07Tiêu đề section dính theo

<main class="stage">
  <header class="head is-demo">
    <span class="head__label head__label--a">A</span>
    <span class="head__label head__label--b">B</span>
  </header>
  <div class="list">
    <p class="list__sentinel" data-label="A"></p>
    <p class="list__row">Amy</p>
    <p class="list__row">Anna</p>
    <p class="list__sentinel" data-label="B"></p>
    <p class="list__row">Ben</p>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-quick;                              // 제목이 교체되는 시간
$easing: $ease-out;
$color: $stage-orange;                              // 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: grid;
  place-items: center;
  background: $subject-ink;
  overflow: hidden;

  // 두 라벨을 겹쳐 두고 opacity + translateY 로 교체한다
  &__label {
    grid-area: 1 / 1;
    color: $subject-cream;
    font-weight: 800;
    font-size: 20px;
    line-height: 1;
    font-family: inherit;
    letter-spacing: .06em;
    opacity: 0;
    transform: translateY(8px);
    transition: opacity $duration $easing, transform $duration $easing;
  }
  &__label.is-current { opacity: 1; transform: translateY(0); }
  &__label--a.is-current ~ &__label--b { opacity: 0; }
}

// 기본 상태: A 가 보인다
.head__label--a { opacity: 1; transform: translateY(0); }

// ── 데모(.is-demo): A ↔ B 2초 루프로 교체 ─────────────────
.head.is-demo .head__label--a { animation: label-a-loop $dur-loop $easing infinite; }
.head.is-demo .head__label--b { animation: label-b-loop $dur-loop $easing infinite; }
@keyframes label-a-loop {
  0%, 20%  { opacity: 1; transform: translateY(0); }
  45%, 75% { opacity: 0; transform: translateY(-8px); }
  95%, 100% { opacity: 1; transform: translateY(0); }
}
@keyframes label-b-loop {
  0%, 20%  { opacity: 0; transform: translateY(8px); }
  45%, 75% { opacity: 1; transform: translateY(0); }
  95%, 100% { opacity: 0; transform: translateY(8px); }
}

// ── 목록: 알파벳 구간이 있는 연락처 흉내 (칸이 좁으면 줄인다) ───
.list { padding: clamp(4px, 3vh, 16px) 24px 0; }
.list__sentinel { height: 1px; margin: 0; }
.list__row {
  margin: 0 0 clamp(3px, 2vh, 12px);
  padding: clamp(3px, 2vh, 10px) 14px;
  border-radius: $r-control;
  background: rgba($subject-ink, .06);
  color: $subject-ink;
  font-weight: 600;
  font-size: 12px;
  line-height: 1.1;
  font-family: inherit;
}

@media (prefers-reduced-motion: reduce) {
  .head__label { transition: none; }
  .head.is-demo .head__label--a, .head.is-demo .head__label--b { animation: none !important; }
}

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: grid;
  place-items: center;
  background: #17141a;
  overflow: hidden;
}
.head__label {
  grid-area: 1/1;
  color: #fff7e6;
  font-weight: 800;
  font-size: 20px;
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.06em;
  opacity: 0;
  transform: translateY(8px);
  transition: opacity 220ms ease-out, transform 220ms ease-out;
}
.head__label.is-current {
  opacity: 1;
  transform: translateY(0);
}
.head__label--a.is-current ~ .head__label--b {
  opacity: 0;
}

.head__label--a {
  opacity: 1;
  transform: translateY(0);
}

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

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

@keyframes label-a-loop {
  0%, 20% {
    opacity: 1;
    transform: translateY(0);
  }
  45%, 75% {
    opacity: 0;
    transform: translateY(-8px);
  }
  95%, 100% {
    opacity: 1;
    transform: translateY(0);
  }
}
@keyframes label-b-loop {
  0%, 20% {
    opacity: 0;
    transform: translateY(8px);
  }
  45%, 75% {
    opacity: 1;
    transform: translateY(0);
  }
  95%, 100% {
    opacity: 0;
    transform: translateY(8px);
  }
}
.list {
  padding: clamp(4px, 3vh, 16px) 24px 0;
}

.list__sentinel {
  height: 1px;
  margin: 0;
}

.list__row {
  margin: 0 0 clamp(3px, 2vh, 12px);
  padding: clamp(3px, 2vh, 10px) 14px;
  border-radius: 12px;
  background: rgba(23, 20, 26, 0.06);
  color: #17141a;
  font-weight: 600;
  font-size: 12px;
  line-height: 1.1;
  font-family: inherit;
}

@media (prefers-reduced-motion: reduce) {
  .head__label {
    transition: none;
  }
  .head.is-demo .head__label--a, .head.is-demo .head__label--b {
    animation: none !important;
  }
}

// 실제 페이지: 지금 통과 중인 구간 센티넬의 글자를 헤더 라벨에 올린다 (IntersectionObserver, 7줄)
var labels = document.querySelectorAll('.head__label');
document.querySelectorAll('.list__sentinel').forEach(function (s) {
  new IntersectionObserver(function (es) {
    if (es[0].isIntersecting) labels.forEach(function (l) { l.classList.toggle('is-current', l.textContent === s.dataset.label); });
  }, { rootMargin: '-56px 0px -80% 0px' }).observe(s);
});

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

export type SectionTitleHeaderProps = {
  /** 구간 이름과, 그 구간이 시작하는 지점의 센티넬 ref */
  sections: { label: string; sentinelRef: { current: HTMLElement | null } }[];
  duration?: number;
  easing?: string;
};

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

export default function SectionTitleHeader({ sections, duration = 220, easing = "ease-out" }: SectionTitleHeaderProps) {
  const [current, setCurrent] = useState(sections[0]?.label ?? "");
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing };
  const observers = useRef<IntersectionObserver[]>([]);

  useEffect(() => {
    observers.current = sections.map(({ label, sentinelRef }) => {
      const io = new IntersectionObserver(
        ([e]) => { if (e.isIntersecting) setCurrent(label); },
        { rootMargin: "-56px 0px -80% 0px" }
      );
      if (sentinelRef.current) io.observe(sentinelRef.current);
      return io;
    });
    return () => observers.current?.forEach((io) => io.disconnect());
  }, [sections]);

  return (
    <header className={styles.head} style={vars}>
      <span key={current} className={styles.label}>{current}</span>
    </header>
  );
}