feat(extension): send nothing to the server while alone in a room

When no other peer is present, heartbeats, force-sync, and episode auto-sync
are now fully suppressed (previously the keepAlive heartbeat, force-sync, and
episode lobby were still broadcast to an empty room):
- keepAlive heartbeat gated on a live peer count
- force-sync is a no-op when solo (no pause-then-timeout freeze, no traffic)
- episode auto-sync skips the lobby entirely and just plays the next episode

The solo state is recomputed live from the current peer list on every event
(never cached), so the instant another peer joins syncing resumes. On that
solo->not-solo transition the background also requests an immediate heartbeat
from the content script so the newcomer sees the current position without
waiting for the next interval.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/background.js

@@ -1081,13 +1081,21 @@ function handleServerEvent(event, data) {
                 if (!Array.isArray(currentRoom.peers)) currentRoom.peers = [];
                 if (data.status === 'joined') {
                     if (!currentRoom.peers.find(p => (p.peerId || p) === data.peerId)) {
+                        const wasSolo = currentRoom.peers.filter(p => (p.peerId || p) !== peerId).length === 0;
                         delete lastSeqBySender[data.peerId];
                         _persistLastSeq();
-                        
+
                         currentRoom.peers.push(createPeerData(data));
                         if (storageInitialized) chrome.storage.session.set({ currentRoom });
                         chrome.runtime.sendMessage({ type: 'PEER_UPDATE', peers: currentRoom.peers }).catch(() => {});
 
+                        // We were alone and now we're not — proactively push our
+                        // current playback state so the newcomer syncs immediately
+                        // instead of waiting up to a full heartbeat interval.
+                        if (wasSolo && currentTabId) {
+                            chrome.tabs.sendMessage(currentTabId, { type: 'REQUEST_HEARTBEAT' }).catch(() => {});
+                        }
+
                         if (episodeLobby && episodeLobby.initiatorPeerId === peerId) {
                             emit(EVENTS.EPISODE_LOBBY, { peerId, expectedTitle: episodeLobby.expectedTitle });
                         }
@@ -1450,14 +1458,18 @@ chrome.alarms.onAlarm.addListener(async (alarm) => {
                 await leaveRoomAfterIdleGrace('Left room after 2 hours without a selected video heartbeat.');
                 return;
             }
-            // Heartbeat Logic: Always include identity metadata
-            const settings = await getSettings();
-            emit(EVENTS.PEER_STATUS, { 
-                peerId, 
-                status: 'heartbeat',
-                username: settings.username,
-                tabTitle: currentTabTitle
-            });
+            // Heartbeat — only broadcast when someone else is in the room.
+            // Recomputed live so a freshly joined peer is picked up immediately.
+            const otherCount = currentRoom && Array.isArray(currentRoom.peers) ? currentRoom.peers.filter(p => (typeof p === 'object' ? p.peerId : p) !== peerId).length : 0;
+            if (otherCount > 0) {
+                const settings = await getSettings();
+                emit(EVENTS.PEER_STATUS, {
+                    peerId,
+                    status: 'heartbeat',
+                    username: settings.username,
+                    tabTitle: currentTabTitle
+                });
+            }
         }
     }
 });
@@ -1706,6 +1718,19 @@ async function handleAsyncMessage(message, sender, sendResponse) {
         });
     } else if (message.type === 'CONTENT_EVENT') {
         const processEvent = () => {
+            // Live solo check — recomputed from the current peer list on every
+            // event (the list is updated synchronously on PEER_STATUS join/leave),
+            // never cached, so the instant a peer joins we resume sending.
+            const otherCount = currentRoom && Array.isArray(currentRoom.peers) ? currentRoom.peers.filter(p => (typeof p === 'object' ? p.peerId : p) !== peerId).length : 0;
+            const hasOtherPeers = otherCount > 0;
+
+            // Force Sync only makes sense with other peers. Solo it is a no-op:
+            // skip the pause/seek + ACK-wait entirely (no freeze, no server traffic).
+            if (message.action === EVENTS.FORCE_SYNC_PREPARE && !hasOtherPeers) {
+                sendResponse({ status: 'ok_solo' });
+                return;
+            }
+
             const timestamp = Date.now();
             localSeq++;
             chrome.storage.session.set({ localSeq });
@@ -1749,11 +1774,8 @@ async function handleAsyncMessage(message, sender, sendResponse) {
                 }, FORCE_SYNC_TIMEOUT);
             }
             addToHistory(message.action, 'You');
-            
+
             const isNonEssentialEvent = message.action === EVENTS.PLAY || message.action === EVENTS.PAUSE || message.action === EVENTS.SEEK;
-            const otherCount = currentRoom && Array.isArray(currentRoom.peers) ? currentRoom.peers.filter(p => (typeof p === 'object' ? p.peerId : p) !== peerId).length : 0;
-            const hasOtherPeers = otherCount > 0;
-            
             if (isNonEssentialEvent && !hasOtherPeers) {
                 sendResponse({ status: 'ok_solo' });
                 return;
@@ -1913,6 +1935,16 @@ async function handleAsyncMessage(message, sender, sendResponse) {
             return;
         }
 
+        // Variant A: alone in the room → no one to wait for. Skip the lobby
+        // entirely so the next episode just plays through (no pause, no traffic).
+        // Live peer check, so the moment someone joins the next transition syncs.
+        const otherCount = currentRoom && Array.isArray(currentRoom.peers) ? currentRoom.peers.filter(p => (typeof p === 'object' ? p.peerId : p) !== peerId).length : 0;
+        if (otherCount === 0) {
+            addLog(`Episode change ("${newTitle}") — alone in room, playing through without a lobby.`, 'info');
+            sendResponse({ status: 'solo_no_lobby' });
+            return;
+        }
+
         // If lobby already exists for this title, just mark self ready
         if (episodeLobby && sameEpisode(episodeLobby.expectedTitle, newTitle)) {
             if (!episodeLobby.readyPeers.includes(peerId)) {

extension/content.js

@@ -538,6 +538,14 @@
             return true;
         }
 
+        // Background asks for an immediate state push (e.g. the first peer just
+        // joined while we were solo) so the newcomer syncs without waiting.
+        if (message.type === 'REQUEST_HEARTBEAT') {
+            sendHeartbeat();
+            sendResponse({ ok: true });
+            return true;
+        }
+
         if (message.type === 'SERVER_COMMAND') {
             const { action, payload } = message;
             let actionCompleted = false;