01固有の名前値で要素を対応させてつなぐ遷移

<main class="stage">
<div class="np is-demo" id="np">
  <div class="np__grid" id="npGrid">
    <button type="button" class="np__card" id="npCard0" data-id="0" aria-label="アルファを開く">
      <span class="np__badge">A</span><span class="np__t">アルファ</span>
    </button>
    <button type="button" class="np__card" id="npCard1" data-id="1" aria-label="ベータを開く">
      <span class="np__badge">B</span><span class="np__t">ベータ</span>
    </button>
    <button type="button" class="np__card" id="npCard2" data-id="2" aria-label="ガンマを開く">
      <span class="np__badge">C</span><span class="np__t">ガンマ</span>
    </button>
  </div>
  <div class="np__detail" id="npDetail" hidden>
    <button type="button" class="np__back" id="npBack" aria-label="一覧へ">戻る</button>
    <span class="np__badge np__badge--lg" id="npDetailBadge">A</span>
    <p class="np__title" id="npDetailTitle">アルファ</p>
    <p class="np__desc">名札の値 <code id="npName">card-0</code>でペアになった</p>
  </div>
</div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-enter;
$easing: $ease-spring;
$color: $subject-blue-deep;

@include stage($stage-ink);

.np { position: relative; width: min(100%, 320px); height: 200px; }

.np__grid {
  display: grid; grid-template-columns: repeat(3, 1fr); gap: $sp-3;
  width: 100%; height: 100%; align-content: center;
}
// [hidden] 은 UA 스타일시트라 같은 특이도의 author .np__grid{display:grid} 에 밀린다 —
// 명시적으로 덮어써야 실제로 사라진다(안 그러면 두 패널이 동시에 렌더돼 이름표 값이 겹친다)
.np__grid[hidden], .np__detail[hidden] { display: none; }
.np__card {
  view-transition-name: none;
  display: flex; flex-direction: column; align-items: center; justify-content: center; gap: $sp-2;
  border: none; border-radius: $r-card; background: $subject-cream; cursor: pointer;
  padding: $sp-4 $sp-2; height: 108px;
}
.np__card:focus-visible { outline: 2px solid $subject-cream; outline-offset: 2px; }

.np__badge {
  display: grid; place-items: center; width: 36px; height: 36px; border-radius: $r-pill;
  background: $color; color: $subject-cream; font: 700 15px/1 -apple-system, "Segoe UI", sans-serif;
}
.np__badge--lg { width: 56px; height: 56px; font-size: 22px; }
.np__t { font: 600 12px/1 -apple-system, "Segoe UI", sans-serif; color: $subject-ink; }

.np__detail {
  position: relative; width: 100%; height: 100%; box-sizing: border-box;
  display: flex; flex-direction: column; align-items: center; justify-content: center; gap: $sp-2;
  border-radius: $r-card; background: $subject-cream; padding: $sp-4;
}
.np__back {
  position: absolute; top: $sp-3; left: $sp-3; border: none; border-radius: $r-pill;
  background: rgba($subject-ink, .1); color: $subject-ink; font: 700 11px/1 -apple-system, sans-serif;
  padding: $sp-1 $sp-3; cursor: pointer;
}
.np__title { margin: 0; font: 700 18px/1 -apple-system, "Segoe UI", sans-serif; color: $subject-ink; }
.np__desc { margin: 0; font: 400 11px/1.4 -apple-system, sans-serif; color: rgba($subject-ink, .72); text-align: center; }
.np__desc code {
  display: inline-block; margin-top: $sp-1; padding: $sp-1 $sp-2; border-radius: $r-xs;
  background: rgba($subject-ink, .1); font: 700 11px/1.2 "Space Mono", monospace; color: $subject-ink;
}

// 실사용 원리: 열릴 카드와 상세 패널에 같은 view-transition-name 값을 순간적으로 실어
// document.startViewTransition() 을 부르면, 브라우저가 두 스냅샷의 위치·크기 차이를 자동으로 보간한다.
::view-transition-group(np-shared) {
  animation-duration: $duration;
  animation-timing-function: $easing;
}

@media (max-width: 340px) {
  .np { height: 160px; }
  .np__card { height: 80px; padding: $sp-2 $sp-1; gap: $sp-1; }
  .np__badge { width: 26px; height: 26px; font-size: 12px; }
  .np__detail { padding: $sp-3; gap: $sp-1; }
  .np__title { font-size: 15px; }
  .np__desc { font-size: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  ::view-transition-group(*),
  ::view-transition-old(*),
  ::view-transition-new(*) { animation: 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);
}

.np {
  position: relative;
  width: min(100%, 320px);
  height: 200px;
}

.np__grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 12px;
  width: 100%;
  height: 100%;
  align-content: center;
}

.np__grid[hidden], .np__detail[hidden] {
  display: none;
}

.np__card {
  view-transition-name: none;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 8px;
  border: none;
  border-radius: 16px;
  background: #fff7e6;
  cursor: pointer;
  padding: 16px 8px;
  height: 108px;
}

