Link video/music playback to the OS own media controls (Windows SMTC, macOS Now Playing, ...)

New mediaSession.ts wraps the standard Media Session API - shared by
Player.ts and MusicBar.ts, which can never both be active at once so
there is no risk of them fighting over this one global. Metadata/
position/playback-state hook into each players own existing single
source of truth (updatePlayPauseBtn/updateProgressBar, setPlayIcon/
updatePositionDisplay) instead of adding parallel call sites. Play/
pause/seek are wired once and read isHost/conn fresh on every
OS-triggered call, so they keep working through a live host handoff;
a non-host play/pause routes through the same requestPause/
requestResume the request toolbar already uses, and seek is gated on
isHost the same way the in-page seek bar already is. Next is handled
separately since a registered nexttrack handler is what makes the OS
show that control at all - its presence tracks whatever the in-page
equivalent already tracks (isHost && hasNextQueued for video, a vote
skip for music). Cleared entirely on leaving a party/page, or just to
idle (metadata/state only) when a listen partys queue runs empty but
the party itself is still open.

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

README.md

@@ -392,6 +392,9 @@ Then put it behind your existing reverse proxy like everything else
 - [x] A mobile banner nudging an install of the PWA — the real
       Chrome/Android install prompt, manual instructions on iOS Safari
       (no such prompt exists there) — see docs/architecture.md
+- [x] Linked whatever's playing (video or music) to the OS's own media
+      controls (Windows' SMTC, macOS' Now Playing, ...) — title/artwork,
+      play/pause/seek/next — see docs/architecture.md
 - [ ] Try a full party end-to-end with two real accounts (still the main gap)
 - [ ] Try casting to a real Chromecast (only the SDK's own bootstrap is
       confirmed here — no real device/session)

