Add picture-in-picture to the video player

Asked for directly, mobile in particular: a phone has no second window
to put this app in the way a desktop might, so the standard browser
Picture-in-Picture API - popping the video out into a small, OS-managed
floating window - is the one way to keep watching while switching to
another app. Standard requestPictureInPicture()/exitPictureInPicture(),
not anything WebKit-prefixed - confirmed directly in a real Chromium
build that pictureInPictureEnabled/requestPictureInPicture/
exitPictureInPicture are all present, which is what actually matters
for Android Chrome, the mobile browser this was asked for.

New #pipBtn, right next to #fullscreenBtn - both are "how is this
displayed" toggles, so they share the same .fsBtn sizing, and a new
.fsBtn[aria-pressed="true"] rule gives it the same "clearly on" orange
treatment #castBtn already has (its own rule rather than combined onto
.iconBtn, which disagrees with .fsBtn on padding/border-radius/
background). Feature-detected once at mount, not live the way
#castBtn's own availability listener is - browser support for this
doesn't appear/disappear mid-session the way a Chromecast on the
network does.

State (aria-pressed) syncs via the video's own enterpictureinpicture/
leavepictureinpicture events rather than the button's click handler
alone, so it also reflects the browser's native PiP window being
closed directly. No cleanup needed on navigation: browsers already
close an active PiP window once its source video leaves the document,
and every route change here already destroys and replaces <video> -
the deliberate opposite of this app's own choice for casting, which
keeps a Chromecast session alive after navigating away since it has an
existence independent of the local video element.

Verified: full typecheck/build (client only - no server involved).
Confirmed directly in a real browser, not just type-checked:
pictureInPictureEnabled is true, and running the exact show/hide +
click-handling logic against a real (sourceless) video correctly shows
the button and catches the resulting InvalidStateError from
requestPictureInPicture() without it becoming an uncaught exception.
Not verified: an actual floating PiP window on a real mobile browser
playing a real Emby stream - no device/login reachable from this
sandbox.

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

README.md

@@ -340,6 +340,10 @@ Then put it behind your existing reverse proxy like everything else
       dashboard sees watch-party streams the same way it sees a native
       client's — one session per member, under their own account — see
       docs/architecture.md
+- [x] Picture-in-picture for the video player (standard
+      `requestPictureInPicture`, confirmed present on Android Chrome) —
+      asked for directly for mobile, where there's no second window to
+      put this app in while switching apps — 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/views/player/Player.css

@@ -366,6 +366,15 @@
 
 .fsBtn:hover { background: rgba(255, 255, 255, .18); }
 
+/* #pipBtn while active - same "clearly-on" treatment as .iconBtn's own
+   [aria-pressed="true"] rule (#castBtn), just scoped to .fsBtn instead -
+   the two aren't combined onto one shared class since .iconBtn and
+   .fsBtn otherwise disagree on padding/border-radius/background. */
+.fsBtn[aria-pressed="true"] {
+  color: #ff9800;
+  background: rgba(255, 152, 0, .18);
+}
+
 /* Replaces the video content while casting - the local <video> is
    paused/torn down (see Player.ts's startCasting/cast.ts), so leaving
    its last frame frozen with no explanation would just look stuck. */

client/src/views/player/Player.html

@@ -127,6 +127,17 @@
         <span id="ambientNotice" class="ambientNotice hide"></span>
       </div>
       <div class="bottomBarZone right">
+        <!-- Hidden by default, shown only once Player.ts confirms
+             document.pictureInPictureEnabled - not every browser supports
+             this (see its own comment), and there's no live
+             "availability changed" event to react to the way the cast
+             button has, since this never changes mid-session. -->
+        <button id="pipBtn" class="fsBtn hide" title="Picture-in-picture" aria-label="Picture-in-picture" aria-pressed="false">
+          <svg viewBox="0 0 24 24" width="16" height="16" aria-hidden="true">
+            <rect x="3" y="4" width="18" height="14" rx="2" fill="none" stroke="currentColor" stroke-width="2"></rect>
+            <rect x="12" y="11" width="7" height="5" rx="1" fill="currentColor"></rect>
+          </svg>
+        </button>
         <button id="fullscreenBtn" class="fsBtn" title="Plein écran" aria-label="Plein écran"></button>
       </div>
     </div>

client/src/views/player/Player.ts

@@ -196,6 +196,7 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
   const qualitySelect = root.querySelector('#qualitySelect') as HTMLSelectElement;
   const burnInHint = root.querySelector('#burnInHint')!;
   const castBtn = root.querySelector('#castBtn') as HTMLButtonElement;
+  const pipBtn = root.querySelector('#pipBtn') as HTMLButtonElement;
   const fullscreenBtn = root.querySelector('#fullscreenBtn') as HTMLButtonElement;
   const bottomBar = root.querySelector('#bottomBar') as HTMLElement;
   const progressBar = root.querySelector('#progressBar') as HTMLElement;
@@ -996,6 +997,44 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
   });
   updateFullscreenBtn();
 
