Shell: the ＋ picker opens at once, and tiles share their streams

The Add-tile menu would stop appearing once enough tiles were on the
dashboard — a different count each time. The button's handler awaited
GET /api/modules before revealing the menu, and that request could never
complete: browsers allow about six HTTP/1.1 connections per host, an open
EventSource holds one for as long as it lives, and every stream-fed tile
(ProPresenter, Timers, PCO Plan, ProdCom) opened its own. With the shell's
own /api/events that is the whole allowance at five or six such tiles, and
every request after that — the picker's module list, a new tile's /state,
its stream — sat in the browser's queue for a connection that was never
handed back. The count varied because a stream in its 3-second retry loop
frees a connection for a moment, and a clock tile holds none.

Reproduced with six stream-fed tiles: the sixth stream and the picker's
/api/modules both pend forever, and the menu stays hidden after the click.

Three changes, all in the shell:

- Dropdowns open the moment they are clicked, from what the page already
  knows, and slot in what their rebuild fetches when it lands. A rebuild
  that fails leaves the previous contents rather than an invisible menu,
  and a generation token stops a late fetch from repainting a menu that has
  since been closed or reopened (the old await also let two quick clicks
  cancel each other out). The ＋ picker repaints only when the module list
  actually changed; the layout menu shows Save and Reset at once and
  inserts the server's layouts above Reset when they arrive.
- Tiles asking for the same stream share one connection. moduleApi.sse()
  now subscribes to a shell-wide pool with one EventSource per URL, fanning
  events out to every subscriber and reconnecting on the same 3-second
  timer while anyone listens; a tile joining an open stream is told `open`
  straight away, so its backfill runs as before. Six tiles of three modules
  now hold three connections instead of six. Modules see no difference.
- /api/modules and /api/layouts fetches give up after 8 s instead of
  pending forever, so nothing awaiting them can hang.

Verified in the browser pane against a scratch server: the same six-tile
layout now opens the picker instantly with all five modules listed and
/api/modules completes; with the pool deliberately starved (three extra
EventSources) the picker still opens from its cached list.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/shell.js

@@ -71,12 +71,34 @@ function promptModal({ title, placeholder = '', value = '', okLabel = 'Save' })
 
 /* ── dropdown menus ─────────────────────────────────────────────────── */
 
+/* A menu opens the moment its button is clicked, drawn from what the page
+   already knows; whatever its rebuild has to fetch is slotted in when it
+   lands. It used to open only once the rebuild had finished, which made the
+   ＋ picker hostage to a network round trip — one that never completes when
+   the tiles' streams hold every connection the browser allows (see the
+   shared-stream pool below). rebuild(menu, stillOpen) may be async;
+   stillOpen() says whether the open it was started for is still current,
+   so a late fetch can't repaint a menu that has since closed or reopened. */
 function wireDropdown(btnId, menuId, rebuild) {
   const btn = document.getElementById(btnId);
   const menu = document.getElementById(menuId);
-  btn.addEventListener('click', async () => {
+  let generation = 0;
-    if (menu.hidden) await rebuild(menu);
+  btn.addEventListener('click', () => {
-    menu.hidden = !menu.hidden;
+    if (!menu.hidden) {
+      menu.hidden = true;
+      return;
+    }
+    const gen = ++generation;
+    const stillOpen = () => gen === generation && !menu.hidden;
+    menu.hidden = false;
+    // Whatever the rebuild does, the menu is on screen; a failure (now or
+    // later) leaves its previous contents rather than an invisible menu.
+    const failed = (err) => console.error(`[shell] ${menuId} could not be rebuilt:`, err);
+    try {
+      Promise.resolve(rebuild(menu, stillOpen)).catch(failed);
+    } catch (err) {
+      failed(err);
+    }
   });
   document.addEventListener('click', (e) => {
     if (!menu.hidden && !e.target.closest('#' + btnId) && !e.target.closest('#' + menuId)) {
@@ -106,11 +128,22 @@ function menuItem(label, onClick, { sub = '', danger = false } = {}) {
 
 let registry = new Map(); // module id -> manifest from GET /api/modules
 
+/** fetch that gives up after `ms`. A request the browser has queued behind
+    held connections would otherwise pend forever, and so would anything
+    awaiting it. */
+function fetchWithTimeout(url, ms, opts = {}) {
+  const ctl = new AbortController();
+  const timer = setTimeout(() => ctl.abort(), ms);
+  return fetch(url, { ...opts, signal: ctl.signal }).finally(() => clearTimeout(timer));
+}
+
 /** Refresh the enabled-module list. On failure the last known list is kept
     (the server may just be restarting mid-service). Returns true on success. */
 async function loadRegistry() {
   try {
-    const res = await fetch('/api/modules');
+    // Slow modules' tiles() are cut off server-side at 2 s, so a healthy
+    // answer is well inside this.
+    const res = await fetchWithTimeout('/api/modules', 8000);
     if (!res.ok) throw new Error('modules ' + res.status);
     const body = await res.json();
     registry = new Map((body.modules || []).map((m) => [m.id, m]));
@@ -753,6 +786,84 @@ function tileMessage(entry, text) {
   entry.body.appendChild(msg);
 }
 
