asked to encode *before* it decides
how the session will actually be handled, not just told afterward.
Unit-tested (see session history): the negotiation call now measurably
carries `SubtitleMethod=Encode` whenever a subtitle is requested, same as
the final URL always did.

Also added `server/src/logger.js`'s `streamLog` (`[watchparty:stream]`,
alongside the party-lifecycle `partyLog` added separately - see "Server-side
logging" below) specifically because two rounds of blind, reasoned-but-
unverified fixes for this one bug is the point where guessing stops being
efficient: one `master.m3u8` request now logs its `itemId` and the
resolved `audio`/`subtitle`/`subtitleMethod`/`quality` values (never the
full URL or params blob - Emby's own `TranscodingUrl` can carry an
`api_key`, confirmed kept out of the log line by a dedicated test case),
so the next retest can confirm from `docker logs`/journalctl alone whether
a requested override actually reached Emby, without needing a devtools
session on whatever machine runs the server. If subtitles still don't show
once this log line confirms `subtitleMethod=Encode` really was sent, the
fault has been narrowed to Emby's own handling of that parameter rather
than anything before it in this app.

## Assumptions to verify

**Confirmed against the real server** (not a guess — actually exercised,
see session history): `POST /Users/AuthenticateByName` really is reachable
from Node's server-side `fetch` with no CORS issue at all (server-to-server
has no CORS concept), returns a real 401 for bad credentials, and the whole
login round-trip (browser → our backend → real Emby → real rejection → our
error message) renders correctly end to end. The `--theme-primary-color-*`
etc. custom properties `client/css/app.css` is built from were read directly
off this exact server's live dark theme, not guessed.

**Login and basic usage are now confirmed for real** — updated once actual
bug reports started coming from real usage rather than mocked testing
(overlay/pause-request feedback, the series-detail request, and the
track-selection bug above, which is only reachable once a stream is already
playing). That means the following are confirmed working end to end against
this real server, not just reasoned about, even though none were checked in
isolation: response field names (`AccessToken`, `User.Id`/`Name`, item
`Id`/`Name`/`Overview`/`ProductionYear`/`Type`, `Views`' `CollectionType`,
`Shows/{id}/Seasons`/`Episodes`, `Items/Latest`'s plain-array shape,
`PlaybackInfo`'s `MediaStreams`), the `X-Emby-Authorization`/`X-Emby-Token`
header format, `rewritePlaylist` against a real Emby-generated playlist, and
`PlaybackInfo` → `TranscodingUrl` → HLS producing a real watchable stream.
None of this was independently exercised - it's inferred from "the features
built on top of it work in practice, and nobody's reported the classes of
failure that would follow from any of it being wrong" - worth keeping in
mind if something in this area breaks in a way that doesn't fit whatever
specific bug is being chased at the time.

**Genuinely not yet confirmed:**

- **`/Users/{userId}` for token validation** was dropped — the proxy
  redesign no longer needs it at all (sessions are validated once at login
  and cached server-side, not re-checked against Emby per-request).
- **Player track selection** (`AudioStreamIndex`/`SubtitleStreamIndex`
  actually taking effect) — see the section just above; a fix is in place,
  reasoned from how Emby's API is known to work, but not yet re-tested for
  real.
- Whether `TranscodingUrl` is reliably present for every source type (a
  direct-play-eligible source might not transcode at all, which `hls.js` on
  the client wouldn't handle — see Known limitations), and whether it's
  always HLS.
- **The Emby web client deep-link shape**
  (`/web/index.html#!/item?id=X&serverId=Y`) used for "Voir sur Emby" —
  matches the long-standing convention, not confirmed against this exact
  build's routing (nobody's reported clicking it yet either way).

## UI: Emby-like shell, per-type browsing, episode queue

Reworked to look and navigate more like Emby itself, and to treat each
collection type (movies / TV / music) as its own experience rather than one
generic grid:

