02Reply Collapse That Springs to a Measured Height

<main class="stage">
<div class="ch is-demo" id="chRoot">
  <div class="ch__card">
    <div class="ch__row">
      <span class="ch__avatar" aria-hidden="true"></span>
      <div class="ch__body">
        <p class="ch__meta"><strong class="ch__name">CoralBooks</strong></p>
        <p class="ch__text">This thread has quite a pile of replies — let me expand and read them.</p>
        <button type="button" class="ch__toggle" id="chToggle" aria-expanded="false" aria-controls="chPanel">
          <svg class="ch__toggleIcon" id="chIcon" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="m208 96l-80 80l-80-80Z" opacity=".2"/><path d="M215.39 92.94A8 8 0 0 0 208 88H48a8 8 0 0 0-5.66 13.66l80 80a8 8 0 0 0 11.32 0l80-80a8 8 0 0 0 1.73-8.72M128 164.69L67.31 104h121.38Z"/></g></svg>
          <span class="ch__toggleLabel">View 3 replies</span>
        </button>
      </div>
    </div>
    <div class="ch__panel" id="chPanel">
      <div class="ch__panelInner" id="chPanelInner">
        <div class="ch__reply">
          <span class="ch__avatar ch__avatar--sm" aria-hidden="true"></span>
          <p class="ch__text">I kept missing where the save button was — it's clear now.</p>
        </div>
        <div class="ch__reply">
          <span class="ch__avatar ch__avatar--sm" aria-hidden="true"></span>
          <p class="ch__text">Agreed, looking forward to the next update.</p>
        </div>
        <div class="ch__reply">
          <span class="ch__avatar ch__avatar--sm" aria-hidden="true"></span>
          <p class="ch__text">It would be great if you could add screenshots too.</p>
        </div>
      </div>
    </div>
  </div>
  <p class="ch__sr" id="chStatus" aria-live="polite"></p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-ink);

.ch {
  position: relative;
  width: min(92vw, calc(160vh - 12.8vw));
  aspect-ratio: 480 / 300;
  margin: 0 auto;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
  background: $tick-panel;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.ch__card {
  padding: $sp-4;
  box-sizing: border-box;
  height: 100%;
  overflow: hidden;
}

.ch__row { display: flex; align-items: flex-start; gap: $sp-2; }

.ch__avatar {
  flex: none;
  width: 24px;
  height: 24px;
  border-radius: 50%;
  background: linear-gradient(160deg, $subject-pink, $subject-lilac);
  margin-top: 4px;

  &--sm { width: 18px; height: 18px; }
}

.ch__body { min-width: 0; }
.ch__meta { margin: 0 0 4px; }
.ch__name { color: #fff; font-size: 12.5px; font-weight: 700; }
.ch__text { margin: 0 0 4px; color: rgba(255, 255, 255, .86); font-size: 12.5px; line-height: 1.5; }

.ch__toggle {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  border: 0;
  background: transparent;
  color: $subject-blue-light;
  font-size: 11.5px;
  font-weight: 600;
  padding: $sp-1 $sp-2 $sp-1 0;
  cursor: pointer;
  border-radius: $r-xs;
}

.ch__toggle:focus-visible { outline: 2px solid $subject-blue-light; outline-offset: 2px; }

.ch__toggleIcon { width: 12px; height: 12px; transform-origin: center; }

.ch__panel {
  overflow: hidden;
  margin-left: 32px;
}

.ch__panelInner {
  display: flex;
  flex-direction: column;
  gap: $sp-2;
  padding-top: $sp-2;
}

.ch__reply {
  display: flex;
  align-items: flex-start;
  gap: $sp-2;
  padding: $sp-2;
  background: rgba(255, 255, 255, .05);
  border-radius: $r-sm;

  .ch__text { margin: 0; }
}

.ch__sr {
  position: absolute;
  width: 1px; height: 1px;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

@media (max-width: 340px) {
  .ch__card { padding: $sp-3; }
  .ch__text, .ch__name { font-size: 11.5px; }
}

@media (prefers-reduced-motion: reduce) {
  .ch__panel, .ch__toggleIcon { transition: none !important; }
}

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);
}

.ch {
  position: relative;
  width: min(92vw, 160vh - 12.8vw);
  aspect-ratio: 480/300;
  margin: 0 auto;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  background: #231f28;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.ch__card {
  padding: 16px;
  box-sizing: border-box;
  height: 100%;
  overflow: hidden;
}

.ch__row {
  display: flex;
  align-items: flex-start;
  gap: 8px;
}

.ch__avatar {
  flex: none;
  width: 24px;
  height: 24px;
  border-radius: 50%;
  background: linear-gradient(160deg, #ff9ebb, #b9a0ff);
  margin-top: 2px;
}
.ch__avatar--sm {
  width: 18px;
  height: 18px;
}