+/* ── shared event streams ───────────────────────────────────────────── */
+
+/* Browsers allow about six HTTP/1.1 connections to one host, and an open
+   EventSource holds one of them for as long as it lives. One stream per
+   tile plus the shell's own /api/events reaches that ceiling at five or six
+   tiles, and from then on every further request — the ＋ picker's module
+   list, a new tile's /state, its stream — sits in the browser's queue
+   waiting for a connection that is never handed back. So tiles that ask
+   for the same stream share one connection: the pool keeps one EventSource
+   per URL, fans its events out to every subscriber, and reconnects it (on
+   the same 3-second timer the per-tile version used) while anyone is still
+   listening. A tile joining an already-open stream is told `open` straight
+   away, so its backfill runs exactly as if the connection were its own. */
+
+const ssePool = new Map(); // url -> shared source
+
+function sseSubscribe(url, handlers = {}) {
+  let shared = ssePool.get(url);
+  if (!shared) {
+    shared = { subs: new Set(), es: null, retryTimer: null, names: new Set() };
+    const fanOut = (kind, e) => {
+      for (const sub of [...shared.subs]) {
+        const fn = kind === 'event' ? sub.handlers.events?.[e.type] : sub.handlers[kind];
+        try { fn?.(e); } catch (err) { console.error('[shell] stream handler failed:', err); }
+      }
+    };
+    shared.listen = (name) => shared.es.addEventListener(name, (e) => fanOut('event', e));
+    shared.connect = () => {
+      clearTimeout(shared.retryTimer);
+      shared.retryTimer = null;
+      try { shared.es?.close(); } catch { /* already closed */ }
+      const es = new EventSource(url);
+      shared.es = es;
+      es.onopen = (e) => fanOut('open', e);
+      es.onmessage = (e) => fanOut('message', e);
+      es.onerror = (e) => {
+        fanOut('error', e);
+        // EventSource retries transient drops itself but gives up for good on
+        // a completed non-SSE response (a 502 while the upstream is down).
+        if (es.readyState === EventSource.CLOSED && shared.subs.size) {
+          clearTimeout(shared.retryTimer);
+          shared.retryTimer = setTimeout(shared.connect, 3000);
+        }
+      };
+      for (const name of shared.names) shared.listen(name);
+    };
+    ssePool.set(url, shared);
+  }
+
+  const sub = { handlers };
+  shared.subs.add(sub);
+  for (const name of Object.keys(handlers.events || {})) {
+    if (shared.names.has(name)) continue;
+    shared.names.add(name);
+    if (shared.es) shared.listen(name);
+  }
+  if (!shared.es || shared.es.readyState === EventSource.CLOSED) {
+    // first subscriber, or the stream had given up while nobody listened
+    shared.connect();
+  } else if (shared.es.readyState === EventSource.OPEN) {
+    // Joining mid-flight: report the open the way a fresh connection would
+    // have, a moment later, so the caller's own setup finishes first.
+    setTimeout(() => {
+      if (!shared.subs.has(sub)) return;
+      try { handlers.open?.(new Event('open')); } catch (err) { console.error('[shell] stream handler failed:', err); }
+    }, 0);
+  }
+
+  return {
+    close() {
+      if (!shared.subs.delete(sub) || shared.subs.size) return;
+      clearTimeout(shared.retryTimer);
+      try { shared.es?.close(); } catch { /* already closed */ }
+      ssePool.delete(url);
+    },
+  };
+}
+
 /**
  * The API handed to each module instance — the whole surface a module may
  * touch outside its root element. See docs/MODULE-GUIDE.md.
@@ -798,40 +909,17 @@ function buildModuleApi(tile, man, entry) {
     },
 
