07対応可否の値を先に確認して退避する遷移

<main class="stage">
<div class="sf is-demo" id="sf">
  <div class="sf__card" id="sfCard">
    <p class="sf__num" id="sfNum">01</p>
  </div>
  <p class="sf__meta">
    <span class="sf__pill" id="sfSupport">対応値：あり</span>
    <span class="sf__pill sf__pill--path" id="sfPath">トランジション使用</span>
  </p>
  <button type="button" class="sf__btn" id="sfBtn" aria-label="対応状況の値を反転させて経路を切り替える">環境を切り替える</button>
</div>
</main>

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

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

@include stage($stage-orange);

.sf { width: min(100%, 260px); display: flex; flex-direction: column; align-items: center; gap: $sp-3; }

.sf__card {
  view-transition-name: sf-card;
  width: 120px; height: 90px; border-radius: $r-card; background: $subject-cream;
  display: grid; place-items: center;
}
.sf__card.is-alt { background: $color; }
.sf__num { margin: 0; font: 700 26px/1 "Space Mono", monospace; color: $color; }
.sf__card.is-alt .sf__num { color: $subject-cream; }

::view-transition-old(sf-card),
::view-transition-new(sf-card) {
  animation-duration: $duration;
  animation-timing-function: $easing;
}

.sf__meta { margin: 0; display: flex; gap: $sp-2; }
.sf__pill {
  border-radius: $r-pill; background: rgba($subject-ink, .1); color: $subject-ink;
  font: 700 10px/1 -apple-system, sans-serif; padding: $sp-1 $sp-3;
}
.sf__pill--path.is-fallback { background: $subject-ink; color: $subject-cream; }

.sf__btn {
  border: none; border-radius: $r-pill; cursor: pointer; background: $subject-ink;
  color: $subject-cream; font: 700 11px/1 -apple-system, sans-serif; padding: $sp-2 $sp-4;
}
.sf__btn:focus-visible { outline: 2px solid $subject-ink; outline-offset: 2px; }

@media (max-width: 340px) {
  .sf__card { width: 96px; height: 72px; }
  .sf__num { font-size: 20px; }
  .sf__pill { font-size: 9px; padding: $sp-1 $sp-2; }
}

@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: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sf {
  width: min(100%, 260px);
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.sf__card {
  view-transition-name: sf-card;
  width: 120px;
  height: 90px;
  border-radius: 16px;
  background: #fff7e6;
  display: grid;
  place-items: center;
}

.sf__card.is-alt {
  background: #17141a;
}

.sf__num {
  margin: 0;
  font: 700 26px/1 "Space Mono", monospace;
  color: #17141a;
}

.sf__card.is-alt .sf__num {
  color: #fff7e6;
}

::view-transition-old(sf-card),
::view-transition-new(sf-card) {
  animation-duration: 600ms;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
}

.sf__meta {
  margin: 0;
  display: flex;
  gap: 8px;
}

.sf__pill {
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.1);
  color: #17141a;
  font: 700 10px/1 -apple-system, sans-serif;
  padding: 4px 12px;
}

.sf__pill--path.is-fallback {
  background: #17141a;
  color: #fff7e6;
}

.sf__btn {
  border: none;
  border-radius: 999px;
  cursor: pointer;
  background: #17141a;
  color: #fff7e6;
  font: 700 11px/1 -apple-system, sans-serif;
  padding: 8px 16px;
}

.sf__btn:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .sf__card {
    width: 96px;
    height: 72px;
  }
  .sf__num {
    font-size: 20px;
  }
  .sf__pill {
    font-size: 9px;
    padding: 4px 8px;
  }
}
@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('sf');
  var card = document.getElementById('sfCard');
  var numEl = document.getElementById('sfNum');
  var supportEl = document.getElementById('sfSupport');
  var pathEl = document.getElementById('sfPath');
  var btn = document.getElementById('sfBtn');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var STATES = ['01', '02', '03'];
  var si = 0;
  var isDemo = true;
  // 데모 전용: 실제로는 지원하는 환경이지만, 지원하지 않는 환경의 물러나는 경로를 눈에 보이도록
  // 번갈아 흉내 낸다. 판정 코드 자체는 실제 SUPPORTS_VT 값을 그대로 쓴다.
  var simulateUnsupported = false;

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

  function renderLabels(usedTransition) {
    supportEl.textContent = simulateUnsupported ? '対応値：なし' : '対応値：あり';
    pathEl.textContent = usedTransition ? 'トランジション使用' : '即時切り替え（代替）';
    pathEl.classList.toggle('is-fallback', !usedTransition);
  }

  function applyState() {
    si = (si + 1) % STATES.length;
    numEl.textContent = STATES[si];
    card.classList.toggle('is-alt', si % 2 === 1);
  }

