Tighten worker shutdown and poll helper cleanup

template_helpers.py

@@ -37,15 +37,32 @@ BROWSER_API_HELPER = r"""
 BROWSER_POLL_HELPER = r"""
     function startSerialPoll(callback, intervalMs) {
       let inFlight = false;
-      return window.setInterval(async () => {
-        if (inFlight) return;
+      let active = true;
+      let timer = null;
+      const stop = () => {
+        active = false;
+        if (timer !== null) {
+          window.clearTimeout(timer);
+          timer = null;
+        }
+      };
+      const tick = async () => {
+        if (!active || inFlight) {
+          if (active) timer = window.setTimeout(tick, intervalMs);
+          return;
+        }
         inFlight = true;
         try {
           await callback();
         } finally {
           inFlight = false;
+          if (active) timer = window.setTimeout(tick, intervalMs);
         }
-      }, intervalMs);
+      };
+      window.addEventListener("pagehide", stop, { once: true });
+      window.addEventListener("beforeunload", stop, { once: true });
+      timer = window.setTimeout(tick, intervalMs);
+      return { stop };
     }
 """