02Form height that measures each step and flows to it

<main class="stage">
  <section class="ah is-demo" id="ah" aria-label="Application form whose steps hold different field counts">
    <div class="ah__bar" id="ah-bar" role="progressbar" aria-valuemin="1" aria-valuemax="3" aria-valuenow="1" aria-label="Step progress">
      <span class="ah__seg" data-seg="1"><span class="ah__seg-f"></span></span>
      <span class="ah__seg" data-seg="2"><span class="ah__seg-f"></span></span>
      <span class="ah__seg" data-seg="3"><span class="ah__seg-f"></span></span>
    </div>
    <div class="ah__box" id="ah-box">
      <div class="ah__panel is-on" data-step="1">
        <p class="ah__t">1 / 3 · Account</p>
        <input class="ah__f" type="text" placeholder="Email">
        <input class="ah__f" type="text" placeholder="Password">
      </div>
      <div class="ah__panel" data-step="2">
        <p class="ah__t">2 / 3 · Delivery</p>
        <input class="ah__f" type="text" placeholder="Recipient">
        <input class="ah__f" type="text" placeholder="Address">
        <input class="ah__f" type="text" placeholder="Phone">
      </div>
      <div class="ah__panel" data-step="3">
        <p class="ah__t">3 / 3 · Review</p>
        <input class="ah__f" type="text" placeholder="Coupon code">
        <p class="ah__note">Check what you typed on earlier steps before sending. Go back if something needs a fix.</p>
      </div>
    </div>
    <div class="ah__foot">
      <button class="ah__btn" type="button" id="ah-prev">Back</button>
      <button class="ah__btn ah__btn--go" type="button" id="ah-next">Next</button>
    </div>
  </section>
</main>

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

$duration: $dur-base;
$easing: $ease-spring;
$color: $subject-blue;

@include stage($stage-yellow);

$line: color.mix($subject-ink, $stage-yellow, 22%);
$soft: color.mix($subject-ink, $stage-yellow, 8%);

// 단계마다 다른 상자 높이. 길이형으로 등록해야 보간된다 —
// 등록하지 않은 커스텀 속성은 중간값이 없어 높이가 툭 튄다
@property --h {
  syntax: "<length>";
  inherits: true;
  initial-value: 63px;
}

// 패널 셋의 실측 높이(측정값과 맞춰 둔 루프용 값)
$h1: 63px;
$h2: 87px;
$h3: 70px;

.ah {
  box-sizing: border-box;
  width: min(304px, 92vw);
  padding: $sp-3;
  border: 1px solid $line;
  border-radius: $r-card;
  background: $subject-cream;
  color: $subject-ink;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  font-size: 12px;
  line-height: 1.3;
}

// ── 진행 막대 ─────────────────────────────────────
.ah__bar {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: $sp-1;
  margin-bottom: $sp-2;
}
.ah__seg {
  height: 4px;
  border-radius: $r-pill;
  background: $soft;
  overflow: hidden;
}
.ah__seg-f {
  display: block;
  height: 100%;
  border-radius: inherit;
  background: $color;
  transform: scaleX(0);
  transform-origin: left center; // 규칙8: 단계 막대 좌측에서 채움
  transition: transform $duration $easing;
}
// 정적(사람이 조작하는) 경로 — .is-demo 를 떼도 진행 상태가 보여야 한다
.ah__seg.is-done .ah__seg-f { transform: scaleX(1); }

// ── 높이가 흐르는 상자 ─────────────────────────────
.ah__box {
  --h: #{$h1};
  display: grid;
  align-items: start;
  height: var(--h);
  overflow: hidden;
  transition: --h $duration $easing;
}

.ah__panel {
  display: grid;
  grid-area: 1 / 1;
  align-content: start;
  gap: $sp-1;
  opacity: 0;
  visibility: hidden;
}
.ah__panel.is-on { opacity: 1; visibility: visible; }

.ah__t {
  margin: 0;
  color: rgba($subject-ink, .72);
  font-size: 11px;
  font-weight: 700;
  letter-spacing: .04em;
}

.ah__f {
  box-sizing: border-box;
  width: 100%;
  height: 20px;
  margin: 0;
  padding: 0 $sp-2;
  border: 1px solid $line;
  border-radius: $r-sm;
  background: $stage-paper;
  color: $subject-ink;
  font-family: inherit;
  font-size: 11px;
  line-height: 1.3;
}
.ah__f:focus-visible { outline: 2px solid $color; outline-offset: 2px; }

