useAgentState: re-arm the poll loop from refresh() instead of killing it

argus's review on the original fix: refresh() did a parallel one-off
fetch instead of calling the mount effect's self-rescheduling poll(),
so it never re-armed timerRef after firing. Wiring refresh() to run on
every visibilitychange-to-visible meant the very first tab-switch back
into focus would kill periodic polling for the rest of the session --
reproducing the exact staleness bug this branch set out to fix, just
delayed by one tab switch instead of immediate.

Fix: hoist poll into a ref set by the mount effect so refresh() invokes
the same self-rescheduling function rather than a parallel fetch that
drops the loop. Also gave the visibilitychange effect an empty
dependency array per the review's second note -- it only closes over
stable refs/setters, and now that Root's 1s ticker re-renders the
calling component every second, a deps-less effect would tear down and
reattach the listener that often for no reason.

frontend/packages/agent/src/hooks/useAgentState.ts

@@ -23,6 +23,15 @@ export function useAgentState(): UseAgentStateResult {
   const [error, setError] = useState<string | null>(null);
   const timerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
   const stoppedRef = useRef(false);
+  // Holds the mount effect's self-rescheduling `poll` so `refresh()` can
+  // invoke the *same* function instead of a parallel one-off fetch — a
+  // fresh call still runs through `poll`'s own `setTimeout(poll, …)` tail,
+  // so the loop stays alive afterwards instead of dying silently the
+  // first time something calls `refresh()` (see the review discussion on
+  // this file's PR: an earlier version of `refresh()` did its own
+  // one-off fetch with no reschedule, which permanently killed periodic
+  // polling the first time a caller invoked it).
+  const pollRef = useRef<() => void>(() => {});
 
   useEffect(() => {
     stoppedRef.current = false;
@@ -42,6 +51,7 @@ export function useAgentState(): UseAgentStateResult {
         timerRef.current = setTimeout(poll, RETRY_MS);
       }
     }
+    pollRef.current = poll;
     poll();
 
     return () => {
@@ -52,20 +62,8 @@ export function useAgentState(): UseAgentStateResult {
 
   function refresh() {
     if (timerRef.current) clearTimeout(timerRef.current);
-    // Re-trigger the same poll loop immediately; the effect's closure
-    // owns `poll`, so the simplest safe re-trigger from outside it is a
-    // fresh fetch here rather than reaching back into the effect.
     stoppedRef.current = false;
-    fetch('api/state')
-      .then((resp) => {
-        if (!resp.ok) throw new Error(`http ${resp.status}`);
-        return resp.json();
-      })
-      .then((s: AgentState) => {
-        setState(s);
-        setError(null);
-      })
-      .catch((err) => setError(err instanceof Error ? err.message : String(err)));
+    pollRef.current();
   }
 
   // A backgrounded/hidden tab has its `setTimeout` chain throttled by the
@@ -74,15 +72,19 @@ export function useAgentState(): UseAgentStateResult {
   // stalls, and everything derived from `state` (not just the fetch
   // itself) goes stale until it happens to fire again. Force a resync the
   // moment the tab becomes visible so switching back doesn't leave a
-  // reading that's minutes old. `refresh` closes over stable refs/setters
-  // only, so a fresh instance each render is safe to use here.
+  // reading that's minutes old. Empty deps: `refresh` only closes over
+  // refs and setState setters, both stable across renders, so the first
+  // render's closure stays correct for the component's whole lifetime —
+  // and, unlike re-subscribing on every render, doesn't re-attach this
+  // listener every second now that `Root`'s own 1s ticker re-renders the
+  // component that calls this hook.
   useEffect(() => {
     function onVisible() {
       if (document.visibilityState === 'visible') refresh();
     }
     document.addEventListener('visibilitychange', onVisible);
     return () => document.removeEventListener('visibilitychange', onVisible);
-  });
+  }, []);
 
   return { state, error, refresh };
 }