refactor(Hero): replace scroll listener with IntersectionObserver

- Replaced window scroll event listener with IntersectionObserver for better performance.
- Added a sentinel element to track scroll position relative to the top.
- Updated tests to mock IntersectionObserver and verify behavior without scroll events.

src/components/sections/Hero.test.tsx

@@ -1,4 +1,5 @@
-import { render, fireEvent, act, cleanup } from "@testing-library/react";
+// @vitest-environment jsdom
+import { render, act, cleanup } from "@testing-library/react";
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import { Hero } from "./Hero";
 import { MemoryRouter } from "react-router-dom";
@@ -30,13 +31,25 @@ vi.mock("../effects", () => ({
   GradientBlinds: () => <div data-testid="gradient-blinds" />,
 }));
 
-// Mock Link from react-router-dom to avoid needing Router context for just Link,
-// OR just use MemoryRouter in the render. MemoryRouter is safer.
+// IntersectionObserver Mock
+const mockObserve = vi.fn();
+const mockDisconnect = vi.fn();
+let intersectCallback: (entries: Partial<IntersectionObserverEntry>[]) => void = () => {};
+
+class MockIntersectionObserver {
+  constructor(callback: (entries: Partial<IntersectionObserverEntry>[]) => void) {
+    intersectCallback = callback;
+  }
+  observe = mockObserve;
+  disconnect = mockDisconnect;
+  unobserve = vi.fn();
+  takeRecords = vi.fn();
+}
+
+window.IntersectionObserver = MockIntersectionObserver as any;
 
 describe("Hero Component", () => {
   beforeEach(() => {
-    // Reset scroll position
-    window.scrollY = 0;
     vi.clearAllMocks();
     vi.useFakeTimers();
   });
@@ -57,7 +70,7 @@ describe("Hero Component", () => {
     expect(getByText("Scroll Down")).toBeDefined();
   });
 
-  it("hides scroll indicator when scrolled down", () => {
+  it("hides scroll indicator when scrolled down (not intersecting)", () => {
     const { queryByText } = render(
       <MemoryRouter>
         <Hero />
@@ -66,17 +79,15 @@ describe("Hero Component", () => {
 
     expect(queryByText("Scroll Down")).toBeDefined();
 
-    // Simulate scroll
+    // Simulate intersection change (scrolled down -> not intersecting)
     act(() => {
-      window.scrollY = 100;
-      window.dispatchEvent(new Event("scroll"));
-      vi.runAllTimers();
+      intersectCallback([{ isIntersecting: false }]);
     });
 
     expect(queryByText("Scroll Down")).toBeNull();
   });
 
-  it("shows scroll indicator again when scrolled back up", () => {
+  it("shows scroll indicator again when scrolled back up (intersecting)", () => {
     const { queryByText } = render(
       <MemoryRouter>
         <Hero />
@@ -85,51 +96,24 @@ describe("Hero Component", () => {
 
     // Initial scroll down
     act(() => {
-      window.scrollY = 100;
-      window.dispatchEvent(new Event("scroll"));
-      vi.runAllTimers();
+      intersectCallback([{ isIntersecting: false }]);
     });
     expect(queryByText("Scroll Down")).toBeNull();
 
     // Scroll back up
     act(() => {
-      window.scrollY = 0;
-      window.dispatchEvent(new Event("scroll"));
-      vi.runAllTimers();
+      intersectCallback([{ isIntersecting: true }]);
     });
     expect(queryByText("Scroll Down")).toBeDefined();
   });
 
-  it("throttles scroll events using requestAnimationFrame", () => {
-    const rafSpy = vi.spyOn(window, "requestAnimationFrame");
+  it("uses IntersectionObserver to track visibility", () => {
     render(
       <MemoryRouter>
         <Hero />
       </MemoryRouter>
     );
 
-    // Trigger multiple scroll events rapidly
-    act(() => {
-      window.scrollY = 10;
-      window.dispatchEvent(new Event("scroll"));
-      window.dispatchEvent(new Event("scroll"));
-      window.dispatchEvent(new Event("scroll"));
-    });
-
-    // Should call rAF only once before timers run (because of ticking flag)
-    expect(rafSpy).toHaveBeenCalledTimes(1);
-
-    // Run timers to reset ticking
-    act(() => {
-      vi.runAllTimers();
-    });
-
-    // Trigger again
-    act(() => {
-      window.dispatchEvent(new Event("scroll"));
-    });
-
-    // Should call rAF again
-    expect(rafSpy).toHaveBeenCalledTimes(2);
+    expect(mockObserve).toHaveBeenCalled();
   });
 });

src/components/sections/Hero.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from "react";
+import { useState, useEffect, useRef } from "react";
 import { Link } from "react-router-dom";
 import { motion } from "motion/react";
 import { useTranslation } from "../../i18n";
@@ -12,26 +12,21 @@ const GRADIENT_COLORS = ["#26a269", "#8ff0a4"];
 export function Hero() {
   const { t } = useTranslation();
   const [showScrollIndicator, setShowScrollIndicator] = useState(true);
+  const sentinelRef = useRef<HTMLDivElement>(null);
 
   useEffect(() => {
-    let ticking = false;
-    let rafId: number;
+    const observer = new IntersectionObserver(
+      ([entry]) => {
+        setShowScrollIndicator(entry.isIntersecting);
+      },
+      { threshold: 0 }
+    );
 
-    const handleScroll = () => {
-      if (!ticking) {
-        rafId = window.requestAnimationFrame(() => {
-          setShowScrollIndicator(window.scrollY < 50);
-          ticking = false;
-        });
-        ticking = true;
-      }
-    };
+    if (sentinelRef.current) {
+      observer.observe(sentinelRef.current);
+    }
 
-    window.addEventListener("scroll", handleScroll, { passive: true });
-    return () => {
-      window.removeEventListener("scroll", handleScroll);
-      if (rafId) window.cancelAnimationFrame(rafId);
-    };
+    return () => observer.disconnect();
   }, []);
 
   const { text } = useTypingEffect({
@@ -43,6 +38,17 @@ export function Hero() {
 
   return (
     <section className={styles.hero}>
+      <div
+        ref={sentinelRef}
+        style={{
+          position: "absolute",
+          top: 0,
+          height: "50px",
+          width: "1px",
+          pointerEvents: "none",
+          opacity: 0,
+        }}
+      />
       <div className={styles.backgroundContainer}>
         <GradientBlinds
           gradientColors={GRADIENT_COLORS}