- **`client/js/shell.js`** renders the persistent chrome once per login: a
  sidebar (one plain-text link per library from `GET /api/catalog/libraries`
  — no icons, no emoji prefix, per an explicit earlier request) + a
  `#mainContent` element the router (`router.js`) swaps on every navigation.
  `GET /api/catalog/libraries` itself only ever returns libraries whose
  `CollectionType` is `movies`/`tvshows`/`music`/`musicvideos`
  (`VISIBLE_COLLECTION_TYPES` in `server/src/index.js`) — Emby's `/Views` can
  return all sorts of things this app has no UI for at all (boxset
  collections, playlists, live TV, photos, books, a mixed-content folder
  with no `CollectionType`), and filtering once at the source means the
  sidebar, the home page's per-library carousels, and direct navigation to
  `/library/:id` are all restricted the same way without repeating the
  check in each place — a stale link to a since-hidden library's id just
  renders `library.js`'s existing "Bibliothèque introuvable" state, since
  it looks the id up in this same filtered list. `router.js` was made
  idempotent
  (`start(root)` can be called again safely) specifically so login → shell
  boot can happen more than once in a page's lifetime without piling up
  duplicate `hashchange` listeners — the same class of bug the earlier Emby
  plugin work ran into with its page lifecycle, worth remembering.
- **`views/home.js`** (route `/home`): a horizontal carousel of every active
  party server-wide (the live lobby subscription, not a fetch — see "Live
  lobby, host handoff, no join codes" above) plus one more carousel per
  library of its recently-added items (`GET /api/catalog/libraries/:id/recent`,
  Emby's own `/Users/{id}/Items/Latest`) — skipping music (see below).
- **`views/library.js`** (route `/library/:id`): dispatches purely on the
  library's `collectionType`. `movies`/`tvshows`/`musicvideos` all get a tile
  grid (`GET /api/catalog/libraries/:id/items`, `Recursive=false` — Movies
  and music videos as themselves, TV as Series tiles); `music` gets a tile
  grid too, just of artists (`GET /api/catalog/libraries/:id/artists`) —
  see "Music: participative playlist" below for the rest of that hierarchy.
- **Tiles** (`client/js/tiles.js`, shared by the home carousels and the
  library grid so they can't drift apart): poster + name, hover-revealed
  overlay with "Lancer une watch party" (only for directly-playable types —
  `WATCH_PARTY_TYPES` in `server/src/index.js`, which had to grow a
  `MusicVideo` entry once `musicvideos` became a visible library type,
  otherwise every tile in it would render with no way to start a party at
  all; a Series tile gets "Voir les épisodes" instead, linking to
  `/series/:id`) and "Voir sur Emby" (opens Emby's own web client via a
  `embyUrl` the backend builds from `EMBY_SERVER_URL` + the session's
  `serverId`). `:hover`/`:focus-within` reveal it on mouse/keyboard;
  `@media (hover: none)` keeps it always visible on touch devices, which
  have no hover state at all.

  Bug: "Voir les épisodes" used to render as a bare, default-blue link,
  much less visible than the other two buttons next to it. The global
  `button {}` rule (`app.css`) that gives "Lancer une watch party" its
  solid green look is tag-scoped, so it never applied to "Voir les
  épisodes" — the one `.tileBtn` that's an `<a>` (a plain hash link) rather
  than a `<button>`. "Voir sur Emby" avoided the same fate only because it
  additionally carries `.tileBtnLink`, which explicitly sets a background/
  color for exactly this reason. Fixed by moving the solid-button look onto
  `.tileBtn` itself, tag-agnostic, with `.tileBtnLink` now overriding just
  the two properties that make it read as the secondary action. This also
  happens to be the semantically correct fix, not just a visual patch: for
  a Series tile "Voir les épisodes" *is* the primary action (a Series is
  never itself directly playable, so the two buttons never compete for
  primacy on the same tile) - it should look like one.
- **`views/series.js`** (route `/series/:id`): a detail header — poster,
  title + year, overview, a "Voir sur Emby" link — above season tabs
  (`GET /api/catalog/series/:id/seasons`, Emby's `/Shows/{id}/Seasons`) and
  an episode list per season (`GET /api/catalog/series/:id/episodes`, Emby's
  `/Shows/{id}/Episodes?SeasonId=...`), each episode startable as its own
  party. The header is just `toItemDto`'s existing fields
  (`GET /api/catalog/item/:id`, the same DTO shape as everything else) —
  the season/episode browsing itself predates this pass and was already
  wired end to end (library grid → Series tile → this route → "Lancer une
  watch party" per episode); what this pass added was making the page
  actually look like a detail page instead of a bare `<h1>` above the tabs.
  `.linklike`'s CSS selector was widened from `button.linklike` to
  `.linklike` so the new "Voir sur Emby" `<a>` picks it up too — there's no
  global `a` reset in `app.css`, so an unclassed anchor would otherwise
  fall back to the browser's default blue/underline against this dark theme.

