01height:auto via interpolate-size

<main class="stage">
  <section class="accordion">
    <button class="accordion__header" type="button" aria-expanded="false">
      <span class="accordion__title">Refund policy</span>
      <span class="accordion__chevron" aria-hidden="true"></span>
    </button>
    <div class="accordion__panel is-demo">
      <p class="accordion__body">Refunds within 7 days of purchase.</p>
    </div>
  </section>
</main>

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

$duration: $dur-base;      // 열리는 데 걸리는 시간
$easing: $ease-spring;     // 전환 곡선
$color: $subject-blue;     // 강조 색

@include stage($stage-ink);

.accordion {
  width: clamp(220px, 78vw, 340px);
  background: $subject-cream;
  border-radius: $r-card;
  box-shadow: $shadow-raised;
  overflow: hidden;
  font-family: inherit;
}
.accordion__header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  width: 100%;
  border: 0;
  background: none;
  padding: clamp(12px, 3vw, 18px) clamp(14px, 4vw, 20px);
  cursor: pointer;
  color: $subject-ink;
  font-weight: 600;
  font-size: clamp(14px, 3.6vw, 17px);
  line-height: 1.3;
  font-family: inherit;
  text-align: left;
}
.accordion__chevron {
  flex: none;
  width: 9px;
  height: 9px;
  border-right: 2px solid currentColor;
  border-bottom: 2px solid currentColor;
  transform: rotate(45deg);
}
.accordion__body {
  margin: 0;
  padding: 0 clamp(14px, 4vw, 20px) clamp(12px, 3vw, 18px);
  color: $subject-ink;
  opacity: .86;
  font-weight: 400;
  font-size: clamp(12px, 3.2vw, 14.5px);
  line-height: 1.5;
  font-family: inherit;
}

.accordion__panel {
  height: 0;
  overflow: hidden;
  interpolate-size: allow-keywords;
}

// 실사용: 클릭하면 열림 상태(.is-open)가 붙고 목표 높이까지 전환된다
.accordion.is-open .accordion__panel {
  height: calc-size(auto, size);
  transition: height $duration $easing;
}
@supports not (height: calc-size(auto, size)) {
  .accordion.is-open .accordion__panel { height: auto; }
}

// 데모: 2초 주기로 스스로 열고 닫힌다. WAAPI 시계로 재생 위치를 잡으려면
// transition 이 아니라 keyframes 여야 한다 (blog-demo 측정 규칙)
.accordion__panel.is-demo {
  animation: accordion-loop $dur-loop $easing infinite;
}
@keyframes accordion-loop {
  0%, 8%   { height: 0; }
  42%, 58% { height: calc-size(auto, size); }
  92%, 100% { height: 0; }
}
@supports not (height: calc-size(auto, size)) {
  @keyframes accordion-loop {
    0%, 8%   { height: 0; }
    42%, 58% { height: 220px; }
    92%, 100% { height: 0; }
  }
}

@media (prefers-reduced-motion: reduce) {
  .accordion__panel,
  .accordion__panel.is-demo,
  .accordion__chevron,
  .accordion__body,
  .accordion.is-open .accordion__panel {
    animation: none !important;
    transition: none !important;
  }
  .accordion__panel.is-demo { height: auto; }
}

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);
}

.accordion {
  width: clamp(220px, 78vw, 340px);
  background: #fff7e6;
  border-radius: 16px;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  overflow: hidden;
  font-family: inherit;
}

.accordion__header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  width: 100%;
  border: 0;
  background: none;
  padding: clamp(12px, 3vw, 18px) clamp(14px, 4vw, 20px);
  cursor: pointer;
  color: #17141a;
  font-weight: 600;
  font-size: clamp(14px, 3.6vw, 17px);
  line-height: 1.3;
  font-family: inherit;
  text-align: left;
}

.accordion__chevron {
  flex: none;
  width: 9px;
  height: 9px;
  border-right: 2px solid currentColor;
  border-bottom: 2px solid currentColor;
  transform: rotate(45deg);
}

.accordion__body {
  margin: 0;
  padding: 0 clamp(14px, 4vw, 20px) clamp(12px, 3vw, 18px);
  color: #17141a;
  opacity: 0.86;
  font-weight: 400;
  font-size: clamp(12px, 3.2vw, 14.5px);
  line-height: 1.5;
  font-family: inherit;
}

.accordion__panel {
  height: 0;
  overflow: hidden;
  interpolate-size: allow-keywords;
}

.accordion.is-open .accordion__panel {
  height: calc-size(auto, size);
  transition: height 300ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

@supports not (height: calc-size(auto, size)) {
  .accordion.is-open .accordion__panel {
    height: auto;
  }
}
.accordion__panel.is-demo {
  animation: accordion-loop 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@keyframes accordion-loop {
  0%, 8% {
    height: 0;
  }
  42%, 58% {
    height: calc-size(auto, size);
  }
  92%, 100% {
    height: 0;
  }
}
@supports not (height: calc-size(auto, size)) {
  @keyframes accordion-loop {
    0%, 8% {
      height: 0;
    }
    42%, 58% {
      height: 220px;
    }
    92%, 100% {
      height: 0;
    }
  }
}
@media (prefers-reduced-motion: reduce) {
  .accordion__panel,
  .accordion__panel.is-demo,
  .accordion__chevron,
  .accordion__body,
  .accordion.is-open .accordion__panel {
    animation: none !important;
    transition: none !important;
  }
  .accordion__panel.is-demo {
    height: auto;
  }
}

(function () {
  document.querySelectorAll('.accordion').forEach(function (acc) {
    var header = acc.querySelector('.accordion__header');
    var panel = acc.querySelector('.accordion__panel');
    header.addEventListener('click', function () {
      panel.classList.remove('is-demo');
      var open = acc.classList.toggle('is-open');
      header.setAttribute('aria-expanded', String(open));
    });
  });
})();

import { useState } from "react";
import type { CSSProperties, ReactNode } from "react";
import styles from "./AccordionHeightAuto.module.scss";

export type AccordionHeightAutoProps = {
  /** 헤더에 보이는 제목 */
  header?: ReactNode;
  /** 열렸을 때 보이는 본문 */
  children?: ReactNode;
  /** height:auto 전환 시간 (기본 300ms) */
  duration?: number;
  /** CSS easing */
  easing?: string;
  /** 강조 색 */
  color?: string;
  defaultOpen?: boolean;
};

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

export default function AccordionHeightAuto({
  header = "Refund policy",
  children = "Refunds within 7 days of purchase.",
  duration = 300,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#2f6df6",
  defaultOpen = false,
}: AccordionHeightAutoProps) {
  const [open, setOpen] = useState(defaultOpen);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  return (
    <section className={styles.accordion} style={vars}>
      <button
        type="button"
        className={styles.header}
        aria-expanded={open}
        onClick={() => setOpen((v) => !v)}
      >
        <span>{header}</span>
        <span className={`${styles.chevron} ${open ? styles.chevronOpen : ""}`} aria-hidden="true" />
      </button>
      <div className={`${styles.panel} ${open ? styles.panelOpen : ""}`}>
        <p className={styles.body}>{children}</p>
      </div>
    </section>
  );
}