.ch__body {
  min-width: 0;
}

.ch__meta {
  margin: 0 0 2px;
}

.ch__name {
  color: #fff;
  font-size: 12.5px;
  font-weight: 700;
}

.ch__text {
  margin: 0 0 4px;
  color: rgba(255, 255, 255, 0.86);
  font-size: 12.5px;
  line-height: 1.5;
}

.ch__toggle {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  border: 0;
  background: transparent;
  color: #5b8cf8;
  font-size: 11.5px;
  font-weight: 600;
  padding: 4px 8px 4px 0;
  cursor: pointer;
  border-radius: 4px;
}

.ch__toggle:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

.ch__toggleIcon {
  width: 12px;
  height: 12px;
  transform-origin: center;
}

.ch__panel {
  overflow: hidden;
  margin-left: 32px;
}

.ch__panelInner {
  display: flex;
  flex-direction: column;
  gap: 8px;
  padding-top: 8px;
}

.ch__reply {
  display: flex;
  align-items: flex-start;
  gap: 8px;
  padding: 8px;
  background: rgba(255, 255, 255, 0.05);
  border-radius: 8px;
}
.ch__reply .ch__text {
  margin: 0;
}

.ch__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip: rect(0 0 0 0);
  white-space: nowrap;
}

@media (max-width: 340px) {
  .ch__card {
    padding: 12px;
  }
  .ch__text, .ch__name {
    font-size: 11.5px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ch__panel, .ch__toggleIcon {
    transition: none !important;
  }
}

(function () {
  // 기법: 패널을 height:0 으로 접어 두고 펼칠 때 height:auto 로 잠깐 바꿔 scrollHeight 를
  // 실측한 뒤(레이아웃은 그 순간 한 번만 재계산), 그 실측값을 목표로 스프링 이징(선형 샘플
  // 곡선, 100% 를 살짝 넘겼다가 되돌아오는 값들)으로 height 를 직접 움직인다. 같은 이징·같은
  // 진행률로 화살표를 0→180도 회전시켜 "펼침 비율만큼 회전"을 구현한다.
  var root = document.getElementById('chRoot');
  var toggle = document.getElementById('chToggle');
  var icon = document.getElementById('chIcon');
  var panel = document.getElementById('chPanel');
  var inner = document.getElementById('chPanelInner');
  var status = document.getElementById('chStatus');
  var label = toggle.querySelector('.ch__toggleLabel');

  // $ease-lerp-settle 곡선(site/static/demo-tokens.scss)과 같은 샘플 — 1 을 넘겼다가(최대 1.126) 되돌아온다.
  var SPRING_SETTLE_EASING = 'linear(0.0, 0.051, 0.176, 0.341, 0.518, 0.685, 0.831, 0.947, 1.032, 1.087, 1.117, 1.126, 1.121, 1.105, 1.085, 1.062, 1.041, 1.022, 1.008, 0.997, 1)';

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) root.classList.remove('is-demo');

  var open = false;
  var panelAnim = null;
  var innerAnim = null;
  var iconAnim = null;
  var bgAnim = null;

  function measure() {
    panel.style.height = 'auto';
    inner.style.opacity = '1';
    var h = panel.scrollHeight;
    return h;
  }

  function setOpen(next, animate, delayMs) {
    open = next;
    toggle.setAttribute('aria-expanded', String(open));
    label.textContent = open ? 'Hide replies' : 'View 3 replies';
    status.textContent = open ? '3 replies expanded.' : 'Replies collapsed.';
    if (panelAnim) panelAnim.cancel();
    if (innerAnim) innerAnim.cancel();
    if (iconAnim) iconAnim.cancel();
    if (bgAnim) bgAnim.cancel();

    var target = measure();
    var from = open ? 0 : target;
    var to = open ? target : 0;
    panel.style.height = from + 'px';
    inner.style.opacity = open ? '0' : '1';