### Episode queue / "next episode"

Starting a party from an episode (`POST /api/parties`) now has the backend
fetch the rest of that series (`/Shows/{seriesId}/Episodes`, no season
filter) and store everything from the clicked episode onward as the party's
`queue` (`server/src/partyManager.js`). The host gets an "Épisode suivant"
button once `hasNext` is true; clicking it (or the video's `ended` event
firing) sends a `"next"` Socket.IO event
(`server/src/socketHandler.js`), host-only, which advances `queueIndex`,
resets every member's known position, and broadcasts `{type:"itemChanged",
itemId, itemName, ...}` — every client (including the host) reloads the
player against the new `itemId` on that message. A movie or a
standalone-started episode just gets a one-item queue, so this is a strict
superset of the old single-item behavior, not a separate code path.

### Player: language, subtitles, quality

`GET /api/catalog/item/:id/playback-options` reads `PlaybackInfo`'s
`MediaSources[0].MediaStreams` and returns the Audio/Subtitle entries as
`{index, label}` pairs for two `<select>`s in the player's host-only
controls. Picking a value reloads the stream
(`/api/catalog/stream/:id/master.m3u8?AudioStreamIndex=&SubtitleStreamIndex=&MaxStreamingBitrate=`)
at the same position (`hls.js` destroyed and rebuilt, `currentTime`/paused
state preserved across the reload) rather than restarting from zero.
"Quality" is a fixed set of `MaxStreamingBitrate` presets (Auto/1080p/720p/
480p) forwarded to `PlaybackInfo` — the same simplification Emby's/
Jellyfin's own web clients use; it's a transcode ceiling, not true
client-side adaptive bitrate switching. Subtitles picked this way get
burned into the transcoded video (Emby's doing, via `SubtitleStreamIndex`)
rather than rendered as a toggleable `<track>` — simpler to implement
correctly than sidecar WebVTT, at the cost of a re-transcode to turn them
on/off.

### Confirmed vs. not, for all of the above

Driving an actual browser against this (see session history) confirms the
**rendering and interaction logic**: sidebar highlighting, home carousels
(parties + one row per non-music library), library grid dispatch (including
a `tvshows` library's Series tiles linking to `/series/:id` both via the
poster and the "Voir les épisodes" button), and the series page's detail
header + season-tab switching + episode list + starting a party from an
episode all render and navigate correctly. That test mocked every
`/api/...` response (canned JSON), since
exercising it for real needs a logged-in session — so what's **not**
confirmed yet is the shape of the real Emby responses this now depends on:
`Views` items actually carrying a `CollectionType` field the way assumed,
`/Shows/{id}/Seasons` and `/Shows/{id}/Episodes`, `Items/Latest`'s
plain-array response shape (handled defensively — see `index.js` — but not
seen for real), and `PlaybackInfo`'s `MediaStreams` shape for the track
pickers. The queue/"next episode" WebSocket flow and the player's
track/quality reload are new protocol surface with **no test coverage at
all yet** (no way to exercise real HLS playback without a live session) —
the first things to check once a real party actually plays something.

## Non-goals

Chat/reactions and OIDC/SSO remain out of scope - see Goals/Non-goals at
the top. Music was in this list for a while (see git history) - it's
implemented now, see its own section below.

## Server-side logging

Requested once the app was running for real on the user's own server:
visibility into what's actually happening, from `docker logs`/journalctl,
without needing to reproduce a bug through the UI first. `server/src/logger.js`
is deliberately tiny - `partyLog(message)` (`[watchparty:party]`) and
`streamLog(message)` (`[watchparty:stream]`, added a bit later for the
streaming route specifically - see "Player track selection" above), both
plain `console.log` with an ISO timestamp, plus a `who(userId, userName)`
formatter shared by every party-lifecycle call site - no logging library,
consistent with this app's general bias against dependencies it doesn't
need.

Covers the party's business lifecycle end to end, called from wherever the
state actually changes so every caller is covered without repeating the
check - `server/src/partyManager.js`'s `create`/`join`/`leave` (which also
covers a host handoff or the party being destroyed, both of which fall out
of `leave`)/`advanceQueue`/`removeIdleParties`, plus
`server/src/socketHandler.js`'s `playstate` and `requestPause` handlers,
since those two mutate member state directly rather than through a
`PartyManager` method.