client/src/mediaSession.ts

@@ -0,0 +1,123 @@
+// System Media Transport Controls integration - Windows' own media-key
+// popup/taskbar overlay (also macOS' Now Playing widget, Android's
+// notification) - see docs/architecture.md. A thin wrapper around the
+// standard Media Session API, not a new abstraction of its own: every
+// function here is a one-line guard (`'mediaSession' in navigator`)
+// around the exact browser call, since Firefox has never implemented
+// this at all and this safely no-ops everywhere else it's missing too.
+// Shared by views/player/Player.ts (video) and musicBar/MusicBar.ts
+// (a listen party) - the two can never be active at once (starting/
+// joining either already leaves the other first), so there's no risk
+// of them fighting over this single global.
+
+export interface MediaSessionActions {
+  onPlay: () => void;
+  onPause: () => void;
+  /** Host-only for video (Player.ts checks isHost itself before acting -
+   *  this just delivers the OS's own seek gesture), unrestricted for
+   *  music (no host concept there). */
+  onSeek?: (seconds: number) => void;
+}
+
+/** Title/artist/album/art shown in the OS's own now-playing UI - artist
+ *  and album are meaningless for a video (pass '' for both), matching
+ *  how a movie has no equivalent fields in this app's own UI either. */
+export function setMediaSessionMetadata(title: string, artist: string, album: string, artworkUrl: string): void {
+  if (!('mediaSession' in navigator)) return;
+  navigator.mediaSession.metadata = new MediaMetadata({
+    title,
+    artist,
+    album,
+    artwork: artworkUrl ? [{ src: artworkUrl, sizes: '512x512', type: 'image/jpeg' }] : []
+  });
+}
+
+export function setMediaSessionPlaybackState(playing: boolean): void {
+  if (!('mediaSession' in navigator)) return;
+  navigator.mediaSession.playbackState = playing ? 'playing' : 'paused';
+}
+
+/** The OS's own scrubber - harmless to skip (not every browser/OS
+ *  combination renders one) but adds real functionality where it does.
+ *  Chrome throws if `duration` isn't a positive finite number, or if
+ *  `position` doesn't satisfy `0 <= position <= duration` - both
+ *  momentarily true the instant an item/track changes but the ticker
+ *  driving this hasn't caught up yet, so this is caught rather than
+ *  guarded field-by-field (the next tick corrects it either way). */
+export function setMediaSessionPositionState(duration: number, position: number): void {
+  if (!('mediaSession' in navigator) || !navigator.mediaSession.setPositionState) return;
+  if (!(duration > 0) || !isFinite(duration)) return;
+  try {
+    navigator.mediaSession.setPositionState({ duration, position: Math.max(0, Math.min(position, duration)), playbackRate: 1 });
+  } catch {
+    // See this function's own comment - a transient mismatch, not a real error.
+  }
+}
+
+/**
+ * Wired once per party lifetime (Player.ts's init(), MusicBar.ts's
+ * initMusicBar()) - the handlers themselves close over whatever local
+ * state they need (isHost, conn, ...) and read it fresh on every
+ * OS-triggered call, so this never needs re-wiring just because that
+ * state changed later (see Player.ts's own isHost, which does exactly
+ * that mid-session on a host handoff).
+ */
+export function setMediaSessionActionHandlers(actions: MediaSessionActions): void {
+  if (!('mediaSession' in navigator)) return;
+  navigator.mediaSession.setActionHandler('play', actions.onPlay);
+  navigator.mediaSession.setActionHandler('pause', actions.onPause);
+  if (actions.onSeek) {
+    const onSeek = actions.onSeek;
+    navigator.mediaSession.setActionHandler('seekto', (details) => {
+      if (details.seekTime != null) onSeek(details.seekTime);
+    });
+  }
+}
+
+/** Shows/hides the OS's own "next" control - called whenever this app's
+ *  own equivalent (Player.ts's #nextBtn, a queue advance in general)
+ *  would also change visibility. A *registered* handler is what makes
+ *  Windows show the button at all, regardless of whether clicking it
+ *  would currently do anything - so this has to track "is there
+ *  actually a next" the same way the in-page button does, not just be
+ *  set once and left there. */
+export function setMediaSessionNextAction(onNext: (() => void) | null): void {
+  if (!('mediaSession' in navigator)) return;
+  try {
+    navigator.mediaSession.setActionHandler('nexttrack', onNext);
+  } catch {
+    // An unrecognized action name on an older browser - safe to skip.
+  }
+}
+
+/** Drops the OS's own now-playing card without touching the play/pause/
+ *  seek handlers underneath it - for "still in the party, just nothing
+ *  queued right now" (MusicBar.ts's own `!music.current`), where those
+ *  handlers stay meaningful (playback can resume the moment a track
+ *  does) and so shouldn't need re-wiring the instant one does. `null`,
+ *  not an empty MediaMetadata - that's the documented way to tell the
+ *  OS there's genuinely nothing loaded, not a track with a blank title. */
+export function setMediaSessionIdle(): void {
+  if (!('mediaSession' in navigator)) return;
+  navigator.mediaSession.metadata = null;
+  navigator.mediaSession.playbackState = 'none';
+}
+
+/** Called when leaving a party/party page entirely - an OS-level "now
+ *  playing" card that outlives the actual party would be actively
+ *  wrong, not just stale, so this clears every piece the functions
+ *  above ever set (not just the now-playing card - see
+ *  setMediaSessionIdle above for that alone) rather than leaving it to
+ *  be silently overwritten by whatever starts next. */
+export function clearMediaSession(): void {
+  if (!('mediaSession' in navigator)) return;
+  setMediaSessionIdle();
+  try {
+    navigator.mediaSession.setActionHandler('play', null);
+    navigator.mediaSession.setActionHandler('pause', null);
+    navigator.mediaSession.setActionHandler('nexttrack', null);
+    navigator.mediaSession.setActionHandler('seekto', null);
+  } catch {
+    // Same tolerance as setMediaSessionNextAction above.
+  }
+}

client/src/musicBar/MusicBar.ts

@@ -10,6 +10,7 @@ import { showToast } from '../toast';
 import type { MusicCurrentPublic, MusicPublicState, PartyPublicState, SocketMember } from '../types';
 import { initCast, localDriver, onCastAvailabilityChanged, onCastSessionEnded, startCast, stopCast } from '../cast';
 import type { PlaybackDriver } from '../cast';
+import { clearMediaSession, setMediaSessionActionHandlers, setMediaSessionIdle, setMediaSessionMetadata, setMediaSessionNextAction, setMediaSessionPlaybackState, setMediaSessionPositionState } from '../mediaSession';
 
