08Header backdrop blur

<main class="stage">
  <div class="photo"></div>
  <div class="sentinel"></div>
  <header class="head is-demo">
    <span class="head__logo">GODRICH</span>
  </header>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-base;                              // 블러가 켜지는 시간
$easing: $ease-out;
$blur: 10px;                                        // 블러 강도

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

// 뒤 화면 흉내 — 단색 배경만으로는 블러가 안 보여 줄무늬를 둔다
.photo {
  position: absolute; inset: 0;
  background: repeating-linear-gradient(120deg, $subject-blue 0 18px, $subject-cream 18px 36px);
}

.head {
  position: sticky;
  top: 0;
  height: 56px;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
  background: rgba(255, 255, 255, .08);
  backdrop-filter: blur(0);
  transition: background $duration $easing, backdrop-filter $duration $easing;

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

  // 실제 사용: 센티넬이 사라지면 반투명 유리 배경이 켜진다
  &.is-scrolled { background: rgba(255, 255, 255, .55); backdrop-filter: blur($blur); }
}

// ── 데모(.is-demo): 블러가 켜졌다 꺼졌다 2초 루프 ──────────
.head.is-demo {
  animation: blur-loop $dur-loop $easing infinite;
}
@keyframes blur-loop {
  0%, 15%  { background: rgba(255, 255, 255, .08); backdrop-filter: blur(0); }
  45%, 70% { background: rgba(255, 255, 255, .55); backdrop-filter: blur($blur); }
  95%, 100% { background: rgba(255, 255, 255, .08); backdrop-filter: blur(0); }
}

@media (prefers-reduced-motion: reduce) {
  .head { transition: none; }
  .head.is-demo { animation: none !important; background: rgba(255, 255, 255, .55); backdrop-filter: blur($blur); }
}

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

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

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

.photo {
  position: absolute;
  inset: 0;
  background: repeating-linear-gradient(120deg, #2f6df6 0 18px, #fff7e6 18px 36px);
}

.head {
  position: sticky;
  top: 0;
  height: 56px;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
  background: rgba(255, 255, 255, 0.08);
  backdrop-filter: blur(0);
  transition: background 300ms ease-out, backdrop-filter 300ms ease-out;
}
.head__logo {
  color: #17141a;
  font-weight: 800;
  font-size: 18px;
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.04em;
}
.head.is-scrolled {
  background: rgba(255, 255, 255, 0.55);
  backdrop-filter: blur(10px);
}

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

@keyframes blur-loop {
  0%, 15% {
    background: rgba(255, 255, 255, 0.08);
    backdrop-filter: blur(0);
  }
  45%, 70% {
    background: rgba(255, 255, 255, 0.55);
    backdrop-filter: blur(10px);
  }
  95%, 100% {
    background: rgba(255, 255, 255, 0.08);
    backdrop-filter: blur(0);
  }
}
@media (prefers-reduced-motion: reduce) {
  .head {
    transition: none;
  }
  .head.is-demo {
    animation: none !important;
    background: rgba(255, 255, 255, 0.55);
    backdrop-filter: blur(10px);
  }
}

// 실제 페이지: 센티넬을 지나면 블러 유리 배경을 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 "./BackdropBlurHeader.module.scss";

export type BackdropBlurHeaderProps = {
  logo: string;
  duration?: number;
  easing?: string;
  /** 블러 강도 (기본 10px) */
  blur?: number;
};

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

export default function BackdropBlurHeader({
  logo,
  duration = 300,
  easing = "ease-out",
  blur = 10,
}: BackdropBlurHeaderProps) {
  const head = useRef<HTMLElement>(null);
  const sentinel = useRef<HTMLDivElement>(null);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--blur": `${blur}px` };

  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>
      </header>
    </>
  );
}