host-display

A host display is a TV or laptop that joins a room as a spectator and shows the shared game. It never takes a seat. Always optional: a room with no display plays exactly as it does without one.

Player flows

• Phone first: host makes a room on the phone; on the TV choose "Join as the host display" and type the code (?room=CODE&role=display).
• TV first: the TV opens a room; the first phone to join becomes the host (as in Track Record and Chameleon). If the host leaves and nobody can take over, the next person in taps "Take control: be the host" (claimHost).

The contract (implemented and tested in room-server/server/base-room.ts)

1. A display joins with screen: true: no seat, never the host, a spectator for gameplay.
2. It gets the same masked view as any spectator (viewer = null). Never build a second "display view".
3. The first player to join a room is its host, TV-first rooms included; a screen or spectator never is. claimHost only works while nobody hosts. The host's Leave button sends {type:'leave'}: host (and the music, unless a TV plays it) passes to the next connected player.
4. Exactly one device plays music (you.music). A connected display takes it, the casting TV (hello.cast, set by the Chromecast receiver) outranks any other display, and the host's phone gets it back only if no display is connected. Use useMusicDevice(audio, snap.you.music) from the audio kit, and create the audio kit with tryAtStart: true so a TV needs no tap.
5. A display is not a player: not in the player list, not in "ready" counts, not in stats (GameRoomDO skips player_joined for screens), and its disconnect never marks anyone away.
6. A display can join mid-game.
7. A display never waits on a button. Whatever needs a tap on a phone must advance by itself on a screen, or be driven by the host's phone, or wait for the players' ready confirmations. "Show on TV" (cast kit) appears on the host's device and on the display.

Pieces

• Stage: lay the table out once at 1280x720 and scale it to the window. Without it a TV shows a tiny phone layout.
• DisplayShell: header with code, QR, Full screen (fullscreen needs a tap and does not exist in iPhone Safari; the button hides itself there).
• Lobby for a display: lobby-host's LobbyScreen already shows the big code and QR when you.screen.
• In-game view for a display: render your normal table component with a spectator prop (no hand, no inputs, every event animated, a turn banner instead of a hand).

Gotchas

• vite build empties dist; restart wrangler dev after each build or assets 404.
• A display opened first must not become host: BaseRoom only makes the first non-screen member host.
• Casting/AirPlay of an arbitrary browser tab cannot be triggered by a web page; a browser on the TV, or the cast kit (Google Cast receiver), is the portable answer.
• Retired: the Firebase displayUid driver is in kits/legacy/host-display-firestore/.