The one thing this needed real care over: the host's player re-sends its
*current* play/pause state every 3 seconds regardless of whether anything
changed (the heartbeat - see "Realtime transport" above), on top of firing
once per actual `play`/`pause`/`seeked` event. Logging every `"playstate"`
message as it arrives would mean one log line every 3 seconds for every
active party, forever - noise that would drown out everything else within
minutes on a server with even a couple of parties running. The handler
instead compares the incoming `isPaused` against the member's *previous*
value and only logs on an actual flip, silently updating position on every
other message - so a party generates exactly one `"playstate"` line per
real pause or resume, never one per heartbeat tick.

Verified directly against the real shipped modules (not reimplemented
mocks) - see session history: a scripted `create`/`join`×2/`advanceQueue`
(twice — once real, once a no-op past the end of the queue, correctly
silent)/`leave`×3 (plain, host handoff, final-member destroy)/idle-sweep
sequence against the real `partyManager.js` produced exactly the expected
line for every step, in the expected order; a fake `io`/`socket` pair
driving the real `socketHandler.js` through a start → 5 unchanged
heartbeats → pause → 3 unchanged heartbeats → resume sequence logged
exactly 3 `"playstate"` lines (not 9) at exactly the right points, and a
throttled repeat `"requestPause"` produced no second `"pauseRequested"`
line - all assertions passed, counted programmatically rather than
eyeballed.

## Music: participative playlist

The last deferred piece from the original UI spec, built once asked for
directly. Three things needed designing before any code: the permission
model for the shared queue (asked, not assumed - see session history), how
audio playback fits the existing party/socket infrastructure, and how
"browse and add a track" works without leaving the player.

**Chosen model: free add, vote-to-skip, no host for playback.** Anyone in
the party can add a track to the shared queue at any time. Anyone can
pause/resume directly (reversible, low-stakes - unlike video's host-only
`"playstate"`). Skipping the *current* track needs a strict majority vote
of currently connected members - the one action asked to be gated, since
unlike adding (purely additive) or pausing (trivially undone), skipping
throws away where the party is now. This is a genuinely different shape
of shared state than video's "one host drives, everyone follows" - not a
size difference, a different model - which is exactly why it's a separate
service (`server/src/musicQueueService.js`, `MusicQueueService`) rather
than more branches inside `partyManager.js`/`socketHandler.js`'s existing
video-shaped logic.

### Data model and server-authoritative timing

`Party` gained a `mediaType: 'video' | 'music'` field
(`PartyManager.create`'s new last argument, inferred server-side in
`POST /api/parties` from the clicked item's Emby `Type` - `'Audio'` means
music, everything else already handled means video - the client never has
to know or say which). A music party's `party.music` object
(`{queue, current, skipVotes, advanceTimer}`) is entirely separate from
the fixed pre-built `party.queue`/`queueIndex` video parties use - a music
queue is live and mutable, not something computed once at creation.

No single client is "the position" for music the way a video host's
heartbeat is. Instead, `party.music.current.startedUtc` (a wall-clock
timestamp) plus the track's known duration is the only state that exists -
`getElapsedSeconds()` computes position on demand
(`Date.now() - startedUtc`, or a frozen `pausedElapsedMs` while paused),
never ticked or stored. This is also what makes auto-advance possible
without any client "reporting" a track ended (which, with several clients'
players all finishing within milliseconds of each other and no single
host, could race): `_scheduleAdvance` sets one `setTimeout` for exactly
when the current track's duration runs out, computed from the same
`startedUtc`. A track with no known duration just has nothing scheduled -
it relies on a manual vote-skip instead, same as it would if the timer
fired.

That timer is the one genuinely new leak surface this feature adds, given
how much this project has already had to fix real memory leaks (see
"Realtime transport" above) - `_clearTimer` runs before every reschedule
and from a new `destroy(party)` hook wired into **both** of
`partyManager.js`'s existing party-destruction paths (`leave`'s last-member
case and `removeIdleParties`' sweep), so a torn-down party can never leave
a timer firing against state that no longer exists in `PartyManager`'s map.

`MusicQueueService` needs to broadcast from inside that timer's callback,
which runs with no socket/request in scope at all - solved with
`attachIo(io, onTrackChanged)`, called once at server startup
(`index.js`, right next to where `attachSocketServer` itself is wired up),
the same "give the singleton what it needs once, up front" shape
`PartyManager`'s own singleton already has. `onTrackChanged` is wired to
`broadcastLobby(io)` specifically so a party's home-page card (preview
image/name) stays current even when the *timer*, not a person clicking
anything, is what changed what's playing.