 /**
  * Persistent, app-lifetime "listen party" bar - mounted ONCE by Shell.ts
@@ -162,14 +163,18 @@ export function initMusicBar(root: HTMLElement): void {
   // as Player.ts's progressBar.
   let isDraggingProgress = false;
 
+  function seekTo(seconds: number): void {
+    if (currentDurationSeconds <= 0) return;
+    activeDriver!.currentTime = Math.min(Math.max(0, seconds), currentDurationSeconds);
+    updatePositionDisplay();
+    conn?.seekMusic(activeDriver!.currentTime);
+  }
+
   function seekFromPointerEvent(e: PointerEvent): void {
     if (currentDurationSeconds <= 0) return;
     const rect = trackProgress.getBoundingClientRect();
     const fraction = Math.min(1, Math.max(0, (e.clientX - rect.left) / rect.width));
-    const seconds = fraction * currentDurationSeconds;
+    seekTo(fraction * currentDurationSeconds);
-    activeDriver!.currentTime = seconds;
-    updatePositionDisplay();
-    conn?.seekMusic(seconds);
   }
 
   trackProgress.addEventListener('pointerdown', (e) => {
@@ -184,6 +189,15 @@ export function initMusicBar(root: HTMLElement): void {
   trackProgress.addEventListener('pointerup', () => { isDraggingProgress = false; });
   trackProgress.addEventListener('pointercancel', () => { isDraggingProgress = false; });
 
+  // Wired once, like Player.ts's own equivalent - no host concept here
+  // (see activeDriver's own comment) so, unlike there, play/pause never
+  // need to branch into a request-instead-of-act path.
+  setMediaSessionActionHandlers({
+    onPlay: () => conn?.setMusicPaused(false),
+    onPause: () => conn?.setMusicPaused(true),
+    onSeek: seekTo
+  });
+
   // Visible to any member the moment a Chromecast is on the network - not
   // gated on anything party-related, same as Player.ts's own castBtn
   // (see its comment: casting is available to any member, and this bar
@@ -219,6 +233,7 @@ function updatePositionDisplay(): void {
   // than its own pointer-seek wiring: dragging is one tap (expand the
   // panel) away, see #mobileMiniBar's own click handler.
   (barEl!.querySelector('#miniProgressFill') as HTMLElement).style.width = percent;
+  setMediaSessionPositionState(currentDurationSeconds, currentTime);
 }
 
 /**
@@ -283,6 +298,7 @@ export function leaveMusicParty(): void {
   currentItemId = null;
   lastMusicState = null;
   latestMembers = [];
+  clearMediaSession();
 
   conn?.leave();
   conn = null;
@@ -336,6 +352,7 @@ function setPlayIcon(isPlaying: boolean): void {
   const icon = isPlaying ? PAUSE_ICON_SVG : PLAY_ICON_SVG;
   (barEl!.querySelector('#playPauseBtn') as HTMLButtonElement).innerHTML = icon;
   (barEl!.querySelector('#miniPlayPauseBtn') as HTMLButtonElement).innerHTML = icon;
+  setMediaSessionPlaybackState(isPlaying);
 }
 
 /** Same pairing as setPlayIcon above, for the track metadata shown in
@@ -429,6 +446,10 @@ function loadTrack(track: MusicCurrentPublic, elapsedSeconds: number, isPaused:
   currentItemId = track.itemId;
   currentDurationSeconds = track.durationSeconds || 0;
   setTrackMeta(track.itemImageUrl || '', track.itemName || '', [track.artistName, track.albumName].filter(Boolean).join(' - '));
+  // Artist/album kept separate here, unlike setTrackMeta's own combined
+  // display line above - the OS's own now-playing UI has real fields for
+  // both, no need to pre-join them into one string for it.
+  setMediaSessionMetadata(track.itemName || '', track.artistName || '', track.albumName || '', track.itemImageUrl || '');
 
   if (isCasting) {
     castTrack([track.itemName, track.artistName].filter(Boolean).join(' - ')).catch((err) => {
@@ -538,6 +559,11 @@ function applyMusicState(music: MusicPublicState): void {
   // "disable, don't just silently no-op" treatment as playPauseBtn above.
   castBtn.disabled = !music.current && !isCasting;
   updateVoteSkipLabel();
+  // A *registered* nexttrack handler is what makes the OS show the
+  // control at all (see setMediaSessionNextAction's own comment) - same
+  // "disable, don't just silently no-op" case as playPauseBtn/castBtn
+  // above, just with no disabled state of its own to set instead.
+  setMediaSessionNextAction(music.current ? () => conn?.voteSkip() : null);
 
