fix: fetch and activate the new service worker on Refresh UI when none is waiting yet

frontend/sw-update.js

@@ -36,8 +36,20 @@
    * @returns {Promise<void>}
    */
   async function applyUpdate({ reg, container, reload, setTimeout: setTimeoutFn }) {
-    const waiting = reg && reg.waiting;
     const scheduleTimeout = setTimeoutFn || (typeof setTimeout !== 'undefined' ? setTimeout : null);
+    let waiting = reg && reg.waiting;
+
+    if (!waiting && reg && typeof reg.update === 'function') {
+      // The banner can be triggered by the server buildTag poll before the
+      // browser has fetched the new sw.js at all. With no waiting worker, a
+      // bare reload would be served the OLD cache-first shell, the new SW
+      // would then install in the background, and the banner would reappear
+      // — the "update available keeps showing" loop. Fetch the update now
+      // and wait (bounded) for it to reach `installed` so a single click
+      // activates the new version.
+      try { await reg.update(); } catch { /* offline / fetch failed — fall through */ }
+      waiting = reg.waiting || (await waitForInstalled(reg, scheduleTimeout, 8000));
+    }
 
     if (!waiting) {
       // Nothing to activate (e.g. banner was shown from a broadcast message
@@ -60,6 +72,25 @@
     waiting.postMessage({ type: 'SKIP_WAITING' });
   }
 
+  /**
+   * Waits for reg.installing to reach the `installed` state (at which point
+   * it becomes reg.waiting), bounded by a timeout. Resolves with the waiting
+   * worker or null.
+   */
+  function waitForInstalled(reg, scheduleTimeout, ms) {
+    return new Promise((resolve) => {
+      const sw = reg.installing;
+      if (!sw || typeof sw.addEventListener !== 'function') { resolve(null); return; }
+      let settled = false;
+      const settle = (v) => { if (!settled) { settled = true; resolve(v); } };
+      sw.addEventListener('statechange', () => {
+        if (sw.state === 'installed') settle(reg.waiting || sw);
+        else if (sw.state === 'redundant') settle(null);
+      });
+      if (scheduleTimeout) scheduleTimeout(() => settle(reg.waiting || null), ms);
+    });
+  }
+
   const SwUpdate = { applyUpdate };
 
   if (typeof module !== 'undefined' && module.exports) {