### Catalog: Artists → Albums → Tracks

Three more dedicated Emby endpoints, same reasoning as `Shows → Seasons →
Episodes`: guessing folder structure via generic recursive `/Items` calls
is fragile, a real endpoint per level isn't. `GET /Artists?ParentId=...`
for a library's artists, `GET /Users/{id}/Items?ArtistIds=...
&IncludeItemTypes=MusicAlbum` for an artist's albums, `GET
/Users/{id}/Items?ParentId=<albumId>&IncludeItemTypes=Audio` for an
album's tracks. `toItemDto` grew `durationTicks`/`indexNumber`/
`artistName`/`albumName` - additive, `undefined` for every other item type,
so nothing existing had to change shape. `WATCH_PARTY_TYPES` grew
`'Audio'` (a track can start a party) - `MusicArtist`/`MusicAlbum` stay
un-playable, drill-down only, joining `Series` in `tiles.js`'s
`DRILL_DOWN` table (generalized from a single `seriesHref` special case to
a small per-type lookup once there were three, not two, drill-down
targets - `/artist/:id` → `artist.js`, `/album/:id` → `album.js`, each a
smaller version of the library-grid/episode-list shape already used
elsewhere).

### Streaming: the same route, one hint

Audio goes through the *exact same* `/api/catalog/stream/:itemId/master.m3u8`
route as video, not a parallel endpoint - Emby's `PlaybackInfo` negotiation
isn't video-specific, so the whole `TranscodingUrl`-first,
hand-built-fallback dance (see "Player track selection" above) applies
identically. The one real difference is the hand-built fallback's path
prefix (`/Videos/` vs `/Audio/`), which the response doesn't reliably
reveal on its own and looking up would cost an extra Emby round trip for
something the client already knows - so the client tags the request
(`streamUrl(itemId, {mediaType:'audio'})`, `audioStreamUrl()` in
`embyApi.js`) and the route reads it back
(`req.query.mediaType === 'audio'`). The music player uses hls.js for the
same reason the video player does: most browsers have no native `.m3u8`
support at all, and there's no reason to assume Emby's negotiation
produces anything *other* than HLS just because the media is audio.

### Player: not theater-mode, and why adding a track doesn't navigate away

`renderPartyPlayer` (route `/party/:id`, `player.js`) is now a small
dispatcher: it fetches the party once (both the video and music renderers
need `mediaType` before they can render anything at all) and delegates to
`renderVideoPlayer` or the new `renderMusicPlayer`
(`views/musicPlayer.js`). The dispatch itself can't be `async` despite
needing to `await` that fetch - `route()`'s contract needs its cleanup
function back *synchronously* (see `router.js`), and an `async` handler
would hand back a `Promise` instead, which `router.js`'s `typeof cleanup
=== 'function'` check silently skips - reintroducing the exact leak class
this project already spent real effort fixing once. Solved with a mutable
box: `renderPartyPlayer` returns a wrapper immediately, which calls
whichever real cleanup function the fetch eventually produces - or nothing
at all, correctly, if the party navigates away before that fetch even
resolves (nothing was set up yet in that case - no socket, no timer,
nothing to leak).

Unlike the video player, the music player is **not** full-viewport/theater
-mode (no `.playerWrap`-style `position:fixed`) - there's no video image to
declutter around, so it renders as a normal page and the sidebar stays up.
That's also what makes "add tracks while listening" simple: rather than
navigating to a separate browsing page and losing the player's own state
(and having to thread "which party am I adding to" through a chain of
query params across three route levels - considered, rejected as needless
complexity for what browsing-with-context would have bought), the player
has its own small embedded Artist → Album → Track browser
(`musicPlayer.js`'s `loadMusicLibraries`/`loadArtists`/`loadAlbums`/
`loadTracks`), reusing the exact same catalog endpoints, rendered inline
in a toggleable panel. Adding a track is REST
(`POST /api/parties/:partyId/tracks`, `addTrackToQueue()` in `party.js`),
not a socket event like `voteSkip`/`musicPlaystate` - deliberately, since
it needs to work from this same panel regardless of realtime-connection
state, and `MusicQueueService.addTrack`'s own `"queueUpdated"` broadcast
(via the `attachIo` wiring above) reaches everyone actually connected
either way, so nothing about the realtime side needed a second code path.

