03Color invert on scroll

<main class="stage">
  <header class="head is-demo">
    <span class="head__layer head__layer--dark"></span>
    <span class="head__layer head__layer--light"></span>
    <span class="head__logo">GODRICH</span>
  </header>
  <div class="hero">
    <div class="hero__sentinel"></div>
    <p class="hero__title">Hero section</p>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-base;                              // 색이 뒤집히는 시간
$easing: $ease-out;
$color: $subject-ink;                              // 반전 뒤 헤더 배경

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

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

  // 두 배경 레이어를 겹쳐 두고 opacity 만 크로스페이드한다 (배경색 자체는 애니메이션하지 않는다)
  &__layer { position: absolute; inset: 0; }
  &__layer--dark  { background: transparent; opacity: 1; transition: opacity $duration $easing; }
  &__layer--light { background: $color; opacity: 0; transition: opacity $duration $easing; }
  &.is-inverted &__layer--dark  { opacity: 0; }
  &.is-inverted &__layer--light { opacity: 1; }

  // 로고도 헤더와 같이 뒤집힌다 — 주황(투명) 구간에선 잉크, 먹 구간에선 크림이라야 둘 다 AA 를 넘는다
  &__logo { position: relative; color: $subject-ink; font-weight: 800; font-size: 18px; line-height: 1; font-family: inherit; letter-spacing: .04em; transition: color $duration $easing; }
  &.is-inverted &__logo { color: $subject-cream; }
}

// ── 데모(.is-demo): 뒤집혔다 돌아왔다 2초 루프 ─────────────
.head.is-demo .head__layer--light {
  animation: invert-loop $dur-loop $easing infinite;
}
.head.is-demo .head__logo {
  animation: logo-invert $dur-loop $easing infinite;
}
@keyframes invert-loop {
  0%, 15%  { opacity: 0; }
  45%, 70% { opacity: 1; }
  95%, 100% { opacity: 0; }
}
@keyframes logo-invert {
  0%, 15%   { color: $subject-ink; }
  45%, 70%  { color: $subject-cream; }
  95%, 100% { color: $subject-ink; }
}

// ── 히어로: 어두운 배경 위 흰 글자 자리 ───────────────────
.hero { position: relative; height: clamp(100px, 40vh, 220px); display: grid; place-items: center; }
.hero__sentinel { position: absolute; bottom: 0; height: 1px; width: 1px; }
.hero__title { color: $subject-ink; font-weight: 700; font-size: clamp(14px, 4vw, 18px); line-height: 1; font-family: inherit; margin: 0; }

@media (prefers-reduced-motion: reduce) {
  .head__layer { transition: none; }
  .head__logo { transition: none; }
  .head.is-demo .head__layer--light { animation: none !important; opacity: 1; }
  .head.is-demo .head__logo { animation: none !important; color: $subject-cream; }
}

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

.stage {
  min-height: 100vh;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.head {
  position: sticky;
  top: 0;
  height: 56px;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
}
.head__layer {
  position: absolute;
  inset: 0;
}
.head__layer--dark {
  background: transparent;
  opacity: 1;
  transition: opacity 300ms ease-out;
}
.head__layer--light {
  background: #17141a;
  opacity: 0;
  transition: opacity 300ms ease-out;
}
.head.is-inverted .head__layer--dark {
  opacity: 0;
}
.head.is-inverted .head__layer--light {
  opacity: 1;
}
.head__logo {
  position: relative;
  color: #17141a;
  font-weight: 800;
  font-size: 18px;
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.04em;
  transition: color 300ms ease-out;
}
.head.is-inverted .head__logo {
  color: #fff7e6;
}

.head.is-demo .head__layer--light {
  animation: invert-loop 2s ease-out infinite;
}

.head.is-demo .head__logo {
  animation: logo-invert 2s ease-out infinite;
}

@keyframes invert-loop {
  0%, 15% {
    opacity: 0;
  }
  45%, 70% {
    opacity: 1;
  }
  95%, 100% {
    opacity: 0;
  }
}
@keyframes logo-invert {
  0%, 15% {
    color: #17141a;
  }
  45%, 70% {
    color: #fff7e6;
  }
  95%, 100% {
    color: #17141a;
  }
}
.hero {
  position: relative;
  height: clamp(100px, 40vh, 220px);
  display: grid;
  place-items: center;
}

.hero__sentinel {
  position: absolute;
  bottom: 0;
  height: 1px;
  width: 1px;
}

.hero__title {
  color: #17141a;
  font-weight: 700;
  font-size: clamp(14px, 4vw, 18px);
  line-height: 1;
  font-family: inherit;
  margin: 0;
}

@media (prefers-reduced-motion: reduce) {
  .head__layer {
    transition: none;
  }
  .head__logo {
    transition: none;
  }
  .head.is-demo .head__layer--light {
    animation: none !important;
    opacity: 1;
  }
  .head.is-demo .head__logo {
    animation: none !important;
    color: #fff7e6;
  }
}

// 실제 페이지: 히어로 하단 센티넬을 지나면 색이 뒤집힌다 (IntersectionObserver, 6줄)
var head = document.querySelector('.head');
new IntersectionObserver(function (es) {
  head.classList.toggle('is-inverted', !es[0].isIntersecting);
}).observe(document.querySelector('.hero__sentinel'));

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

export type ColorInvertHeaderProps = {
  logo: string;
  heroSentinelRef: { current: HTMLElement | null };
  /** 색이 뒤집히는 시간 (기본 300ms) */
  duration?: number;
  easing?: string;
  color?: string;
};

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

export default function ColorInvertHeader({
  logo,
  heroSentinelRef,
  duration = 300,
  easing = "ease-out",
  color = "#17141a",
}: ColorInvertHeaderProps) {
  const head = useRef<HTMLElement>(null);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

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

  return (
    <header ref={head} className={styles.head} style={vars}>
      <span className={styles.layerDark} />
      <span className={styles.layerLight} />
      <span className={styles.logo}>{logo}</span>
    </header>
  );
}