.ah__note {
  margin: 0;
  color: color.mix($subject-ink, $subject-cream, 62%);
  font-size: 11px;
  line-height: 1.25;
}

// ── 발판 ─────────────────────────────────────────
.ah__foot {
  display: grid;
  grid-template-columns: auto auto;
  justify-content: space-between;
  gap: $sp-2;
  margin-top: $sp-2;
}
.ah__btn {
  margin: 0;
  padding: $sp-1 $sp-3;
  border: 1px solid $line;
  border-radius: $r-pill;
  background: transparent;
  color: color.mix($subject-ink, $subject-cream, 70%);
  font-family: inherit;
  font-size: 11px;
  font-weight: 700;
  line-height: 1.3;
  cursor: pointer;
}
.ah__btn--go { border-color: $subject-blue-deep; background: $subject-blue-deep; color: $subject-cream; }
.ah__btn:focus-visible { outline: 2px solid $color; outline-offset: 2px; }

// ── 격자 미리보기 전용 2초 루프 ─────────────────────
// --h 는 등록돼 있으니 키프레임 사이가 보간된다(상자가 흐른다).
// 패널 교체만 steps(1, end) 로 한 프레임에 자른다 — 반투명 유령 금지
@keyframes ah-h {
  0%, 18% { --h: #{$h1}; }
  30%, 46% { --h: #{$h2}; }
  58%, 72% { --h: #{$h3}; }
  86%, 100% { --h: #{$h1}; }
}
@keyframes ah-p1 {
  0%, 17% { opacity: 1; visibility: visible; }
  18%, 71% { opacity: 0; visibility: hidden; }
  72%, 100% { opacity: 1; visibility: visible; }
}
@keyframes ah-p2 {
  0%, 17% { opacity: 0; visibility: hidden; }
  18%, 45% { opacity: 1; visibility: visible; }
  46%, 100% { opacity: 0; visibility: hidden; }
}
@keyframes ah-p3 {
  0%, 45% { opacity: 0; visibility: hidden; }
  46%, 71% { opacity: 1; visibility: visible; }
  72%, 100% { opacity: 0; visibility: hidden; }
}
@keyframes ah-seg2 {
  0%, 17% { transform: scaleX(0); }
  18% { transform: scaleX(0); }
  30%, 71% { transform: scaleX(1); }
  84%, 100% { transform: scaleX(0); }
}
@keyframes ah-seg3 {
  0%, 45% { transform: scaleX(0); }
  46% { transform: scaleX(0); }
  58%, 71% { transform: scaleX(1); }
  84%, 100% { transform: scaleX(0); }
}

.ah.is-demo {
  // 루프 중에는 전환을 끈다 — 전환이 살아 있으면 캐스케이드에서 애니메이션을 덮는다
  .ah__box, .ah__seg-f { transition: none; }
  .ah__box {
    animation-name: ah-h;
    animation-duration: $dur-loop;
    animation-timing-function: $easing;
    animation-iteration-count: infinite;
  }
  .ah__panel[data-step="1"] { animation-name: ah-p1; }
  .ah__panel[data-step="2"] { animation-name: ah-p2; }
  .ah__panel[data-step="3"] { animation-name: ah-p3; }
  .ah__panel {
    animation-duration: $dur-loop;
    animation-timing-function: steps(1, end);
    animation-iteration-count: infinite;
  }
  .ah__seg[data-seg="2"] .ah__seg-f { animation-name: ah-seg2; }
  .ah__seg[data-seg="3"] .ah__seg-f { animation-name: ah-seg3; }
  .ah__seg[data-seg="2"] .ah__seg-f,
  .ah__seg[data-seg="3"] .ah__seg-f {
    animation-duration: $dur-loop;
    animation-timing-function: $easing;
    animation-iteration-count: infinite;
  }
}

