perf: throttle scroll event listener in Navbar

Implemented `requestAnimationFrame` throttling for the scroll event listener in `Navbar` to reduce the frequency of state updates and logic execution.

- Wrapped scroll handler in `requestAnimationFrame`.
- Added performance test `src/components/layout/Navbar.test.tsx` verifying logic runs once per frame instead of per event.
- Verified functional correctness of scroll state updates.
- Fixed `useTypingEffect` test environment.

src/components/layout/Navbar.test.tsx

@@ -0,0 +1,91 @@
+// @vitest-environment jsdom
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { render, fireEvent, screen, cleanup, act } from '@testing-library/react';
+import { Navbar } from './Navbar';
+import styles from './Navbar.module.css';
+import { LanguageProvider } from '../../i18n';
+import { MemoryRouter } from 'react-router-dom';
+
+describe('Navbar Performance', () => {
+  let scrollYGetterSpy: ReturnType<typeof vi.fn>;
+
+  beforeEach(() => {
+    // Mock window.scrollY
+    scrollYGetterSpy = vi.fn(() => 0);
+    Object.defineProperty(window, 'scrollY', {
+      configurable: true,
+      get: scrollYGetterSpy,
+    });
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    cleanup();
+    vi.restoreAllMocks();
+    vi.useRealTimers();
+  });
+
+  it('throttles scroll events using requestAnimationFrame', () => {
+    render(
+      <MemoryRouter>
+        <LanguageProvider>
+          <Navbar />
+        </LanguageProvider>
+      </MemoryRouter>
+    );
+
+    const scrollEvents = 50;
+
+    // Simulate 50 scroll events synchronously
+    for (let i = 0; i < scrollEvents; i++) {
+      fireEvent.scroll(window);
+    }
+
+    // Because of throttling, the expensive logic (accessing scrollY)
+    // should not have run immediately for every event.
+    expect(scrollYGetterSpy).toHaveBeenCalledTimes(0);
+
+    // Now advance timers to let RAF callback run
+    vi.runAllTimers();
+
+    // It should have run ONCE (or very few times)
+    expect(scrollYGetterSpy).toHaveBeenCalledTimes(1);
+  });
+
+  it('correctly updates style when scrolled', () => {
+    render(
+      <MemoryRouter>
+        <LanguageProvider>
+          <Navbar />
+        </LanguageProvider>
+      </MemoryRouter>
+    );
+
+    const header = screen.getByRole('banner');
+
+    // Initial state check
+    expect(header.className).not.toContain(styles.scrolled);
+
+    // Simulate scroll > 20
+    scrollYGetterSpy.mockReturnValue(50);
+    fireEvent.scroll(window);
+
+    // Run timers wrapped in act to flush React updates
+    act(() => {
+      vi.runAllTimers();
+    });
+
+    // Now it should be scrolled
+    expect(header.className).toContain(styles.scrolled);
+
+    // Scroll back to top
+    scrollYGetterSpy.mockReturnValue(0);
+    fireEvent.scroll(window);
+
+    act(() => {
+      vi.runAllTimers();
+    });
+
+    expect(header.className).not.toContain(styles.scrolled);
+  });
+});

src/components/layout/Navbar.tsx

@@ -23,8 +23,15 @@ export function Navbar() {
   const activeIndex = navLinks.findIndex((link) => link.path === location.pathname);
 
   useEffect(() => {
+    let ticking = false;
     const handleScroll = () => {
+      if (!ticking) {
+        window.requestAnimationFrame(() => {
           setIsScrolled(window.scrollY > 20);
+          ticking = false;
+        });
+        ticking = true;
+      }
     };
     window.addEventListener('scroll', handleScroll, { passive: true });
     return () => window.removeEventListener('scroll', handleScroll);

src/hooks/useTypingEffect.test.ts

@@ -1,3 +1,4 @@
+// @vitest-environment jsdom
 import { renderHook, act } from '@testing-library/react';
 import { useTypingEffect } from './useTypingEffect';
 import { describe, it, expect, vi, afterEach, beforeEach } from 'vitest';