01Accordion-reveal value toggle — when the button's open value changes, its rounding and shadow values change along, presenting the content below

<main class="stage">
<div class="ba is-demo" id="ba">
  <button type="button" class="ba__btn" id="baBtn" aria-expanded="false" aria-controls="baPanel">
    <span class="ba__label">Expand open value</span>
    <span class="ba__mark" aria-hidden="true">
      <svg viewBox="0 0 16 16" width="14" height="14"><path d="M3 6l5 5 5-5" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/></svg>
    </span>
  </button>
  <div class="ba__panel" id="baPanel" role="region">
    <ul class="ba__list" id="baList">
      <li class="ba__item">Answer value 1</li>
      <li class="ba__item">Answer value 2</li>
      <li class="ba__item">Answer value 3</li>
    </ul>
  </div>
</div>
</main>

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

// 계산 겹: 하위 항목 노출 높이를 getBoundingClientRect 로 실측해 --panel-h 로 넘기고(index.html),
// 항목마다 순서 * STAGGER_MS 산술로 transition-delay 를 준다.
@include stage($stage-ink);

.ba {
  width: 100%;
  max-width: 300px;
}

.ba__btn {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: $sp-3;
  width: 100%;
  box-sizing: border-box;
  padding: $sp-3 $sp-4;
  border: 0;
  border-radius: $r-control $r-control 0 0;
  background: $subject-cream;
  color: $subject-ink;
  font-weight: 600;
  font-size: 15px;
  box-shadow: $shadow-press;
  transition: border-radius $dur-base $ease-out, box-shadow $dur-base $ease-out;
  cursor: pointer;
}

.ba__mark {
  display: inline-flex;
  transition: transform $dur-base $ease-out;
}

.ba__panel {
  overflow: hidden;
  max-height: 0;
  border-radius: 0 0 $r-control $r-control;
  background: rgba($subject-cream, .92);
  transition: max-height $dur-enter $ease-spring;
}

.ba__list {
  list-style: none;
  margin: 0;
  padding: $sp-3 $sp-4;
  display: grid;
  gap: $sp-2;
}

.ba__item {
  color: $subject-ink;
  font-size: 13px;
  opacity: 0;
  transform: translateY(-6px);
  transition: opacity $dur-quick $ease-out, transform $dur-quick $ease-out;
}

.ba--open {
  .ba__btn { border-radius: $r-control $r-control 0 0; box-shadow: $shadow-raised; }
  .ba__mark { transform: rotate(180deg); }
  .ba__panel { max-height: var(--panel-h, 160px); }
  .ba__item { opacity: 1; transform: translateY(0); }
}

@media (max-width: 340px) {
  .ba__btn { padding: $sp-2 $sp-3; font-size: 14px; }
  .ba__list { padding: $sp-2 $sp-3; }
}

@media (prefers-reduced-motion: reduce) {
  .ba__btn, .ba__mark, .ba__panel, .ba__item { transition: none; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.ba {
  width: 100%;
  max-width: 300px;
}

.ba__btn {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  width: 100%;
  box-sizing: border-box;
  padding: 12px 16px;
  border: 0;
  border-radius: 12px 12px 0 0;
  background: #fff7e6;
  color: #17141a;
  font-weight: 600;
  font-size: 15px;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  transition: border-radius 300ms ease-out, box-shadow 300ms ease-out;
  cursor: pointer;
}

.ba__mark {
  display: inline-flex;
  transition: transform 300ms ease-out;
}

.ba__panel {
  overflow: hidden;
  max-height: 0;
  border-radius: 0 0 12px 12px;
  background: rgba(255, 247, 230, 0.92);
  transition: max-height 600ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.ba__list {
  list-style: none;
  margin: 0;
  padding: 12px 16px;
  display: grid;
  gap: 8px;
}

.ba__item {
  color: #17141a;
  font-size: 13px;
  opacity: 0;
  transform: translateY(-6px);
  transition: opacity 220ms ease-out, transform 220ms ease-out;
}