     /**
-     * EventSource scoped the same way, with auto-reconnect: EventSource
+     * EventSource scoped the same way, with auto-reconnect: a stream that
-     * retries transient drops itself but gives up for good when a retry gets
+     * gives up (a 502 while the upstream is down) is recreated on a timer
-     * a completed non-SSE response (a 502 while the upstream is down), so a
+     * until it works again. Tiles asking for the same stream share one
-     * closed stream is recreated on a timer until it works again.
+     * connection — see the pool above; a module sees no difference.
      * handlers: { open(e), error(e), message(e), events: { name: fn } }
      */
     sse(subPath, handlers = {}) {
-      const url = '/api/modules/' + man.id + subPath;
+      const shared = sseSubscribe('/api/modules/' + man.id + subPath, handlers);
-      let es = null;
-      let retryTimer = null;
-      let closed = false;
-      const connect = () => {
-        if (closed) return;
-        try { es?.close(); } catch { /* already closed */ }
-        es = new EventSource(url);
-        if (handlers.open) es.onopen = handlers.open;
-        if (handlers.message) es.onmessage = handlers.message;
-        for (const [name, fn] of Object.entries(handlers.events || {})) {
-          es.addEventListener(name, fn);
-        }
-        es.onerror = (e) => {
-          try { handlers.error?.(e); } catch { /* module's problem */ }
-          if (es.readyState === EventSource.CLOSED) {
-            clearTimeout(retryTimer);
-            retryTimer = setTimeout(connect, 3000);
-          }
-        };
-      };
-      connect();
       const handle = {
         close() {
-          closed = true;
+          shared.close();
-          clearTimeout(retryTimer);
-          try { es?.close(); } catch { /* already closed */ }
           sseHandles.delete(handle);
         },
       };
@@ -1182,8 +1270,21 @@ function renderLayout(list) {
 
 /* ── menus ──────────────────────────────────────────────────────────── */
 
-wireDropdown('add-btn', 'add-menu', async (menu) => {
+/* What the ＋ picker shows, as a string: repainting a menu the user is
-  await loadRegistry();
+   already pointing at is only worth it when something in it changed. */
+function addMenuSignature() {
+  return JSON.stringify([...registry.values()].map((m) => [m.id, m.name, m.description, m.tiles]));
+}
+
+wireDropdown('add-btn', 'add-menu', async (menu, stillOpen) => {
+  // Painted from the list already in hand, then refreshed: a module's tile
+  // list is live data, and Admin may have enabled or disabled something.
+  const shown = addMenuSignature();
+  renderAddMenu(menu);
+  if (await loadRegistry() && stillOpen() && addMenuSignature() !== shown) renderAddMenu(menu);
+});
+
+function renderAddMenu(menu) {
   menu.innerHTML = '';
   const manifests = [...registry.values()];
   if (!manifests.length) {
@@ -1215,9 +1316,9 @@ wireDropdown('add-btn', 'add-menu', async (menu) => {
       }, { sub: man.description || '' }));
     }
   }
-});
+}
 
-wireDropdown('layout-btn', 'layout-menu', async (menu) => {
+wireDropdown('layout-btn', 'layout-menu', async (menu, stillOpen) => {
   menu.innerHTML = '';
 
   menu.appendChild(menuItem('Save as named layout…', async () => {
@@ -1250,19 +1351,29 @@ wireDropdown('layout-btn', 'layout-menu', async (menu) => {
   divider.className = 'menu-divider';
   menu.appendChild(divider);
 
-  // named layouts stored on the server
+  const reset = menuItem('Reset layout', () => {
+    menu.hidden = true;
+    renderLayout([]);
+    saveLayout();
+    toast('Layout cleared');
+  }, { danger: true, sub: 'Remove every tile from this browser' });
+  menu.appendChild(reset);
+
+  // Named layouts stored on the server slot in above Reset once they
+  // arrive; until then (or if they never do) the menu is already usable.
   let layouts = [];
   try {
-    const res = await fetch('/api/layouts');
+    const res = await fetchWithTimeout('/api/layouts', 8000);
     if (res.ok) layouts = (await res.json()).layouts || [];
   } catch { /* server briefly away — the menu just shows none */ }
-  if (layouts.length) {
+  if (!layouts.length || !stillOpen()) return;
+  const block = document.createDocumentFragment();
   const title = document.createElement('div');
   title.className = 'menu-title';
   title.textContent = 'Load from server';
-    menu.appendChild(title);
+  block.appendChild(title);
   for (const l of layouts) {
-      menu.appendChild(menuItem(l.name, async () => {
+    block.appendChild(menuItem(l.name, async () => {
       menu.hidden = true;
       try {
         const res = await fetch('/api/layouts/' + encodeURIComponent(l.name));
@@ -1278,15 +1389,8 @@ wireDropdown('layout-btn', 'layout-menu', async (menu) => {
       }
     }, { sub: `${l.tiles} tile${l.tiles === 1 ? '' : 's'}` }));
   }
-    menu.appendChild(divider.cloneNode());
+  block.appendChild(divider.cloneNode());
-  }
+  menu.insertBefore(block, reset);
-
-  menu.appendChild(menuItem('Reset layout', () => {
-    menu.hidden = true;
-    renderLayout([]);
-    saveLayout();
-    toast('Layout cleared');
-  }, { danger: true, sub: 'Remove every tile from this browser' }));
 });
 
 /* ── menu bar show/hide (notch, like the tile title bars) ───────────── */