01Default cross-fade

<main class="stage">
  <div class="pages is-demo">
    <section class="page page--a">
      <p class="page__tag">ARTICLE</p>
      <h2 class="page__title">Previous post</h2>
      <p class="page__body">Build variants with 3 CSS variables</p>
    </section>
    <section class="page page--b">
      <p class="page__tag">ARTICLE</p>
      <h2 class="page__title">Next post</h2>
      <p class="page__body">Transitions have a clock too</p>
    </section>
  </div>
</main>

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

// ── 변수 3개. 변형은 이것만 바꾼다 (값은 토큰에서 받는다) ──────
$duration: $dur-base;
$easing: $ease-out;
$color: $subject-blue;

// ── 데모 무대 ───────────────────────────────
@include stage($stage-ink);

// ── 두 "페이지" 패널 ───────────────────────
.pages { position: relative; width: min(300px, 88%); height: 150px; border-radius: $r-card; overflow: hidden; box-shadow: $shadow-raised; }
.page {
  position: absolute; inset: 0; box-sizing: border-box;
  display: flex; flex-direction: column; justify-content: center; gap: 6px;
  padding: clamp(12px, 5vw, 20px); text-align: left;
}
.page--a { background: #fff; color: $subject-ink; }
.page--b { background: $color; color: #fff; }
.page__tag { margin: 0; font-weight: 700; font-size: clamp(9px, 2.6vw, 11px); line-height: 1; font-family: inherit; letter-spacing: .12em; opacity: .6; }
.page__title { margin: 0; font-weight: 700; font-size: clamp(14px, 4.2vw, 18px); line-height: 1.3; font-family: inherit; }
.page__body { margin: 0; font-weight: 400; font-size: clamp(11px, 3.2vw, 13px); line-height: 1.5; font-family: inherit; opacity: .82; }

// A 는 보였다 사라지고, B 는 그 자리에서 겹쳐 나타난다. transform·opacity 만 쓴다.
// (실제 전환 속도는 $duration. 데모 루프는 $dur-loop 로 반복 주기를 맞춘다)
.page--a { animation: cross-a $dur-loop $easing infinite; }
.page--b { animation: cross-b $dur-loop $easing infinite; opacity: 0; }

@keyframes cross-a {
  0%, 5% { opacity: 1; }
  50% { opacity: 0; }
  95%, 100% { opacity: 1; }
}
@keyframes cross-b {
  0%, 5% { opacity: 0; }
  50% { opacity: 1; }
  95%, 100% { opacity: 0; }
}

// ── 접근성: 움직임 줄이기 설정이면 최종 상태만 고정으로 ────────
@media (prefers-reduced-motion: reduce) {
  .page--a, .page--b { animation: none !important; }
  .page--a { opacity: 1; }
  .page--b { opacity: 0; }
}

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);
}

.pages {
  position: relative;
  width: min(300px, 88%);
  height: 150px;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.page {
  position: absolute;
  inset: 0;
  box-sizing: border-box;
  display: flex;
  flex-direction: column;
  justify-content: center;
  gap: 6px;
  padding: clamp(12px, 5vw, 20px);
  text-align: left;
}

.page--a {
  background: #fff;
  color: #17141a;
}

.page--b {
  background: #2f6df6;
  color: #fff;
}

.page__tag {
  margin: 0;
  font-weight: 700;
  font-size: clamp(9px, 2.6vw, 11px);
  line-height: 1;
  font-family: inherit;
  letter-spacing: 0.12em;
  opacity: 0.6;
}

.page__title {
  margin: 0;
  font-weight: 700;
  font-size: clamp(14px, 4.2vw, 18px);
  line-height: 1.3;
  font-family: inherit;
}

.page__body {
  margin: 0;
  font-weight: 400;
  font-size: clamp(11px, 3.2vw, 13px);
  line-height: 1.5;
  font-family: inherit;
  opacity: 0.82;
}

.page--a {
  animation: cross-a 2s ease-out infinite;
}

.page--b {
  animation: cross-b 2s ease-out infinite;
  opacity: 0;
}

@keyframes cross-a {
  0%, 5% {
    opacity: 1;
  }
  50% {
    opacity: 0;
  }
  95%, 100% {
    opacity: 1;
  }
}
@keyframes cross-b {
  0%, 5% {
    opacity: 0;
  }
  50% {
    opacity: 1;
  }
  95%, 100% {
    opacity: 0;
  }
}
@media (prefers-reduced-motion: reduce) {
  .page--a, .page--b {
    animation: none !important;
  }
  .page--a {
    opacity: 1;
  }
  .page--b {
    opacity: 0;
  }
}

import type { CSSProperties, ReactNode } from "react";
import styles from "./CrossFadeTransition.module.scss";

export type CrossFadeTransitionProps = {
  /** 어느 페이지가 보이는가. 라우트가 바뀔 때 부모가 이 값을 바꾼다 */
  active: "a" | "b";
  pageA: ReactNode;
  pageB: ReactNode;
  /** 전환이 끝나는 시간 (기본 300ms) */
  duration?: number;
  /** easing (기본 ease-out) */
  easing?: string;
  /** page--b 배경색 */
  color?: string;
  className?: string;
};

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

export default function CrossFadeTransition({
  active,
  pageA,
  pageB,
  duration = 300,
  easing = "ease-out",
  color = "#2f6df6",
  className = "",
}: CrossFadeTransitionProps) {
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };
  return (
    <div className={`${styles.pages} ${className}`} style={vars} data-active={active}>
      <section className={styles.pageA}>{pageA}</section>
      <section className={styles.pageB}>{pageB}</section>
    </div>
  );
}