@media (max-width: 340px) {
  .ah { padding: $sp-2; font-size: 11px; }
  .ah__btn { padding: 0 $sp-2; }
  .ah__note { font-size: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  .ah.is-demo .ah__box,
  .ah.is-demo .ah__panel,
  .ah.is-demo .ah__seg-f { animation: none !important; transition: none !important; }
  // 움직임만 끄고 진행 상태는 남긴다 — 칸 셋짜리 2단계에 맞춰진 상자
  .ah.is-demo .ah__box { --h: #{$h2} !important; }
  .ah.is-demo .ah__panel[data-step="1"],
  .ah.is-demo .ah__panel[data-step="3"] { opacity: 0 !important; visibility: hidden !important; }
  .ah.is-demo .ah__panel[data-step="2"] { opacity: 1 !important; visibility: visible !important; }
  .ah.is-demo .ah__seg[data-seg="1"] .ah__seg-f,
  .ah.is-demo .ah__seg[data-seg="2"] .ah__seg-f { transform: scaleX(1) !important; }
  .ah.is-demo .ah__seg[data-seg="3"] .ah__seg-f { transform: scaleX(0) !important; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

@property --h {
  syntax: "<length>";
  inherits: true;
  initial-value: 63px;
}
.ah {
  box-sizing: border-box;
  width: min(304px, 92vw);
  padding: 12px;
  border: 1px solid rgb(79.9843137255%, 65.9607843137%, 21.5137254902%);
  border-radius: 16px;
  background: #fff7e6;
  color: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  font-size: 12px;
  line-height: 1.3;
}

.ah__bar {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 4px;
  margin-bottom: 8px;
}

.ah__seg {
  height: 4px;
  border-radius: 999px;
  background: rgb(92.7215686275%, 76.3921568627%, 23.5450980392%);
  overflow: hidden;
}

.ah__seg-f {
  display: block;
  height: 100%;
  border-radius: inherit;
  background: #2f6df6;
  transform: scaleX(0);
  transform-origin: left center;
  transition: transform 300ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.ah__seg.is-done .ah__seg-f {
  transform: scaleX(1);
}

.ah__box {
  --h: 63px;
  display: grid;
  align-items: start;
  height: var(--h);
  overflow: hidden;
  transition: --h 300ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.ah__panel {
  display: grid;
  grid-area: 1/1;
  align-content: start;
  gap: 4px;
  opacity: 0;
  visibility: hidden;
}

.ah__panel.is-on {
  opacity: 1;
  visibility: visible;
}

.ah__t {
  margin: 0;
  color: rgba(23, 20, 26, 0.72);
  font-size: 11px;
  font-weight: 700;
  letter-spacing: 0.04em;
}

.ah__f {
  box-sizing: border-box;
  width: 100%;
  height: 20px;
  margin: 0;
  padding: 0 8px;
  border: 1px solid rgb(79.9843137255%, 65.9607843137%, 21.5137254902%);
  border-radius: 8px;
  background: #ffffff;
  color: #17141a;
  font-family: inherit;
  font-size: 11px;
  line-height: 1.3;
}

.ah__f:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

.ah__note {
  margin: 0;
  color: rgb(43.5921568627%, 41.6705882353%, 40.5960784314%);
  font-size: 11px;
  line-height: 1.25;
}

.ah__foot {
  display: grid;
  grid-template-columns: auto auto;
  justify-content: space-between;
  gap: 8px;
  margin-top: 8px;
}

.ah__btn {
  margin: 0;
  padding: 4px 12px;
  border: 1px solid rgb(79.9843137255%, 65.9607843137%, 21.5137254902%);
  border-radius: 999px;
  background: transparent;
  color: rgb(36.3137254902%, 34.5490196078%, 34.1960784314%);
  font-family: inherit;
  font-size: 11px;
  font-weight: 700;
  line-height: 1.3;
  cursor: pointer;
}

.ah__btn--go {
  border-color: #0a4fe8;
  background: #0a4fe8;
  color: #fff7e6;
}