   // Its own .queueRow, NOT views/album's shared .trackRow (see
   // styles/trackList.css) - that one is a CSS grid whose 4 columns
@@ -561,6 +587,7 @@ function applyMusicState(music: MusicPublicState): void {
   if (!music.current) {
     currentItemId = null;
     setTrackMeta('', 'Rien ne joue pour le moment.', '');
+    setMediaSessionIdle();
     if (hls) { hls.destroy(); hls = null; }
     activeDriver!.pause();
     audio.removeAttribute('src'); // harmless no-op while casting - the local element is already srcless then, see startCasting

client/src/views/player/Player.ts

@@ -13,6 +13,7 @@ import type { BurnInChangedMsg, ItemChangedMsg, PartyPublicState, PauseRequested
 import { joinMusicParty, leaveMusicParty } from '../../musicBar/MusicBar';
 import { initCast, localDriver, onCastAvailabilityChanged, onCastSessionEnded, startCast, stopCast } from '../../cast';
 import type { PlaybackDriver } from '../../cast';
+import { clearMediaSession, setMediaSessionActionHandlers, setMediaSessionMetadata, setMediaSessionNextAction, setMediaSessionPlaybackState, setMediaSessionPositionState } from '../../mediaSession';
 
 const TICKS_PER_SECOND = 10000000; // Emby/.NET convention: 1 tick = 100ns
 const DRIFT_TOLERANCE_SECONDS = 1.5;
@@ -414,6 +415,7 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
    *  onDriverStateChanged below. */
   function updatePlayPauseBtn() {
     playPauseBtn.innerHTML = activeDriver.paused ? PLAY_BTN_ICON_SVG : PAUSE_BTN_ICON_SVG;
+    setMediaSessionPlaybackState(!activeDriver.paused);
   }
 
   /** #muteBtn - deliberately reads/writes `video.muted` directly, never
@@ -437,6 +439,7 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
     const current = activeDriver.currentTime;
     progressFill.style.width = duration > 0 ? `${Math.min(100, (current / duration) * 100)}%` : '0%';
     timeLabel.textContent = duration > 0 ? `${formatDuration(current)} / ${formatDuration(duration)}` : '';
+    setMediaSessionPositionState(duration, current);
   }
 
   /**
@@ -517,13 +520,23 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
 
   let isDraggingProgress = false;
 
+  /** Host-only in effect (both call sites already gate on isHost before
+   *  reaching here - the progress bar's own pointerdown handler below,
+   *  and the Media Session seekto action) - this itself just clamps and
+   *  applies, same shape as MusicBar.ts's own seekTo. */
+  function seekTo(seconds: number) {
+    const duration = activeDriver.duration;
+    if (!duration) return;
+    activeDriver.currentTime = Math.min(Math.max(0, seconds), duration);
+    onDriverStateChanged();
+  }
+
   function seekFromPointerEvent(e: PointerEvent) {
     const duration = activeDriver.duration;
     if (!duration) return;
     const rect = progressBar.getBoundingClientRect();
     const fraction = Math.min(1, Math.max(0, (e.clientX - rect.left) / rect.width));
-    activeDriver.currentTime = fraction * duration;
+    seekTo(fraction * duration);
-    onDriverStateChanged();
   }
 
   // Host-only (checked here, not just via the readOnly class - that one
@@ -662,6 +675,7 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
     settingsWrap.classList.remove('hide');
     requestToolbar.classList.add('hide'); // the host acts directly - no requests to make once promoted mid-session
     nextBtn.classList.toggle('hide', !hasNextQueued);
+    setMediaSessionNextAction(hasNextQueued ? goToNextItem : null);
     playPauseBtn.classList.remove('hide');
     progressBar.classList.remove('readOnly');
     updateAmbientNotice();
@@ -1038,8 +1052,10 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
 
