Implement visibility-aware polling to reduce background tab resource usage

- Add usePageVisibility hook to track page visibility state
- Add pauseWhenHidden option to usePolledData (defaults to false for backward compatibility)
- When enabled, polling pauses when page is hidden and resumes with immediate refresh when visible
- Refactor useBlocklistStatus to use usePolledData with pauseWhenHidden=true
- Add comprehensive tests for usePageVisibility hook
- Add polling lifecycle documentation to Web-Development.md

Fixes #36: Polling continues when tab is not visible

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/hooks/__tests__/usePageVisibility.test.ts

@@ -0,0 +1,110 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { renderHook, act } from "@testing-library/react";
+import { usePageVisibility } from "../usePageVisibility";
+
+describe("usePageVisibility", () => {
+  let visibilityChangeListeners: Array<(event: Event) => void> = [];
+
+  beforeEach(() => {
+    visibilityChangeListeners = [];
+
+    // Mock document.hidden
+    Object.defineProperty(document, "hidden", {
+      configurable: true,
+      get: vi.fn(() => false),
+    });
+
+    // Mock addEventListener
+    vi.spyOn(document, "addEventListener").mockImplementation((event, listener) => {
+      if (event === "visibilitychange") {
+        visibilityChangeListeners.push(listener as (event: Event) => void);
+      }
+    });
+
+    vi.spyOn(document, "removeEventListener");
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+    visibilityChangeListeners = [];
+  });
+
+  it("initializes with correct visibility state", () => {
+    const { result } = renderHook(() => usePageVisibility());
+    expect(result.current).toBe(true); // not hidden on init
+  });
+
+  it("returns false when document.hidden is true", () => {
+    (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(true);
+
+    const { result } = renderHook(() => usePageVisibility());
+    expect(result.current).toBe(false);
+  });
+
+  it("attaches visibilitychange listener on mount", () => {
+    renderHook(() => usePageVisibility());
+
+    expect(document.addEventListener).toHaveBeenCalledWith(
+      "visibilitychange",
+      expect.any(Function),
+    );
+  });
+
+  it("removes visibilitychange listener on unmount", () => {
+    const { unmount } = renderHook(() => usePageVisibility());
+
+    unmount();
+
+    expect(document.removeEventListener).toHaveBeenCalledWith("visibilitychange", expect.any(Function));
+  });
+
+  it("updates state when page becomes hidden", () => {
+    const { result } = renderHook(() => usePageVisibility());
+
+    expect(result.current).toBe(true);
+
+    // Simulate visibility change
+    act(() => {
+      (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(true);
+      visibilityChangeListeners.forEach((listener) => {
+        listener(new Event("visibilitychange"));
+      });
+    });
+
+    expect(result.current).toBe(false);
+  });
+
+  it("updates state when page becomes visible", () => {
+    (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(true);
+    const { result } = renderHook(() => usePageVisibility());
+    expect(result.current).toBe(false);
+
+    // Simulate visibility change
+    act(() => {
+      (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(false);
+      visibilityChangeListeners.forEach((listener) => {
+        listener(new Event("visibilitychange"));
+      });
+    });
+
+    expect(result.current).toBe(true);
+  });
+
+  it("handles multiple rapid visibility changes", () => {
+    const { result } = renderHook(() => usePageVisibility());
+
+    act(() => {
+      // Hidden
+      (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(true);
+      visibilityChangeListeners.forEach((listener) => listener(new Event("visibilitychange")));
+      // Visible
+      (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(false);
+      visibilityChangeListeners.forEach((listener) => listener(new Event("visibilitychange")));
+      // Hidden again
+      (Object.getOwnPropertyDescriptor(document, "hidden")?.get as any).mockReturnValue(true);
+      visibilityChangeListeners.forEach((listener) => listener(new Event("visibilitychange")));
+    });
+
+    expect(result.current).toBe(false);
+  });
+});

frontend/src/hooks/useBlocklistStatus.ts

@@ -2,7 +2,7 @@
  * React hook for polling blocklist schedule error state.
  */
 
-import { useEffect, useRef, useState } from "react";
+import { usePolledData } from "./usePolledData";
 import { fetchSchedule } from "../api/blocklist";
 
 const BLOCKLIST_POLL_INTERVAL_MS = 60_000;
@@ -14,36 +14,17 @@ export interface UseBlocklistStatusReturn {
 /**
  * Poll `GET /api/blocklists/schedule` every 60 seconds to detect whether
  * the most recent blocklist import had errors.
+ *
+ * Polling pauses when the page is hidden and resumes immediately when visible.
  */
 export function useBlocklistStatus(): UseBlocklistStatusReturn {
-  const [hasErrors, setHasErrors] = useState(false);
-  const abortRef = useRef<AbortController | null>(null);
+  const { data } = usePolledData({
+    fetcher: (signal) => fetchSchedule(signal),
+    selector: (response) => response.last_run_errors === true,
+    errorMessage: "Failed to fetch blocklist schedule",
+    pollInterval: BLOCKLIST_POLL_INTERVAL_MS,
+    pauseWhenHidden: true,
+  });
 