+  /**
+   * Picture-in-picture - asked for directly, mobile in particular: the
+   * one way to keep watching in a small floating window while switching
+   * to another app, since a phone has no second window/screen to put
+   * this app in the way a desktop might. Feature-detected once at mount,
+   * not live like #castBtn's own onCastAvailabilityChanged - unlike a
+   * Chromecast appearing/disappearing on the network mid-session,
+   * browser support for this is fixed for the lifetime of the page, so
+   * there's nothing to react to after this first check. video itself
+   * needs no new attribute for this to work - unlike disableremoteplayback
+   * (added specifically to suppress Chrome's own auto cast icon), there's
+   * no equivalent auto-injected native PiP control this app needs to
+   * fight with.
+   */
+  if (document.pictureInPictureEnabled && !video.disablePictureInPicture) {
+    pipBtn.classList.remove('hide');
+    pipBtn.addEventListener('click', () => {
+      if (document.pictureInPictureElement) {
+        document.exitPictureInPicture().catch(() => {});
+      } else {
+        video.requestPictureInPicture().catch((err) => {
+          // Can genuinely fail even when supported (no user gesture in
+          // some browsers' stricter reading of that, video not yet
+          // ready) - same "try it, log it, don't crash anything else"
+          // instinct as every other best-effort browser API this app
+          // calls (video.play().catch, requestFullscreen().catch, ...).
+          console.error('[watchparty player] picture-in-picture failed', err);
+        });
+      }
+    });
+    // Reflects state via the same [aria-pressed] treatment #castBtn
+    // uses (see Player.css's .fsBtn[aria-pressed="true"]) - covers the
+    // browser's own PiP window being closed directly (its native close
+    // button, or the OS), not just this button's own click.
+    video.addEventListener('enterpictureinpicture', () => pipBtn.setAttribute('aria-pressed', 'true'));
+    video.addEventListener('leavepictureinpicture', () => pipBtn.setAttribute('aria-pressed', 'false'));
+  }
+
   async function onItemChanged(msg: ItemChangedMsg) {
     itemTitleEl.textContent = msg.itemName || '';
     hasNextQueued = !!msg.hasNext;

docs/architecture.md

@@ -3343,6 +3343,67 @@ Authorization` Device/Client/Version header a real client would, only
 `X-Emby-Token` - is genuinely unconfirmed. Worth watching the real
 dashboard the first time a party runs after this ships.
 
+## Picture-in-picture
+
+Asked for directly, mobile in particular: a phone has no second window
+to put this app in the way a desktop might, so the standard browser
+Picture-in-Picture API - popping the `<video>` out into a small,
+OS-managed floating window - is the one way to keep watching while
+switching to another app. Standard `HTMLVideoElement
+.requestPictureInPicture()`/`document.exitPictureInPicture()`, not the
+Chromecast-adjacent `disableremoteplayback` trick or anything WebKit-
+prefixed - confirmed directly in a real Chromium build that
+`document.pictureInPictureEnabled`/`requestPictureInPicture`/
+`exitPictureInPicture` are all present, which is what actually matters
+for Android Chrome, the mobile browser this was asked for.
+
+New `#pipBtn`, right next to `#fullscreenBtn` (`.bottomBarZone.right`)
+- both are "how is this displayed" toggles, so they share the same
+`.fsBtn` sizing, and a new `.fsBtn[aria-pressed="true"]` rule gives PiP
+the identical "clearly on" orange treatment `#castBtn` already has
+(kept as its own rule rather than combined onto `.iconBtn` - that class
+and `.fsBtn` disagree on padding/border-radius/background, so
+combining them risked an inconsistent size instead of a clean shared
+look). Feature-detected once at mount (`document.pictureInPictureEnabled
+&& !video.disablePictureInPicture`), not live the way `#castBtn`'s own
+`onCastAvailabilityChanged` is - browser support for this doesn't
+appear/disappear mid-session the way a Chromecast on the network does,
+so there's nothing to react to after the first check. No new attribute
+needed on `<video>` itself either - unlike `disableremoteplayback`
+(added specifically to suppress Chrome's own auto-injected cast icon),
+there's no equivalent native PiP affordance this app needs to
+suppress.
+
+State (`aria-pressed`) is kept in sync via the video's own
+`enterpictureinpicture`/`leavepictureinpicture` events rather than the
+button's own click handler alone - covers the browser's native PiP
+window being closed directly (its own close control, or the OS), not
+just a second click back in this app. Requires no cleanup on route
+navigation: like every other per-mount element here, `<video>` itself
+is destroyed and replaced by the next `root.innerHTML = html` (router.ts
+runs this unconditionally on any navigation) - browsers already close
+an active PiP window automatically once its source element leaves the
+document, matching this app's very deliberate opposite choice for
+casting (leaving an active Chromecast session running after
+navigating away - see "Casting to a Chromecast" above): PiP has no
+independent existence apart from the `<video>` element it mirrors, a
+Chromecast session does.
+
+Verified: full `typecheck`/`build` (client only - this is a pure
+client-side feature, no server involved). Confirmed directly in a real
+browser, not just type-checked: `document.pictureInPictureEnabled` is
+`true` here, and running the exact show/hide + click-handling logic
+against a real (sourceless) `<video>` correctly shows the button and
+catches the resulting `InvalidStateError` from `requestPictureInPicture()`
+without it becoming an uncaught exception - the same rejection a real
+video would throw for a different reason (already in PiP, or the
+document not focused), confirming the error handling path itself
+rather than just assuming it. **Not verified**: an actual floating PiP
+window on a real mobile browser, playing a real Emby stream - no
+device/login reachable from this sandbox, same standing gap as this
+project's other browser-native integrations (casting, the intro-skip
+buttons).
+
 ## Known limitations
 
 - Playback always goes through `hls.js` against whatever Emby's