  // 이 함수가 규칙의 핵심이다: 지원 여부 값을 먼저 확인하고(SUPPORTS_VT), 시뮬레이션으로
  // "지원하지 않는 환경"을 흉내 낼 때는 지원 여부와 무관하게 即時切り替え（代替） 경로로 물러난다.
  function step() {
    var willUseTransition = SUPPORTS_VT && !simulateUnsupported;
    if (willUseTransition) {
      document.startViewTransition(applyState);
    } else {
      applyState();
    }
    renderLabels(willUseTransition);
  }

  btn.addEventListener('click', function () { human(); simulateUnsupported = !simulateUnsupported; step(); });
  root.addEventListener('keydown', function (e) {
    if (e.key === 'Enter' || e.key === ' ') { human(); e.preventDefault(); simulateUnsupported = !simulateUnsupported; step(); }
  });
  var HOLD_MS = 600;
  function loop() {
    if (!isDemo) return;
    simulateUnsupported = !simulateUnsupported;
    step();
    window.setTimeout(loop, HOLD_MS);
  }

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

// SkipFallback — vanilla 판(run/401/demo/07-view-transition-skip-fallback/index.html) 의 React 포트.
// 이 함수가 규칙의 핵심이다: 지원 여부 값을 먼저 확인하고(SUPPORTS_VT), 시뮬레이션으로
// "지원하지 않는 환경"을 흉내 낼 때는 지원 여부와 무관하게 즉시 전환(대체) 경로로 물러난다.
import { useEffect, useRef } from "react";
import styles from "./SkipFallback.module.scss";

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

const STATES = ["01", "02", "03"];
const HOLD_MS = 600;

export default function SkipFallback() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const cardRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const numRef = useRef(null) as unknown as Box<HTMLElement>;
  const supportRef = useRef(null) as unknown as Box<HTMLElement>;
  const pathRef = useRef(null) as unknown as Box<HTMLElement>;
  const btnRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  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 si = 0;
    // 데모 전용: 실제로는 지원하는 환경이지만, 지원하지 않는 환경의 물러나는 경로를 눈에 보이도록
    // 번갈아 흉내 낸다. 판정 코드 자체는 실제 SUPPORTS_VT 값을 그대로 쓴다.
    let simulateUnsupported = false;

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

    function renderLabels(usedTransition: boolean) {
      if (supportRef.current) supportRef.current.textContent = simulateUnsupported ? "지원 값 아니오" : "지원 값 예";
      if (pathRef.current) {
        pathRef.current.textContent = usedTransition ? "전환 사용" : "즉시 전환(대체)";
        pathRef.current.classList.toggle(styles.isFallback, !usedTransition);
      }
    }

    function applyState() {
      si = (si + 1) % STATES.length;
      if (numRef.current) numRef.current.textContent = STATES[si];
      cardRef.current?.classList.toggle(styles.isAlt, si % 2 === 1);
    }

    function step() {
      const willUseTransition = SUPPORTS_VT && !simulateUnsupported;
      if (willUseTransition) {
        document.startViewTransition(applyState);
      } else {
        applyState();
      }
      renderLabels(willUseTransition);
    }

    function onClick() { human(); simulateUnsupported = !simulateUnsupported; step(); }
    function onKeydown(e: KeyboardEvent) {
      if (e.key === "Enter" || e.key === " ") { human(); e.preventDefault(); simulateUnsupported = !simulateUnsupported; step(); }
    }

    btnRef.current?.addEventListener("click", onClick);
    rootRef.current?.addEventListener("keydown", onKeydown);

    function loop() {
      if (!isDemo) return;
      simulateUnsupported = !simulateUnsupported;
      step();
      timerRef.current = window.setTimeout(loop, HOLD_MS);
    }

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

    return () => {
      window.clearTimeout(timerRef.current);
      btnRef.current?.removeEventListener("click", onClick);
      rootRef.current?.removeEventListener("keydown", onKeydown);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.sf + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.card} ref={cardRef}>
          <p className={styles.num} ref={numRef}>01</p>
        </div>
        <p className={styles.meta}>
          <span className={styles.pill} ref={supportRef}>지원 값 예</span>
          <span className={styles.pill + " " + styles.pillPath} ref={pathRef}>전환 사용</span>
        </p>
        <button type="button" className={styles.btn} ref={btnRef} aria-label="지원 여부 값을 뒤집어 경로 전환">환경 뒤집기</button>
      </div>
    </main>
  );
}