   async function onItemChanged(msg: ItemChangedMsg) {
     itemTitleEl.textContent = msg.itemName || '';
+    setMediaSessionMetadata(msg.itemName || '', '', '', `/api/catalog/image/${msg.itemId}`);
     hasNextQueued = !!msg.hasNext;
     nextBtn.classList.toggle('hide', !(isHost && hasNextQueued));
+    setMediaSessionNextAction(isHost && hasNextQueued ? goToNextItem : null);
     // A new item's own stream indices/subtitle pick have nothing to do
     // with the old one's - PartyService.advanceQueue already resets this
     // server-side too (see its own comment); this just keeps this client
@@ -1187,22 +1203,42 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
   // cooldown (mirroring the server's own independent per-request
   // throttle in SocketGateway) purely so a rapid double-click doesn't
   // even bother emitting the second request.
-  requestPauseBtn.addEventListener('click', () => {
+  function requestPause() {
     const now = Date.now();
     if (now < pauseRequestCooldownUntil || !conn) return;
     pauseRequestCooldownUntil = now + REQUEST_COOLDOWN_MS;
     conn.requestPause();
-  });
+  }
 
-  requestResumeBtn.addEventListener('click', () => {
+  function requestResume() {
     const now = Date.now();
     if (now < resumeRequestCooldownUntil || !conn) return;
     resumeRequestCooldownUntil = now + REQUEST_COOLDOWN_MS;
     conn.requestResume();
-  });
+  }
+
+  requestPauseBtn.addEventListener('click', requestPause);
+  requestResumeBtn.addEventListener('click', requestResume);
+
+  function goToNextItem() {
+    conn?.next();
+  }
 
   async function init() {
     itemTitleEl.textContent = party.itemName || '';
+    setMediaSessionMetadata(party.itemName || '', '', '', `/api/catalog/image/${party.itemId}`);
+    // Play/pause/seek are wired exactly once - see setMediaSessionActionHandlers'
+    // own comment for why they never need re-wiring on a host handoff.
+    // "next" isn't included here: unlike play/pause (always meaningful,
+    // just routed differently for a follower - see requestPause/Resume
+    // above), a *registered* nexttrack handler is what makes Windows show
+    // the control at all, so it has to track hasNextQueued/isHost the
+    // same way #nextBtn's own visibility does - see becomeHost/onItemChanged.
+    setMediaSessionActionHandlers({
+      onPlay: () => { if (isHost) activeDriver.play(); else requestResume(); },
+      onPause: () => { if (isHost) activeDriver.pause(); else requestPause(); },
+      onSeek: (seconds) => { if (isHost) seekTo(seconds); }
+    });
     hasNextQueued = !!party.hasNext;
     updateAmbientNotice();
     requestToolbar.classList.toggle('hide', party.youAreHost);
@@ -1245,7 +1281,7 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
     });
   }
 
-  nextBtn.addEventListener('click', () => { conn?.next(); });
+  nextBtn.addEventListener('click', goToNextItem);
   (root.querySelector('#leaveBtn') as HTMLButtonElement).addEventListener('click', () => navigate('/home'));
 
   init();
@@ -1260,6 +1296,7 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
     if (requestBannerTimer) clearTimeout(requestBannerTimer);
     clearControlsHideTimer();
     if (hls) hls.destroy();
+    clearMediaSession();
     document.removeEventListener('click', onDocumentClick);
     document.removeEventListener('fullscreenchange', onFullscreenChange);
     playerWrap.removeEventListener('pointermove', showControls);

docs/architecture.md

@@ -4330,6 +4330,91 @@ iOS-flavored copy with no install button. A same-event dispatch into a
 real ≥900px-wide `<iframe>` confirmed the banner never renders there
 at all, regardless of the event firing.
 