    if (!animate || reduce) {
      panel.style.height = to + 'px';
      inner.style.opacity = open ? '1' : '0';
      icon.style.transform = 'rotate(' + (open ? 180 : 0) + 'deg)';
      return;
    }
    // 계산: 패널이 실측한 픽셀 높이가 클수록 늘어나는 데 걸리는 시간도 조금 더 길게 잡는다.
    var dur = Math.max(320, Math.min(680, 300 + target * 0.9));
    panelAnim = panel.animate(
      [{ height: from + 'px' }, { height: to + 'px' }],
      { duration: dur, delay: delayMs || 0, easing: SPRING_SETTLE_EASING, fill: 'forwards' }
    );
    innerAnim = inner.animate(
      [{ opacity: open ? 0 : 1 }, { opacity: open ? 1 : 0 }],
      { duration: Math.round(dur * 0.6), delay: delayMs || 0, easing: 'ease-out', fill: 'forwards' }
    );
    iconAnim = icon.animate(
      [{ transform: 'rotate(' + (open ? 0 : 180) + 'deg)' }, { transform: 'rotate(' + (open ? 180 : 0) + 'deg)' }],
      { duration: dur, delay: delayMs || 0, easing: SPRING_SETTLE_EASING, fill: 'forwards' }
    );
    // 계산: 펼침 비율이 0→1(또는 반대)로 진행되는 같은 구간 동안, 패널 전체(카드 바탕)에도
    // 옅은 색 변화를 얹어 화살표·패널 경계 바깥까지 값을 전파한다(넓은 면적의 근거 있는 변화).
    bgAnim = root.animate(
      [{ backgroundColor: '#231f28' }, { backgroundColor: '#2b3550' }, { backgroundColor: '#231f28' }],
      { duration: Math.round(dur * 1.15), delay: delayMs || 0, easing: 'ease-out' }
    );
  }

  panel.style.height = '0px';
  inner.style.opacity = '0';

  toggle.addEventListener('click', function () {
    root.classList.remove('is-demo');
    setOpen(!open, true, 0);
  });
  toggle.addEventListener('keydown', function (e) {
    if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); toggle.click(); }
  });

  var seqTimer = null;
  function playSequence() {
    if (seqTimer) { window.clearTimeout(seqTimer); seqTimer = null; }
    setOpen(false, false, 0);
    setOpen(true, true, 200);
    seqTimer = window.setTimeout(function () {
      setOpen(false, true, 0);
    }, 1080);
  }

  if (reduce) {
    setOpen(true, false, 0);
  } else {
    playSequence();
    window.setInterval(function () {
      if (!root.classList.contains('is-demo')) return;
      playSequence();
    }, 2600);
  }
})();

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

// $ease-lerp-settle 곡선(site/static/demo-tokens.scss)과 같은 샘플 — 1 을 넘겼다가(최대 1.126) 되돌아온다.
const SPRING_SETTLE_EASING =
  "linear(0.0, 0.051, 0.176, 0.341, 0.518, 0.685, 0.831, 0.947, 1.032, 1.087, 1.117, 1.126, 1.121, 1.105, 1.085, 1.062, 1.041, 1.022, 1.008, 0.997, 1)";

export default function CommentThreadCollapseHeightSpring() {
  const rootRef = useRef<HTMLDivElement>(null);
  const toggleRef = useRef<HTMLButtonElement>(null);
  const iconRef = useRef<SVGSVGElement>(null);
  const panelRef = useRef<HTMLDivElement>(null);
  const innerRef = useRef<HTMLDivElement>(null);
  const labelRef = useRef<HTMLSpanElement>(null);
  const statusRef = useRef<HTMLParagraphElement>(null);

  useEffect(() => {
    const root = rootRef.current;
    const toggle = toggleRef.current;
    const icon = iconRef.current;
    const panel = panelRef.current;
    const inner = innerRef.current;
    const label = labelRef.current;
    const status = statusRef.current;
    if (!root || !toggle || !icon || !panel || !inner || !label || !status) return;

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (reduce) root.classList.remove("is-demo");

    let open = false;
    let panelAnim: Animation | null = null;
    let innerAnim: Animation | null = null;
    let iconAnim: Animation | null = null;
    let bgAnim: Animation | null = null;

    function measure() {
      panel!.style.height = "auto";
      inner!.style.opacity = "1";
      return panel!.scrollHeight;
    }

    function setOpen(next: boolean, animate: boolean, delayMs: number) {
      open = next;
      toggle!.setAttribute("aria-expanded", String(open));
      label!.textContent = open ? "답글 숨기기" : "답글 3개 보기";
      status!.textContent = open ? "답글 3개가 펼쳐졌습니다." : "답글이 접혔습니다.";
      if (panelAnim) panelAnim.cancel();
      if (innerAnim) innerAnim.cancel();
      if (iconAnim) iconAnim.cancel();
      if (bgAnim) bgAnim.cancel();

      const target = measure();
      const from = open ? 0 : target;
      const to = open ? target : 0;
      panel!.style.height = from + "px";
      inner!.style.opacity = open ? "0" : "1";