.ba--open .ba__btn {
  border-radius: 12px 12px 0 0;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}
.ba--open .ba__mark {
  transform: rotate(180deg);
}
.ba--open .ba__panel {
  max-height: var(--panel-h, 160px);
}
.ba--open .ba__item {
  opacity: 1;
  transform: translateY(0);
}

@media (max-width: 340px) {
  .ba__btn {
    padding: 8px 12px;
    font-size: 14px;
  }
  .ba__list {
    padding: 8px 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ba__btn, .ba__mark, .ba__panel, .ba__item {
    transition: none;
  }
}

(function () {
  var root = document.getElementById('ba');
  var btn = document.getElementById('baBtn');
  var panel = document.getElementById('baPanel');
  var items = panel.querySelectorAll('.ba__item');
  var STAGGER_MS = 70; // 계산 겹: 항목 순서 * 이 값만큼 늦게 나타난다

  // 계산 겹: 하위 항목이 실제로 차지하는 노출 높이를 getBoundingClientRect 로 실측해
  // panel 의 --panel-h 로 넘긴다 — 짐작한 고정값이 아니라 지금 폰트·내용 기준 값이다.
  function measure() {
    var h = 0;
    items.forEach(function (li) { h += li.getBoundingClientRect().height; });
    panel.style.setProperty('--panel-h', h + 'px');
  }
  items.forEach(function (li, i) {
    li.style.transitionDelay = (i * STAGGER_MS) + 'ms';
  });

  var open = false;
  function paint() {
    measure();
    root.classList.toggle('ba--open', open);
    btn.setAttribute('aria-expanded', String(open));
  }

  function toggle() {
    open = !open;
    paint();
  }

  btn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    toggle();
  });

  var autoTimer = null;
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  measure();
  if (!reduce) { toggle(); autoTimer = setInterval(toggle, 800); }
  else { open = true; paint(); }
})();

import { useEffect, useRef } from "react";
import styles from "./AccordionReveal.module.scss";

type Box<T> = { current: T };

const STAGGER_MS = 70; // 계산 겹: 항목 순서 * 이 값만큼 늦게 나타난다

export default function AccordionReveal() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement | null>;

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const btn = root.querySelector(`.${styles.btn}`) as HTMLButtonElement | null;
    const panel = root.querySelector(`.${styles.panel}`) as HTMLElement | null;
    const items = root.querySelectorAll(`.${styles.item}`);
    if (!btn || !panel) return;

    items.forEach((li, i) => {
      (li as HTMLElement).style.transitionDelay = (i * STAGGER_MS) + "ms";
    });

    function measure() {
      let h = 0;
      items.forEach((li) => { h += li.getBoundingClientRect().height; });
      panel!.style.setProperty("--panel-h", h + "px");
    }

    let open = false;
    function paint() {
      measure();
      root!.classList.toggle(styles["root--open"], open);
      btn!.setAttribute("aria-expanded", String(open));
    }
    function toggle() {
      open = !open;
      paint();
    }

    function onClick() {
      root!.classList.remove(styles["is-demo"]);
      stopAutoplay();
      toggle();
    }
    btn.addEventListener("click", onClick);

    let autoTimer: number | null = null;
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    measure();
    if (!reduce) { toggle(); autoTimer = window.setInterval(toggle, 800); }
    else { open = true; paint(); }

    return () => {
      stopAutoplay();
      btn.removeEventListener("click", onClick);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.ba} ${styles["is-demo"]}`}>
      <button type="button" className={styles.btn} aria-expanded="false">
        <span className={styles.label}>열림 값 펼치기</span>
        <span className={styles.mark} aria-hidden="true">
          <svg viewBox="0 0 16 16" width="14" height="14"><path d="M3 6l5 5 5-5" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" /></svg>
        </span>
      </button>
      <div className={styles.panel} role="region">
        <ul className={styles.list}>
          <li className={styles.item}>답변 값 1</li>
          <li className={styles.item}>답변 값 2</li>
          <li className={styles.item}>답변 값 3</li>
        </ul>
      </div>
    </div>
  );
}