.np__card:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 2px;
}

.np__badge {
  display: grid;
  place-items: center;
  width: 36px;
  height: 36px;
  border-radius: 999px;
  background: #0a4fe8;
  color: #fff7e6;
  font: 700 15px/1 -apple-system, "Segoe UI", sans-serif;
}

.np__badge--lg {
  width: 56px;
  height: 56px;
  font-size: 22px;
}

.np__t {
  font: 600 12px/1 -apple-system, "Segoe UI", sans-serif;
  color: #17141a;
}

.np__detail {
  position: relative;
  width: 100%;
  height: 100%;
  box-sizing: border-box;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 8px;
  border-radius: 16px;
  background: #fff7e6;
  padding: 16px;
}

.np__back {
  position: absolute;
  top: 12px;
  left: 12px;
  border: none;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.1);
  color: #17141a;
  font: 700 11px/1 -apple-system, sans-serif;
  padding: 4px 12px;
  cursor: pointer;
}

.np__title {
  margin: 0;
  font: 700 18px/1 -apple-system, "Segoe UI", sans-serif;
  color: #17141a;
}

.np__desc {
  margin: 0;
  font: 400 11px/1.4 -apple-system, sans-serif;
  color: rgba(23, 20, 26, 0.72);
  text-align: center;
}

.np__desc code {
  display: inline-block;
  margin-top: 4px;
  padding: 4px 8px;
  border-radius: 4px;
  background: rgba(23, 20, 26, 0.1);
  font: 700 11px/1.2 "Space Mono", monospace;
  color: #17141a;
}

::view-transition-group(np-shared) {
  animation-duration: 600ms;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
}

@media (max-width: 340px) {
  .np {
    height: 160px;
  }
  .np__card {
    height: 80px;
    padding: 8px 4px;
    gap: 4px;
  }
  .np__badge {
    width: 26px;
    height: 26px;
    font-size: 12px;
  }
  .np__detail {
    padding: 12px;
    gap: 4px;
  }
  .np__title {
    font-size: 15px;
  }
  .np__desc {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  ::view-transition-group(*),
  ::view-transition-old(*),
  ::view-transition-new(*) {
    animation: none !important;
  }
}

(function () {
  var SUPPORTS_VT = 'startViewTransition' in document;

  var root = document.getElementById('np');
  var grid = document.getElementById('npGrid');
  var detail = document.getElementById('npDetail');
  var back = document.getElementById('npBack');
  var cards = [0, 1, 2].map(function (i) { return document.getElementById('npCard' + i); });
  var detailBadge = document.getElementById('npDetailBadge');
  var detailTitle = document.getElementById('npDetailTitle');
  var nameEl = document.getElementById('npName');
  var DATA = [{ b: 'A', t: 'アルファ' }, { b: 'B', t: 'ベータ' }, { b: 'C', t: 'ガンマ' }];
  var HOLD_MS = 1000;
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var opened = null;
  var isDemo = true;

  function human() {
    if (isDemo) { root.classList.remove('is-demo'); isDemo = false; }
  }

  // 계산 겹: 名札の値(view-transition-name)을 그 순간 선택된 카드 하나에만 실어 보낸다 —
  // 모든 카드에 상시 부여하면 이름이 겹쳐 매칭이 깨진다. 값은 매번 새로 계산해서 준다.
  function assignName(id, on) {
    cards[id].style.viewTransitionName = on ? 'np-shared' : '';
  }

  function openRender(id) {
    if (opened !== null) assignName(opened, false);
    assignName(id, true);
    detail.style.viewTransitionName = 'np-shared';
    grid.hidden = true;
    detail.hidden = false;
    detailBadge.textContent = DATA[id].b;
    detailTitle.textContent = DATA[id].t;
    nameEl.textContent = 'card-' + id;
    opened = id;
  }

  function closeRender() {
    detail.hidden = true;
    grid.hidden = false;
    if (opened !== null) assignName(opened, false);
    detail.style.viewTransitionName = '';
    opened = null;
  }

  function change(fn) {
    if (SUPPORTS_VT) { document.startViewTransition(fn); }
    else { fn(); }
  }

  function open(id) {
    if (opened === id) return;
    change(function () { openRender(id); });
  }

  function close() {
    if (opened === null) return;
    change(closeRender);
  }

  cards.forEach(function (c, i) {
    c.addEventListener('click', function () { human(); open(i); });
  });
  back.addEventListener('click', function () { human(); close(); });
  root.addEventListener('keydown', function (e) {
    if (e.key === 'Escape') { human(); close(); }
  });

  var seq = [0, 1, 2];
  var si = 0;
  function loop() {
    if (!isDemo) return;
    if (opened === null) { open(seq[si % seq.length]); si++; }
    else { close(); }
    window.setTimeout(loop, HOLD_MS);
  }

  if (!reduce) {
    window.setTimeout(loop, HOLD_MS);
  } else {
    root.classList.remove('is-demo');
  }
})();

// NameTagPairSwap — vanilla 판(run/401/demo/01-view-transition-name-pair-swap/index.html) 의 React 포트.
// 카드와 상세 패널에 같은 view-transition-name 값(np-shared)을 그 순간에만 실어 startViewTransition 을 부른다.
import { useEffect, useRef } from "react";
import styles from "./NameTagPairSwap.module.scss";

