fix(frontend): stop false Server Unreachable alerts while the tab is hidden

frontend/src/lib/serviceStatusStore.test.ts

@@ -82,6 +82,36 @@ describe("serviceStatusStore backend health", () => {
     });
   });
 
+  it("does not probe or accrue failures while the tab is hidden", async () => {
+    const fetchMock = vi.fn<typeof fetch>();
+    vi.stubGlobal("fetch", fetchMock);
+
+    Object.defineProperty(document, "visibilityState", {
+      configurable: true,
+      get: () => "hidden",
+    });
+
+    try {
+      const { useServiceStatusStore } = await import("./serviceStatusStore");
+
+      await useServiceStatusStore.getState().checkBackend();
+
+      // A backgrounded tab is throttled by the browser, so a timed-out probe
+      // would be a false negative. We must not fetch, and must leave the
+      // previously-known-good status untouched.
+      expect(fetchMock).not.toHaveBeenCalled();
+      expect(useServiceStatusStore.getState()).toMatchObject({
+        backend: true,
+        backendFailCount: 0,
+      });
+    } finally {
+      Object.defineProperty(document, "visibilityState", {
+        configurable: true,
+        get: () => "visible",
+      });
+    }
+  });
+
   it("preserves deployment warnings when falling back to the public health probe", async () => {
     const fetchMock = vi
       .fn<typeof fetch>()

frontend/src/lib/serviceStatusStore.ts

@@ -77,6 +77,22 @@ export const useServiceStatusStore = create<ServiceStatusState>((set, get) => {
     backendFailCount: 0,
 
     checkBackend: async () => {
+      // A hidden/backgrounded tab has its timers throttled and network
+      // requests deprioritised by the browser, so the health probe can time
+      // out even while the backend is perfectly healthy (active recording
+      // uploads keep succeeding throughout). Skip probing while hidden so we
+      // never accrue failures — and never raise a false "Server Unreachable"
+      // alert — during a call the user is watching in another window. The
+      // visibilitychange handler in ServiceStatusAlerts re-checks immediately
+      // on return to the foreground.
+      if (
+        typeof document !== "undefined" &&
+        document.visibilityState === "hidden"
+      ) {
+        scheduleNextBackend();
+        return;
+      }
+
       const apiBaseUrl = (process.env.NEXT_PUBLIC_API_URL || "/api").replace(
         /\/$/,
         "",