### A real bug found while testing, not just the mock's fault

`applyMusicState` (the "queueUpdated"/initial-render handler) used to
update the queue list and vote button *after* the current-track branch,
which calls `loadTrack()` - and `loadTrack()`'s hls.js setup can throw (a
bad manifest, hls.js failing to load from the CDN, any of it). Because
JavaScript doesn't roll back a function's earlier statements when a later
one throws, but *does* abort everything after the throw in the same call,
an exception there took the queue list and vote tally down with it even
though they have nothing to do with whether one specific track's audio
loaded - and worse, on the very first render, it escaped `renderMusicPlayer`
entirely into the dispatcher's `.catch()`, which alerts and navigates back
to `/home`, discarding the whole view. Fixed two ways: reordered
`applyMusicState` so the queue/vote/icon updates - which can't fail - run
*before* the current-track branch that can (mirroring `player.js`'s
`onItemChanged`, which already updates the title/next-button before
calling `loadVideo` for exactly this reason), and wrapped `loadTrack`'s
hls.js setup in its own `try/catch` so a failure there logs and stops,
rather than propagating out of the handler at all. Found by a test whose
own mock happened to be missing `Hls.Events` - which pointed at a real
fragility in the shipped code, not just an incomplete mock, so it's fixed
in `musicPlayer.js` itself, not worked around in the test.

### Verified

Server-side, directly against the real shipped modules, not reimplemented
mocks (see session history): a scripted sequence against the real
`musicQueueService.js` alone - init auto-starts the first track, a second
add while one plays queues rather than jumping the queue, a two-vote
majority skips and clears the tally, pause/resume freezes and correctly
resumes elapsed time, a short real track auto-advances on its own via a
genuine `setTimeout` after its duration, and `destroy()` clears a pending
timer (18 checks, instrumented `setTimeout`/`clearTimeout` counting to
catch anything left dangling - same technique used for the client-side
router-cleanup leak earlier in this project). A second pass through the
real `socketHandler.js` + `partyManager.js` together (fake `io`/sockets)
confirmed `voteSkip`/`musicPlaystate` are no-ops on a video party, a
non-host member can pause and resume a music party's track directly, a
two-connected-member vote-skip needs exactly two votes, and the resulting
`"queueUpdated"` really reaches the party's socket room (7 checks).

Client-side, against the real running local server with mocked
`fetch`/`Hls`/`io` (see session history): the full library → artist →
album browsing chain renders correct tiles and drill-down links at each
level; starting a party from a track correctly creates one and navigates;
the dispatcher correctly renders the music player (not the video one) for
a `mediaType:'music'` party and still renders the video player when
`mediaType` is absent (back-compat); the now-playing header, vote tally
(computed against the live member list, matching `musicQueueService.js`'s
own majority math), and queue list all render correctly and update
correctly on a `"queueUpdated"` push including a live track change (new
audio source loaded); play/pause and vote-skip clicks emit the right
socket events; the inline browse-and-add panel correctly walks
artist → album → tracks and POSTs the exact right payload shape to
`/api/parties/:partyId/tracks`; and the crash fixed above was confirmed
fixed by re-running the same broken-`Hls.Events` scenario through the
full dispatcher afterward - no alert, no navigation away, the queue/track
info render correctly regardless.

**Not verified** - the usual limit of testing without real Emby access:
whether `/Artists`/the album-tracks query really return what's assumed
here, whether a music item's `PlaybackInfo`/`TranscodingUrl` behaves the
same way a video's does (reasoned to, per "Streaming" above, not
observed), and the real audio quality/gapless-ness of playback through a
transcode. First things to check once this gets a real retest.

## Known limitations

- Playback always goes through `hls.js` against whatever Emby's
  `PlaybackInfo` suggests. If that ever comes back as a direct-play source
  with no `TranscodingUrl` (no transcoding needed/possible), the client has
  no fallback path yet - it'll try to hand hls.js something that isn't HLS.
- One fixed item/episode-queue per **video** party (a music party's queue
  is the live, participative one - see "Music" above).
- A video party member needs a browser tab open and connected for sync to
  apply - there's no "catch up automatically when you come back" beyond
  whatever the next heartbeat corrects. Music has no equivalent heartbeat
  at all (a deliberate simplification, not an oversight - see "Music"
  above): position only resyncs on a discrete event (add/skip/pause/
  resume), so two listeners' `<audio>` elements could drift apart slightly
  during a long stretch with no such event - accepted for v1 given music's
  more forgiving tolerance for exact sync compared to video+subtitles.
- No persistence: server restart or long idle time ends every party.
- No free seeking within a music track (only vote-skip to the *next* one)
  - not asked for, and would need its own protocol addition
  (`MusicQueueService` has no notion of "jump to position X" today, only
  play/pause and advance).