-  useEffect(() => {
-    const poll = (): void => {
-      abortRef.current?.abort();
-      const controller = new AbortController();
-      abortRef.current = controller;
-
-      fetchSchedule(controller.signal)
-        .then((info) => {
-          if (controller.signal.aborted) {
-            return;
-          }
-          setHasErrors(info.last_run_errors === true);
-        })
-        .catch(() => {
-          // Silently swallow network errors — do not change indicator state.
-        });
-    };
-
-    poll();
-    const id = window.setInterval(poll, BLOCKLIST_POLL_INTERVAL_MS);
-    return (): void => {
-      abortRef.current?.abort();
-      window.clearInterval(id);
-    };
-  }, []);
-
-  return { hasErrors };
+  return { hasErrors: data ?? false };
 }

frontend/src/hooks/usePageVisibility.ts

@@ -0,0 +1,32 @@
+/**
+ * Hook that tracks whether the current page is visible to the user.
+ *
+ * Returns true when the page is active and visible, false when the user has switched
+ * to another tab, window, or minimized the browser.
+ *
+ * Uses the Document.hidden API with the `visibilitychange` event.
+ * See: https://developer.mozilla.org/en-US/docs/Web/API/Document/visibilitychange_event
+ */
+import { useEffect, useState } from "react";
+
+/**
+ * Track whether the page is currently visible to the user.
+ *
+ * @returns True if the page is visible, false if hidden
+ */
+export function usePageVisibility(): boolean {
+  const [isVisible, setIsVisible] = useState(!document.hidden);
+
+  useEffect(() => {
+    const handleVisibilityChange = (): void => {
+      setIsVisible(!document.hidden);
+    };
+
+    document.addEventListener("visibilitychange", handleVisibilityChange);
+    return (): void => {
+      document.removeEventListener("visibilitychange", handleVisibilityChange);
+    };
+  }, []);
+
+  return isVisible;
+}

frontend/src/hooks/usePolledData.ts

@@ -6,6 +6,7 @@
  */
 import { useEffect, useRef } from "react";
 import { useFetchData } from "./useFetchData";
+import { usePageVisibility } from "./usePageVisibility";
 import type { FetchError } from "../types/api";
 
 export interface UsePolledDataOptions<TResponse, TData> {
@@ -23,6 +24,12 @@ export interface UsePolledDataOptions<TResponse, TData> {
   pollInterval?: number;
   /** If true, automatically refetch when browser window regains focus. Defaults to true. */
   refetchOnWindowFocus?: boolean;
+  /**
+   * If true, pause polling when the page is hidden (user switched to another tab).
+   * Polling resumes immediately when the page becomes visible, with an immediate refresh.
+   * Defaults to false for backward compatibility.
+   */
+  pauseWhenHidden?: boolean;
 }
 
 export interface UsePolledDataResult<TData> {
@@ -47,6 +54,9 @@ export interface UsePolledDataResult<TData> {
  * - `"network_error"`: Network, DNS, or JSON parse failure
  * - `"abort_error"`: Request was cancelled (typically silently ignored by hook)
  *
+ * When `pauseWhenHidden` is enabled, polling pauses when the page becomes hidden
+ * and resumes immediately with a fresh fetch when the page becomes visible again.
+ *
  * @param options - Configuration options
  * @returns Data, loading state, typed error, and refresh callback
  */
@@ -61,6 +71,7 @@ export function usePolledData<TResponse, TData>(
     initialData,
     pollInterval,
     refetchOnWindowFocus = true,
+    pauseWhenHidden = false,
   } = options;
 
   const { data, loading, error, refresh } = useFetchData({
@@ -72,25 +83,61 @@ export function usePolledData<TResponse, TData>(
   });
 
   const refreshRef = useRef(refresh);
+  const intervalIdRef = useRef<number | null>(null);
+  const previousVisibilityRef = useRef<boolean | null>(null);
+  const isVisible = usePageVisibility();
 
   useEffect(() => {
     refreshRef.current = refresh;
   }, [refresh]);
 
-  // Polling effect: set up interval if pollInterval is provided
+  // Polling effect: set up interval if pollInterval is provided and page is visible
   useEffect(() => {
     if (!pollInterval) {
       return;
     }
 
-    const id = setInterval((): void => {
+    // If pauseWhenHidden is enabled and page is hidden, clear any existing interval
+    if (pauseWhenHidden && !isVisible) {
+      if (intervalIdRef.current !== null) {
+        clearInterval(intervalIdRef.current);
+        intervalIdRef.current = null;
+      }
+      return;
+    }
+
+    const id = window.setInterval((): void => {
       refreshRef.current();
     }, pollInterval);
 
+    intervalIdRef.current = id;
+
     return (): void => {
-      clearInterval(id);
+      if (intervalIdRef.current === id) {
+        clearInterval(id);
+        intervalIdRef.current = null;
+      }
     };
-  }, [pollInterval]);
+  }, [pollInterval, pauseWhenHidden, isVisible]);
+
+  // Visibility effect: handle pause/resume when page visibility changes
+  useEffect(() => {
+    if (!pauseWhenHidden || !pollInterval) {
+      return;
+    }
+
+    // Only refresh if this is a transition from hidden to visible, not on mount
+    if (previousVisibilityRef.current === false && isVisible) {
+      // Page became visible: clear pending interval and refresh immediately
+      if (intervalIdRef.current !== null) {
+        clearInterval(intervalIdRef.current);
+        intervalIdRef.current = null;
+      }
+      refreshRef.current();
+    }
+
+    previousVisibilityRef.current = isVisible;
+  }, [isVisible, pauseWhenHidden, pollInterval]);
 
   // Window focus: optional refetch on regain focus
   useEffect(() => {