09Vào màn hình bằng @starting-style

<main class="stage">
  <div class="scene">
    <p class="scene__label">Auto-save</p>
    <div class="toast" id="toast">Saved</div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-quick;   // 220ms — @starting-style 진입 전환 길이 (반복 재생은 JS 가 담당, index.html 참고)
$easing: $ease-pop;
$color: #9fbed0;

@include stage($stage-ink);

.scene {
  box-sizing: border-box;
  width: clamp(220px, 62vmin, 300px);
  min-height: 160px;
  display: grid;
  place-items: center;
  gap: $sp-4;
}

.scene__label {
  margin: 0;
  color: rgba($subject-cream, .75);
  font-weight: 700;
  font-size: clamp(11px, 3vw, 12px);
  line-height: 1;
  font-family: inherit;
  letter-spacing: .08em;
  text-transform: uppercase;
}

.toast {
  padding: $sp-3 $sp-6;
  border-radius: $r-pill;
  background: $color;
  color: $stage-ink;
  font-weight: 800;
  font-size: clamp(13px, 3.6vw, 14px);
  line-height: 1;
  font-family: inherit;
  box-shadow: $shadow-raised;
}

// Chrome 117+ 만 지원 — 미지원 브라우저는 이 블록 밖의 기본 상태(항상 보임, 전환 없음)로 조용히 대체된다
@supports (transition-behavior: allow-discrete) {
  .toast {
    transition: display $duration allow-discrete, opacity $duration $easing, transform $duration $easing;
  }

  .toast.is-hidden {
    display: none;
    opacity: 0;
    transform: translateY($sp-3) scale(.92);
  }

  // display:none → 보임으로 "새로 렌더" 되는 그 순간의 시작 상태. 키프레임 없이 CSS 만으로 진입 전환을 만든다
  @starting-style {
    .toast { opacity: 0; transform: translateY($sp-3) scale(.92); }
  }
}

@media (prefers-reduced-motion: reduce) {
  .toast { 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);
}

.scene {
  box-sizing: border-box;
  width: clamp(220px, 62vmin, 300px);
  min-height: 160px;
  display: grid;
  place-items: center;
  gap: 16px;
}

.scene__label {
  margin: 0;
  color: rgba(255, 247, 230, 0.75);
  font-weight: 700;
  font-size: clamp(11px, 3vw, 12px);
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.toast {
  padding: 12px 24px;
  border-radius: 999px;
  background: #9fbed0;
  color: #17141a;
  font-weight: 800;
  font-size: clamp(13px, 3.6vw, 14px);
  line-height: 1;
  font-family: inherit;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

@supports (transition-behavior: allow-discrete) {
  .toast {
    transition: display 220ms allow-discrete, opacity 220ms cubic-bezier(0.2, 1.4, 0.4, 1), transform 220ms cubic-bezier(0.2, 1.4, 0.4, 1);
  }
  .toast.is-hidden {
    display: none;
    opacity: 0;
    transform: translateY(12px) scale(0.92);
  }
  @starting-style {
    .toast {
      opacity: 0;
      transform: translateY(12px) scale(0.92);
    }
  }
}
@media (prefers-reduced-motion: reduce) {
  .toast {
    transition: none !important;
  }
}

// @starting-style 는 요소가 "새로 렌더" 될 때만 발동한다. 데모에서 반복 재현하려면
// display:none 으로 뺐다가(is-hidden) 두 번의 requestAnimationFrame 뒤에 다시 넣어
// 매번 새로 렌더된 것처럼 만든다 — 8줄.
(function () {
  var t = document.getElementById('toast');
  function loop() {
    t.classList.add('is-hidden');
    requestAnimationFrame(function () { requestAnimationFrame(function () { t.classList.remove('is-hidden'); }); });
  }
  setTimeout(function () { loop(); setInterval(loop, 350); }, 300);
})();

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

export type StartingStyleLoadProps = {
  label?: string;
  duration?: number;
  easing?: string;
  color?: string;
};

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

export default function StartingStyleLoad({
  label = "Saved",
  duration = 220,
  easing = "cubic-bezier(.2,1.4,.4,1)",
  color = "#38b6ff",
}: StartingStyleLoadProps) {
  const [hidden, setHidden] = useState(false);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  // @starting-style 는 요소가 "새로 렌더" 될 때만 발동한다 — 실서비스라면 실제 저장 이벤트가
  // 이 토글을 부르고, 여기서는 데모용으로 짧은 간격을 두고 반복 재현한다
  useEffect(() => {
    const id = setInterval(() => {
      setHidden(true);
      requestAnimationFrame(() => requestAnimationFrame(() => setHidden(false)));
    }, 350);
    return () => clearInterval(id);
  }, []);

  return (
    <div className={styles.scene} style={vars}>
      <p className={styles.label}>Auto-save</p>
      <div className={`${styles.toast} ${hidden ? styles.hidden : ""}`}>{label}</div>
    </div>
  );
}