02Shadow appears on scroll

<main class="stage">
  <div class="sentinel"></div>
  <header class="head is-demo">
    <span class="head__logo">GODRICH</span>
    <span class="head__shadow" aria-hidden="true"></span>
  </header>
  <div class="page">
    <p class="page__line"></p>
    <p class="page__line"></p>
    <p class="page__line short"></p>
  </div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-quick;                             // 그림자가 번지는 시간
$easing: $ease-out;
$color: $stage-paper;                              // 헤더 배경

// ── 데모 무대 ──────────────────────────────────────────
html, body { margin: 0; overflow: hidden; }
.stage {
  min-height: 100vh;
  margin: 0;
  background: $stage-yellow;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}
.sentinel { position: absolute; top: 0; height: 1px; width: 1px; }

.head {
  position: sticky;
  top: 0;
  height: 56px;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
  background: $color;

  &__logo { color: $stage-ink; font-weight: 800; font-size: 18px; line-height: 1; font-family: inherit; letter-spacing: .04em; }

  // 그림자는 요소를 새로 그리지 않고 미리 그려둔 그라데이션 레이어의 opacity 만 켠다 (범위·대비를 키워 눈에 띄게)
  &__shadow {
    position: absolute;
    left: 0; right: 0; bottom: -48px;
    height: 48px;
    background: linear-gradient(rgba($stage-ink, .85), rgba($stage-ink, 0));
    opacity: 0;
    transition: opacity $duration $easing;
  }

  // 실제 사용: 센티넬이 사라지면 JS 가 .is-scrolled 를 붙인다
  &.is-scrolled &__shadow { opacity: 1; }
}

// ── 데모(.is-demo): 그림자가 켜졌다 꺼졌다 2초 루프 ────────
.head.is-demo .head__shadow {
  animation: shadow-loop $dur-loop $easing infinite;
}
@keyframes shadow-loop {
  0%, 5%   { opacity: 0; }
  40%, 60% { opacity: 1; }
  95%, 100% { opacity: 0; }
}

// ── 배경: 문서처럼 보이는 자리 ─────────────────────────
.page { padding: 24px 24px 0; }
.page__line {
  height: 12px;
  margin: 0 0 14px;
  border-radius: 4px;
  background: rgba($stage-ink, .12);
  &.short { width: 60%; }
}

@media (prefers-reduced-motion: reduce) {
  .head__shadow { transition: none; }
  .head.is-demo .head__shadow { animation: none !important; opacity: 1; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  min-height: 100vh;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.sentinel {
  position: absolute;
  top: 0;
  height: 1px;
  width: 1px;
}

.head {
  position: sticky;
  top: 0;
  height: 56px;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
  background: #ffffff;
}
.head__logo {
  color: #17141a;
  font-weight: 800;
  font-size: 18px;
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.04em;
}
.head__shadow {
  position: absolute;
  left: 0;
  right: 0;
  bottom: -48px;
  height: 48px;
  background: linear-gradient(rgba(23, 20, 26, 0.85), rgba(23, 20, 26, 0));
  opacity: 0;
  transition: opacity 220ms ease-out;
}
.head.is-scrolled .head__shadow {
  opacity: 1;
}

.head.is-demo .head__shadow {
  animation: shadow-loop 2s ease-out infinite;
}

@keyframes shadow-loop {
  0%, 5% {
    opacity: 0;
  }
  40%, 60% {
    opacity: 1;
  }
  95%, 100% {
    opacity: 0;
  }
}
.page {
  padding: 24px 24px 0;
}

.page__line {
  height: 12px;
  margin: 0 0 14px;
  border-radius: 4px;
  background: rgba(23, 20, 26, 0.12);
}
.page__line.short {
  width: 60%;
}

@media (prefers-reduced-motion: reduce) {
  .head__shadow {
    transition: none;
  }
  .head.is-demo .head__shadow {
    animation: none !important;
    opacity: 1;
  }
}

// 실제 페이지: 맨 위 1px 센티넬이 화면을 벗어나는 순간 그림자를 켠다 (IntersectionObserver, 6줄)
var head = document.querySelector('.head');
new IntersectionObserver(function (es) {
  head.classList.toggle('is-scrolled', !es[0].isIntersecting);
}).observe(document.querySelector('.sentinel'));

import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./ShadowOnScrollHeader.module.scss";

export type ShadowOnScrollHeaderProps = {
  logo: string;
  /** 그림자가 번지는 시간 (기본 220ms) */
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };

export default function ShadowOnScrollHeader({
  logo,
  duration = 220,
  easing = "ease-out",
  color = "#fff",
}: ShadowOnScrollHeaderProps) {
  const head = useRef<HTMLElement>(null);
  const sentinel = useRef<HTMLDivElement>(null);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  useEffect(() => {
    if (!sentinel.current) return;
    const io = new IntersectionObserver(([e]) => {
      head.current?.classList.toggle("is-scrolled", !e.isIntersecting);
    });
    io.observe(sentinel.current);
    return () => io.disconnect();
  }, []);

  return (
    <>
      <div ref={sentinel} className={styles.sentinel} />
      <header ref={head} className={styles.head} style={vars}>
        <span className={styles.logo}>{logo}</span>
        <span className={styles.shadow} aria-hidden="true" />
      </header>
    </>
  );
}