.ah__btn:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

@keyframes ah-h {
  0%, 18% {
    --h: 63px;
  }
  30%, 46% {
    --h: 87px;
  }
  58%, 72% {
    --h: 70px;
  }
  86%, 100% {
    --h: 63px;
  }
}
@keyframes ah-p1 {
  0%, 17% {
    opacity: 1;
    visibility: visible;
  }
  18%, 71% {
    opacity: 0;
    visibility: hidden;
  }
  72%, 100% {
    opacity: 1;
    visibility: visible;
  }
}
@keyframes ah-p2 {
  0%, 17% {
    opacity: 0;
    visibility: hidden;
  }
  18%, 45% {
    opacity: 1;
    visibility: visible;
  }
  46%, 100% {
    opacity: 0;
    visibility: hidden;
  }
}
@keyframes ah-p3 {
  0%, 45% {
    opacity: 0;
    visibility: hidden;
  }
  46%, 71% {
    opacity: 1;
    visibility: visible;
  }
  72%, 100% {
    opacity: 0;
    visibility: hidden;
  }
}
@keyframes ah-seg2 {
  0%, 17% {
    transform: scaleX(0);
  }
  18% {
    transform: scaleX(0);
  }
  30%, 71% {
    transform: scaleX(1);
  }
  84%, 100% {
    transform: scaleX(0);
  }
}
@keyframes ah-seg3 {
  0%, 45% {
    transform: scaleX(0);
  }
  46% {
    transform: scaleX(0);
  }
  58%, 71% {
    transform: scaleX(1);
  }
  84%, 100% {
    transform: scaleX(0);
  }
}
.ah.is-demo .ah__box, .ah.is-demo .ah__seg-f {
  transition: none;
}
.ah.is-demo .ah__box {
  animation-name: ah-h;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
}
.ah.is-demo .ah__panel[data-step="1"] {
  animation-name: ah-p1;
}
.ah.is-demo .ah__panel[data-step="2"] {
  animation-name: ah-p2;
}
.ah.is-demo .ah__panel[data-step="3"] {
  animation-name: ah-p3;
}
.ah.is-demo .ah__panel {
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.ah.is-demo .ah__seg[data-seg="2"] .ah__seg-f {
  animation-name: ah-seg2;
}
.ah.is-demo .ah__seg[data-seg="3"] .ah__seg-f {
  animation-name: ah-seg3;
}
.ah.is-demo .ah__seg[data-seg="2"] .ah__seg-f,
.ah.is-demo .ah__seg[data-seg="3"] .ah__seg-f {
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
}

@media (max-width: 340px) {
  .ah {
    padding: 8px;
    font-size: 11px;
  }
  .ah__btn {
    padding: 0 8px;
  }
  .ah__note {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ah.is-demo .ah__box,
  .ah.is-demo .ah__panel,
  .ah.is-demo .ah__seg-f {
    animation: none !important;
    transition: none !important;
  }
  .ah.is-demo .ah__box {
    --h: 87px !important;
  }
  .ah.is-demo .ah__panel[data-step="1"],
  .ah.is-demo .ah__panel[data-step="3"] {
    opacity: 0 !important;
    visibility: hidden !important;
  }
  .ah.is-demo .ah__panel[data-step="2"] {
    opacity: 1 !important;
    visibility: visible !important;
  }
  .ah.is-demo .ah__seg[data-seg="1"] .ah__seg-f,
  .ah.is-demo .ah__seg[data-seg="2"] .ah__seg-f {
    transform: scaleX(1) !important;
  }
  .ah.is-demo .ah__seg[data-seg="3"] .ah__seg-f {
    transform: scaleX(0) !important;
  }
}

// 실제 동작: 단계를 바꿀 때마다 그 패널의 실제 높이를 재서 --h 에 넣는다.
// --h 는 길이형으로 등록돼 있어 상자가 그 값까지 흐른다(짐작한 고정 높이가 아니다).
var ah = document.getElementById('ah');
var ahBox = document.getElementById('ah-box');
var ahBar = document.getElementById('ah-bar');
var ahPanels = [].slice.call(ah.querySelectorAll('.ah__panel'));
var ahSegs = [].slice.call(ah.querySelectorAll('.ah__seg'));
var ahStep = 1;
function ahGo(n) {
  ahStep = Math.max(1, Math.min(ahPanels.length, n));
  ahPanels.forEach(function (p, i) { p.classList.toggle('is-on', i + 1 === ahStep); });
  ahSegs.forEach(function (s, i) { s.classList.toggle('is-done', i + 1 <= ahStep); });
  var h = Math.ceil(ahPanels[ahStep - 1].getBoundingClientRect().height);
  ahBox.style.setProperty('--h', h + 'px');
  ahBar.setAttribute('aria-valuenow', String(ahStep));
}
function ahMove(d) { ah.classList.remove('is-demo'); ahGo(ahStep + d); }
document.getElementById('ah-next').addEventListener('click', function () { ahMove(1); });
document.getElementById('ah-prev').addEventListener('click', function () { ahMove(-1); });
ah.addEventListener('keydown', function (e) {
  if (e.key === 'ArrowRight') { e.preventDefault(); ahMove(1); }
  if (e.key === 'ArrowLeft') { e.preventDefault(); ahMove(-1); }
});
ahGo(1);