+## OS-level media controls (Windows' SMTC, macOS' Now Playing, ...)
+
+Asked for directly: linking whatever's playing to Windows' own media
+popup (the one media keys/the volume flyout show) - the standard
+Media Session API (`navigator.mediaSession`), which the same code path
+also surfaces to macOS' Now Playing widget and Android's notification
+for free, on whichever OS/browser combination actually implements it
+(Chromium family; Firefox has never implemented it at all).
+
+New `mediaSession.ts`, a thin wrapper - every export is a one-line
+`'mediaSession' in navigator` guard around the exact browser call, not
+a new abstraction of its own - shared by `views/player/Player.ts`
+(video) and `musicBar/MusicBar.ts` (a listen party), which can never
+both be active at once (starting/joining either already leaves the
+other first - see docs above), so there's no risk of the two fighting
+over this one global.
+
+**Metadata/position/playback state** hook into each player's own
+existing single-source-of-truth functions rather than adding parallel
+call sites everywhere: Player.ts's `updatePlayPauseBtn`/
+`updateProgressBar` (already the one place `#playPauseBtn`'s icon and
+the seek bar's fill/label get synced, from every driver-changed event,
+the heartbeat, and the ticker alike) also call
+`setMediaSessionPlaybackState`/`setMediaSessionPositionState` now;
+MusicBar.ts's `setPlayIcon`/`updatePositionDisplay` do the same. Title/
+artwork update at both places an item/track actually changes
+(Player.ts's `init`/`onItemChanged`, MusicBar.ts's `loadTrack`) -
+artist/album are left empty for video (meaningless there, same as this
+app's own UI already treats them).
+
+**Play/pause/seek** are wired once per party (not re-wired on state
+changes) via `setMediaSessionActionHandlers` - the handler functions
+themselves close over local state (`isHost`, `conn`, ...) and read it
+fresh on every OS-triggered call, the same way any other closure in
+either file already does, so a handler set at party-join time keeps
+working correctly straight through a live host handoff without needing
+to know that happened. Two real permission models, unchanged by this:
+video's play/pause is host-only - a non-host's OS-level play/pause
+routes to the exact same `requestPause`/`requestResume` (now factored
+out so both the in-page buttons and this can call them) the request
+toolbar already uses, never touching the video directly - and seeking
+is gated on `isHost` the same way the in-page seek bar already is
+(factored into a shared `seekTo`, reused by both the pointer handler
+and this). Music has neither gate (no host concept at all - see
+MusicQueueService's own reasoning), so its handlers act directly.
+
+**"Next"** is handled differently from play/pause/seek because a
+*registered* `nexttrack` handler is what makes the OS show that
+control at all, regardless of whether clicking it would currently do
+anything - so unlike the other three, its presence has to track
+whatever the in-page equivalent's own visibility already tracks, via a
+separate `setMediaSessionNextAction(fn | null)` called alongside it:
+Player.ts's `#nextBtn` (`isHost && hasNextQueued`, in `becomeHost` and
+`onItemChanged`) and MusicBar.ts's vote-skip button (`music.current`,
+in `applyMusicState` - clicking the OS's own "next" casts one skip
+vote, same as the in-page button, not an instant skip).
+
+**Clearing**: `clearMediaSession()` (metadata, playback state, *and*
+every action handler) runs on Player.ts's route cleanup and
+MusicBar.ts's `leaveMusicParty` - an OS-level "now playing" card that
+outlives the actual party would be actively wrong, not stale. A
+lighter `setMediaSessionIdle()` (metadata/state only, action handlers
+left alone) covers MusicBar.ts's own "queue ran empty but still in the
+party" case instead - play/pause/seek stay meaningful the instant
+something plays again, so there's nothing to re-wire there.
+
+Verified: full `typecheck`/`build` (client only). `mediaSession.ts`
+itself checked directly against the real API in a real Chromium
+browser (this sandbox's own): metadata resolves to a genuine
+`MediaMetadata` with the right fields and a resolved artwork URL (or
+none for an empty one), playback state and position state both apply
+without throwing, an invalid (non-positive/`NaN`) duration is silently
+skipped rather than reaching the browser call that would throw on it,
+action-handler registration doesn't throw with or without `onSeek`
+supplied, and both `setMediaSessionIdle`/`clearMediaSession` reset
+metadata to `null` and playback state to `'none'`. Player.ts's/
+MusicBar.ts's own wiring is one-line calls into already-verified
+functions at points each file already treats as its single source of
+truth for that piece of state (see above) - not independently
+exercised end to end against a live party (would need mocking the
+entire socket/party layer for no more real signal than reading the
+call sites already gives), and the actual OS-level popup itself - the
+whole point - isn't something this sandbox can display or click
+regardless. Worth a real Windows/Chrome check once deployed.
+
 ## Known limitations
 
 - Playback always goes through `hls.js` against whatever Emby's