      if (!animate || reduce) {
        panel!.style.height = to + "px";
        inner!.style.opacity = open ? "1" : "0";
        icon!.style.transform = `rotate(${open ? 180 : 0}deg)`;
        return;
      }
      const dur = Math.max(320, Math.min(680, 300 + target * 0.9));
      panelAnim = panel!.animate(
        [{ height: from + "px" }, { height: to + "px" }],
        { duration: dur, delay: delayMs, easing: SPRING_SETTLE_EASING, fill: "forwards" }
      );
      innerAnim = inner!.animate(
        [{ opacity: open ? 0 : 1 }, { opacity: open ? 1 : 0 }],
        { duration: Math.round(dur * 0.6), delay: delayMs, easing: "ease-out", fill: "forwards" }
      );
      iconAnim = icon!.animate(
        [{ transform: `rotate(${open ? 0 : 180}deg)` }, { transform: `rotate(${open ? 180 : 0}deg)` }],
        { duration: dur, delay: delayMs, easing: SPRING_SETTLE_EASING, fill: "forwards" }
      );
      // 계산: 펼침 비율이 0→1(또는 반대)로 진행되는 같은 구간 동안, 패널 전체(카드 바탕)에도
      // 옅은 색 변화를 얹어 화살표·패널 경계 바깥까지 값을 전파한다(넓은 면적의 근거 있는 변화).
      bgAnim = root!.animate(
        [{ backgroundColor: "#231f28" }, { backgroundColor: "#2b3550" }, { backgroundColor: "#231f28" }],
        { duration: Math.round(dur * 1.15), delay: delayMs, easing: "ease-out" }
      );
    }

    panel.style.height = "0px";
    inner.style.opacity = "0";

    const onActivate = () => { root.classList.remove("is-demo"); setOpen(!open, true, 0); };
    toggle.addEventListener("click", onActivate);
    toggle.addEventListener("keydown", (e) => {
      if (e.key === "Enter" || e.key === " ") { e.preventDefault(); onActivate(); }
    });

    let seqTimer: number | undefined;
    function playSequence() {
      if (seqTimer) { window.clearTimeout(seqTimer); seqTimer = undefined; }
      setOpen(false, false, 0);
      setOpen(true, true, 200);
      seqTimer = window.setTimeout(() => { setOpen(false, true, 0); }, 1080);
    }

    let timer: number | undefined;
    if (reduce) {
      setOpen(true, false, 0);
    } else {
      playSequence();
      timer = window.setInterval(() => {
        if (!root.classList.contains("is-demo")) return;
        playSequence();
      }, 2600);
    }

    return () => { if (timer) window.clearInterval(timer); if (seqTimer) window.clearTimeout(seqTimer); };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={`${styles.ch} is-demo`} ref={rootRef}>
        <div className={styles.ch__card}>
          <div className={styles.ch__row}>
            <span className={styles.ch__avatar} aria-hidden="true" />
            <div className={styles.ch__body}>
              <p className={styles.ch__meta}><strong className={styles.ch__name}>코랄북스</strong></p>
              <p className={styles.ch__text}>이 스레드 답글이 꽤 쌓였네요, 펼쳐서 볼게요.</p>
              <button type="button" className={styles.ch__toggle} ref={toggleRef} aria-expanded="false" aria-controls="chPanel">
                <svg className={styles.ch__toggleIcon} ref={iconRef} viewBox="0 0 256 256" aria-hidden="true">
                  <g fill="currentColor">
                    <path d="m208 96l-80 80l-80-80Z" opacity=".2" />
                    <path d="M215.39 92.94A8 8 0 0 0 208 88H48a8 8 0 0 0-5.66 13.66l80 80a8 8 0 0 0 11.32 0l80-80a8 8 0 0 0 1.73-8.72M128 164.69L67.31 104h121.38Z" />
                  </g>
                </svg>
                <span ref={labelRef}>답글 3개 보기</span>
              </button>
            </div>
          </div>
          <div className={styles.ch__panel} id="chPanel" ref={panelRef}>
            <div className={styles.ch__panelInner} ref={innerRef}>
              <div className={styles.ch__reply}>
                <span className={`${styles.ch__avatar} ${styles["ch__avatar--sm"]}`} aria-hidden="true" />
                <p className={styles.ch__text}>저장 버튼 위치가 헷갈렸는데 이제 명확해요.</p>
              </div>
              <div className={styles.ch__reply}>
                <span className={`${styles.ch__avatar} ${styles["ch__avatar--sm"]}`} aria-hidden="true" />
                <p className={styles.ch__text}>동감이에요, 다음 업데이트도 기대할게요.</p>
              </div>
              <div className={styles.ch__reply}>
                <span className={`${styles.ch__avatar} ${styles["ch__avatar--sm"]}`} aria-hidden="true" />
                <p className={styles.ch__text}>스크린샷도 같이 올려주시면 좋을 것 같아요.</p>
              </div>
            </div>
          </div>
        </div>
        <p className={styles.ch__sr} ref={statusRef} aria-live="polite" />
      </div>
    </main>
  );
}