06サブナビが固定される

<main class="stage">
  <header class="head">GODRICH</header>
  <div class="sentinel"></div>
  <nav class="subnav is-demo">
    <span class="subnav__tab is-active">レディース</span>
    <span class="subnav__tab">メンズ</span>
    <span class="subnav__tab">キッズ</span>
  </nav>
  <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 ──────────────────────────────────────────
$head-h: 56px;                                     // 메인 헤더 높이 — 서브 내비가 이 값 아래 붙는다
$duration: $dur-quick;                              // 테두리가 켜지는 시간
$color: $subject-ink;                               // 서브 내비 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 { height: 1px; }

.head {
  position: sticky;
  top: 0;
  z-index: 2;
  height: $head-h;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
  background: $stage-ink;
  color: $subject-cream;
  font-weight: 800;
  font-size: 18px;
  line-height: 1;
  font-family: inherit;
  letter-spacing: .04em;
}

// 서브 내비는 메인 헤더 바로 아래에 두 번째 줄로 붙는다 (이중 sticky, 겹침 없음)
.subnav {
  position: sticky;
  top: $head-h;
  z-index: 1;
  display: flex;
  gap: 4px;
  padding: clamp(6px, 3vh, 10px) clamp(16px, 5vw, 24px);
  background: $color;
  border-bottom: 4px solid transparent;
  transition: border-color $duration $ease-out;

  &.is-stuck { border-bottom-color: $subject-cream; }

  &__tab {
    padding: 6px 14px;
    border-radius: $r-pill;
    font-weight: 600;
    font-size: 13px;
    line-height: 1;
    font-family: inherit;
    color: rgba(255, 255, 255, .6);
    &.is-active { color: $stage-ink; background: $subject-cream; }
  }
}

// ── 데모(.is-demo): 테두리가 켜졌다 꺼졌다 2초 루프 ────────
.subnav.is-demo {
  animation: stuck-loop $dur-loop $ease-out infinite;
}
@keyframes stuck-loop {
  0%, 5%   { border-bottom-color: transparent; }
  40%, 60% { border-bottom-color: $subject-cream; }
  95%, 100% { border-bottom-color: transparent; }
}

// ── 배경: 문서처럼 보이는 자리 (칸이 좁으면 줄인다) ────────────
.page { padding: clamp(8px, 4vh, 20px) 24px 0; }
.page__line {
  height: 10px;
  margin: 0 0 clamp(6px, 3vh, 14px);
  border-radius: 4px;
  background: rgba($stage-ink, .14);
  &.short { width: 60%; }
}

@media (prefers-reduced-motion: reduce) {
  .subnav { transition: none; }
  .subnav.is-demo { animation: none !important; border-bottom-color: $subject-cream; }
}

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 {
  height: 1px;
}

.head {
  position: sticky;
  top: 0;
  z-index: 2;
  height: 56px;
  display: flex;
  align-items: center;
  padding: 0 clamp(16px, 5vw, 24px);
  background: #17141a;
  color: #fff7e6;
  font-weight: 800;
  font-size: 18px;
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.04em;
}

.subnav {
  position: sticky;
  top: 56px;
  z-index: 1;
  display: flex;
  gap: 4px;
  padding: clamp(6px, 3vh, 10px) clamp(16px, 5vw, 24px);
  background: #17141a;
  border-bottom: 4px solid transparent;
  transition: border-color 220ms ease-out;
}
.subnav.is-stuck {
  border-bottom-color: #fff7e6;
}
.subnav__tab {
  padding: 6px 14px;
  border-radius: 999px;
  font-weight: 600;
  font-size: 13px;
  line-height: 1;
  font-family: inherit;
  color: rgba(255, 255, 255, 0.6);
}
.subnav__tab.is-active {
  color: #17141a;
  background: #fff7e6;
}

.subnav.is-demo {
  animation: stuck-loop 2s ease-out infinite;
}

@keyframes stuck-loop {
  0%, 5% {
    border-bottom-color: transparent;
  }
  40%, 60% {
    border-bottom-color: #fff7e6;
  }
  95%, 100% {
    border-bottom-color: transparent;
  }
}
.page {
  padding: clamp(8px, 4vh, 20px) 24px 0;
}

.page__line {
  height: 10px;
  margin: 0 0 clamp(6px, 3vh, 14px);
  border-radius: 4px;
  background: rgba(23, 20, 26, 0.14);
}
.page__line.short {
  width: 60%;
}

@media (prefers-reduced-motion: reduce) {
  .subnav {
    transition: none;
  }
  .subnav.is-demo {
    animation: none !important;
    border-bottom-color: #fff7e6;
  }
}

// 실제 페이지: 서브 내비가 헤더 밑에 닿는 순간 테두리를 켠다 (IntersectionObserver, 6줄)
var sub = document.querySelector('.subnav');
new IntersectionObserver(function (es) {
  sub.classList.toggle('is-stuck', !es[0].isIntersecting);
}, { rootMargin: '-56px 0px 0px 0px', threshold: 1 }).observe(document.querySelector('.sentinel'));

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

export type StickySubNavProps = {
  tabs: string[];
  activeIndex?: number;
  /** 메인 헤더 높이 (기본 56px) — 서브 내비가 이 값 아래 붙는다 */
  headHeight?: number;
  duration?: number;
  color?: string;
};

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

export default function StickySubNav({
  tabs,
  activeIndex = 0,
  headHeight = 56,
  duration = 220,
  color = "#17141a",
}: StickySubNavProps) {
  const nav = useRef<HTMLElement>(null);
  const sentinel = useRef<HTMLDivElement>(null);
  const vars: Vars = { "--head-h": `${headHeight}px`, "--duration": `${duration}ms`, "--color": color };

  useEffect(() => {
    if (!sentinel.current) return;
    const io = new IntersectionObserver(
      ([e]) => nav.current?.classList.toggle("is-stuck", !e.isIntersecting),
      { rootMargin: `-${headHeight}px 0px 0px 0px`, threshold: 1 }
    );
    io.observe(sentinel.current);
    return () => io.disconnect();
  }, [headHeight]);

  return (
    <>
      <div ref={sentinel} className={styles.sentinel} />
      <nav ref={nav} className={styles.subnav} style={vars}>
        {tabs.map((t, i) => (
          <span key={t} className={i === activeIndex ? `${styles.tab} ${styles.active}` : styles.tab}>{t}</span>
        ))}
      </nav>
    </>
  );
}