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

export type AutoHeightStep = {
  /** "1 / 3 · Account" */
  title: string;
  /** placeholder of each field on this step */
  fields: string[];
  note?: string;
};

export type AutoHeightTransitionProps = {
  steps?: AutoHeightStep[];
  prevLabel?: string;
  nextLabel?: string;
  groupLabel?: string;
  progressLabel?: string;
  /** 격자 미리보기용 2초 자동 루프. 사람이 손대면 꺼진다 */
  demo?: boolean;
  duration?: number;
  easing?: string;
  color?: string;
};

type KeyEvt = { key: string; preventDefault(): void };
type Box<T> = { current: T };

const DEFAULT_STEPS: AutoHeightStep[] = [
  { title: "1 / 3 · Account", fields: ["Email", "Password"] },
  { title: "2 / 3 · Delivery", fields: ["Recipient", "Address", "Phone"] },
  {
    title: "3 / 3 · Review",
    fields: ["Coupon code"],
    note: "Check what you typed on earlier steps before sending. Go back if something needs a fix.",
  },
];

export default function AutoHeightTransition({
  steps = DEFAULT_STEPS,
  prevLabel = "Back",
  nextLabel = "Next",
  groupLabel = "Application form with uneven field counts",
  progressLabel = "Step progress",
  demo = true,
  duration = 300,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#2f6df6",
}: AutoHeightTransitionProps) {
  const [step, setStep] = useState(1);
  const [live, setLive] = useState(demo);
  const [h, setH] = useState<number | null>(null);
  const panels = useRef<HTMLDivElement[]>([]) as Box<HTMLDivElement[]>;

  // 다음 패널의 실제 높이를 재서 --h 에 넣는다 — 짐작한 고정 높이가 아니다
  const measure = useCallback((n: number) => {
    const el = panels.current[n - 1];
    if (el) setH(Math.ceil(el.getBoundingClientRect().height));
  }, []);

  useEffect(() => {
    measure(step);
  }, [measure, step]);

  function move(d: number) {
    setLive(false);
    setStep((p) => Math.max(1, Math.min(steps.length, p + d)));
  }

  function onKey(e: KeyEvt) {
    if (e.key === "ArrowRight") {
      e.preventDefault();
      move(1);
    }
    if (e.key === "ArrowLeft") {
      e.preventDefault();
      move(-1);
    }
  }

  const vars: CSSProperties = {
    "--duration": `${duration}ms`,
    "--easing": easing,
    "--color": color,
  };
  const boxVars: CSSProperties = h === null ? {} : { "--h": `${h}px` };
  const cls = [styles.ah, live ? styles.isDemo : ""].filter(Boolean).join(" ");

  return (
    <section className={cls} style={vars} aria-label={groupLabel} onKeyDown={onKey}>
      <div
        className={styles.bar}
        role="progressbar"
        aria-valuemin={1}
        aria-valuemax={steps.length}
        aria-valuenow={step}
        aria-label={progressLabel}
      >
        {steps.map((s, i) => (
          <span
            key={s.title}
            className={[styles.seg, i + 1 <= step ? styles.isDone : ""].filter(Boolean).join(" ")}
            data-seg={i + 1}
          >
            <span className={styles.segF} />
          </span>
        ))}
      </div>
      <div className={styles.box} style={boxVars}>
        {steps.map((s, i) => (
          <div
            key={s.title}
            className={[styles.panel, i + 1 === step ? styles.isOn : ""].filter(Boolean).join(" ")}
            data-step={i + 1}
            ref={(el: HTMLDivElement | null) => {
              if (el) panels.current[i] = el;
            }}
          >
            <p className={styles.t}>{s.title}</p>
            {s.fields.map((f) => (
              <input key={f} className={styles.f} type="text" placeholder={f} />
            ))}
            {s.note ? <p className={styles.note}>{s.note}</p> : null}
          </div>
        ))}
      </div>
      <div className={styles.foot}>
        <button className={styles.btn} type="button" onClick={() => move(-1)}>
          {prevLabel}
        </button>
        <button className={[styles.btn, styles.btnGo].join(" ")} type="button" onClick={() => move(1)}>
          {nextLabel}
        </button>
      </div>
    </section>
  );
}