01height:auto transition

<main class="stage">
  <nav class="menu is-demo">
    <button class="menu__trigger" type="button" aria-expanded="false">Menu</button>
    <div class="menu__panel">
      <ul class="menu__list">
        <li><a href="#">Announcements</a></li>
        <li><a href="#">FAQ</a></li>
        <li><a href="#">Support</a></li>
      </ul>
    </div>
  </nav>
</main>

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

$duration: $dur-base;                              // 패널이 늘어나는 시간
$easing: $ease-out;                                // 시작 빠르고 끝은 부드럽게
$color: $subject-blue-deep;                        // 트리거 버튼 색 (크림 글자가 얹히므로 진한 파랑)

@include stage($stage-ink);

.menu {
  position: relative;
  width: min(220px, 90vw);
  font-weight: 500;
  font-size: clamp(13px, 3.6vw, 16px);
  line-height: 1.4;
  font-family: inherit;

  &__trigger {
    width: 100%;
    border: 0;
    border-radius: $r-control;
    padding: clamp(9px, 3vw, 13px) clamp(16px, 4vw, 22px);
    font-weight: 700;
    font-family: inherit;
    color: $subject-cream;
    background: $color;
    cursor: pointer;
  }

  &__panel {
    display: grid;
    grid-template-rows: 0fr;                        // height:auto 를 흉내내는 트릭. 0fr↔1fr 만 움직인다
    transition: grid-template-rows $duration $easing;
    margin-top: 8px;
  }

  &__list {
    overflow: hidden;                                // grid-template-rows 트릭엔 자식에 overflow:hidden 이 필수
    min-height: 0;
    margin: 0;
    padding: 0;
    list-style: none;
    background: $subject-cream;
    border-radius: $r-card;
  }

  &__list li a {
    display: block;
    padding: clamp(8px, 2.6vw, 12px) clamp(12px, 3.6vw, 18px);
    color: $stage-ink;
    text-decoration: none;
    border-bottom: 1px solid rgba($stage-ink, .08);
  }
  &__list li:last-child a { border-bottom: 0; }

  &.is-demo &__panel { animation: reveal-loop $dur-loop $easing infinite; }
  &.is-open &__panel { animation: none; grid-template-rows: 1fr; }
}

@keyframes reveal-loop {
  0%, 30%   { grid-template-rows: 0fr; }
  45%, 80%  { grid-template-rows: 1fr; }
  95%, 100% { grid-template-rows: 0fr; }
}

@media (prefers-reduced-motion: reduce) {
  .menu__panel { transition: none; }
  .menu.is-demo .menu__panel { animation: none; grid-template-rows: 0fr; }
}

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);
}

.menu {
  position: relative;
  width: min(220px, 90vw);
  font-weight: 500;
  font-size: clamp(13px, 3.6vw, 16px);
  line-height: 1.4;
  font-family: inherit;
}
.menu__trigger {
  width: 100%;
  border: 0;
  border-radius: 12px;
  padding: clamp(9px, 3vw, 13px) clamp(16px, 4vw, 22px);
  font-weight: 700;
  font-family: inherit;
  color: #fff7e6;
  background: #0a4fe8;
  cursor: pointer;
}
.menu__panel {
  display: grid;
  grid-template-rows: 0fr;
  transition: grid-template-rows 300ms ease-out;
  margin-top: 8px;
}
.menu__list {
  overflow: hidden;
  min-height: 0;
  margin: 0;
  padding: 0;
  list-style: none;
  background: #fff7e6;
  border-radius: 16px;
}
.menu__list li a {
  display: block;
  padding: clamp(8px, 2.6vw, 12px) clamp(12px, 3.6vw, 18px);
  color: #17141a;
  text-decoration: none;
  border-bottom: 1px solid rgba(23, 20, 26, 0.08);
}
.menu__list li:last-child a {
  border-bottom: 0;
}
.menu.is-demo .menu__panel {
  animation: reveal-loop 2s ease-out infinite;
}
.menu.is-open .menu__panel {
  animation: none;
  grid-template-rows: 1fr;
}

@keyframes reveal-loop {
  0%, 30% {
    grid-template-rows: 0fr;
  }
  45%, 80% {
    grid-template-rows: 1fr;
  }
  95%, 100% {
    grid-template-rows: 0fr;
  }
}
@media (prefers-reduced-motion: reduce) {
  .menu__panel {
    transition: none;
  }
  .menu.is-demo .menu__panel {
    animation: none;
    grid-template-rows: 0fr;
  }
}

// 실제 클릭으로도 열고 닫힌다. 데모(.is-demo)는 스스로 반복하다가 사람이 누르면 멈추고 수동 토글로 바뀐다.
var m = document.querySelector('.menu');
var t = m.querySelector('.menu__trigger');
t.addEventListener('click', function () {
  m.classList.remove('is-demo');
  var open = m.classList.toggle('is-open');
  t.setAttribute('aria-expanded', open);
});

import { useState } from "react";
import type { CSSProperties } from "react";
import styles from "./HeightAutoDropdown.module.scss";

export type HeightAutoDropdownProps = {
  triggerLabel: string;
  items: string[];
  /** 패널이 늘어나는 시간 (기본 300ms) */
  duration?: number;
  easing?: string;
  color?: string;
};

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

export default function HeightAutoDropdown({
  triggerLabel,
  items,
  duration = 300,
  easing = "ease-out",
  color = "#0a4fe8",
}: HeightAutoDropdownProps) {
  const [open, setOpen] = useState(false);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  return (
    <nav className={styles.menu} style={vars}>
      <button
        type="button"
        className={styles.trigger}
        aria-expanded={open}
        onClick={() => setOpen((v) => !v)}
      >
        {triggerLabel}
      </button>
      <div className={open ? `${styles.panel} ${styles.panelOpen}` : styles.panel}>
        <ul className={styles.list}>
          {items.map((label) => (
            <li key={label}><a href="#">{label}</a></li>
          ))}
        </ul>
      </div>
    </nav>
  );
}