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

const DATA = [
  { b: "A", t: "알파" },
  { b: "B", t: "베타" },
  { b: "C", t: "감마" },
];
const HOLD_MS = 1000;

export default function NameTagPairSwap() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const gridRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const detailRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const cardRefs = useRef([null, null, null]) as unknown as Box<(HTMLButtonElement | null)[]>;
  const badgeRef = useRef(null) as unknown as Box<HTMLElement>;
  const titleRef = useRef(null) as unknown as Box<HTMLElement>;
  const nameRef = useRef(null) as unknown as Box<HTMLElement>;
  const timerRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const SUPPORTS_VT = "startViewTransition" in document;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let isDemo = true;
    let opened: number | null = null;

    function human() {
      if (isDemo) { rootRef.current?.classList.remove(styles.isDemo); isDemo = false; }
    }

    // 계산 겹: 이름표 값(view-transition-name)을 그 순간 선택된 카드 하나에만 실어 보낸다 —
    // 모든 카드에 상시 부여하면 이름이 겹쳐 매칭이 깨진다. 값은 매번 새로 계산해서 준다.
    function assignName(id: number, on: boolean) {
      const c = cardRefs.current[id];
      if (c) c.style.viewTransitionName = on ? "np-shared" : "";
    }

    function openRender(id: number) {
      if (opened !== null) assignName(opened, false);
      assignName(id, true);
      const detail = detailRef.current;
      const grid = gridRef.current;
      if (detail) detail.style.viewTransitionName = "np-shared";
      grid?.setAttribute("hidden", "");
      detail?.removeAttribute("hidden");
      if (badgeRef.current) badgeRef.current.textContent = DATA[id].b;
      if (titleRef.current) titleRef.current.textContent = DATA[id].t;
      if (nameRef.current) nameRef.current.textContent = "card-" + id;
      opened = id;
    }

    function closeRender() {
      detailRef.current?.setAttribute("hidden", "");
      gridRef.current?.removeAttribute("hidden");
      if (opened !== null) assignName(opened, false);
      if (detailRef.current) detailRef.current.style.viewTransitionName = "";
      opened = null;
    }

    function change(fn: () => void) {
      if (SUPPORTS_VT) { document.startViewTransition(fn); }
      else { fn(); }
    }

    function open(id: number) {
      if (opened === id) return;
      change(() => openRender(id));
    }
    function close() {
      if (opened === null) return;
      change(closeRender);
    }

    function onCardClick(i: number) { return () => { human(); open(i); }; }
    const clickHandlers = [0, 1, 2].map(onCardClick);
    cardRefs.current.forEach((c, i) => c?.addEventListener("click", clickHandlers[i]));
    function onBackClick() { human(); close(); }
    detailRef.current; // noop keep ref referenced
    const backBtn = rootRef.current?.querySelector<HTMLButtonElement>("[data-back]");
    backBtn?.addEventListener("click", onBackClick);
    function onKeydown(e: KeyboardEvent) {
      if (e.key === "Escape") { human(); close(); }
    }
    rootRef.current?.addEventListener("keydown", onKeydown);

    const seq = [0, 1, 2];
    let si = 0;
    function loop() {
      if (!isDemo) return;
      if (opened === null) { open(seq[si % seq.length]); si++; }
      else { close(); }
      timerRef.current = window.setTimeout(loop, HOLD_MS);
    }

    if (!reduce) {
      timerRef.current = window.setTimeout(loop, HOLD_MS);
    } else {
      rootRef.current?.classList.remove(styles.isDemo);
    }

    return () => {
      window.clearTimeout(timerRef.current);
      cardRefs.current.forEach((c, i) => c?.removeEventListener("click", clickHandlers[i]));
      backBtn?.removeEventListener("click", onBackClick);
      rootRef.current?.removeEventListener("keydown", onKeydown);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.np + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.grid} ref={gridRef}>
          <button type="button" className={styles.card} aria-label="알파 열기" ref={(el: HTMLButtonElement | null) => { cardRefs.current[0] = el; }}>
            <span className={styles.badge}>A</span><span className={styles.t}>알파</span>
          </button>
          <button type="button" className={styles.card} aria-label="베타 열기" ref={(el: HTMLButtonElement | null) => { cardRefs.current[1] = el; }}>
            <span className={styles.badge}>B</span><span className={styles.t}>베타</span>
          </button>
          <button type="button" className={styles.card} aria-label="감마 열기" ref={(el: HTMLButtonElement | null) => { cardRefs.current[2] = el; }}>
            <span className={styles.badge}>C</span><span className={styles.t}>감마</span>
          </button>
        </div>
        <div className={styles.detail} ref={detailRef} hidden>
          <button type="button" className={styles.back} data-back aria-label="목록으로">뒤로</button>
          <span className={styles.badge + " " + styles.badgeLg} ref={badgeRef}>A</span>
          <p className={styles.title} ref={titleRef}>알파</p>
          <p className={styles.desc}>이름표 값 <code ref={nameRef}>card-0</code>로 짝지어졌다</p>
        </div>
      </div>
    </main>
  );
}