01Slow-moving background

<main class="stage is-demo">
  <div class="bgslow">
    <div class="bgslow__bg" aria-hidden="true"></div>
    <h2 class="bgslow__title">Only the background slows down</h2>
  </div>
</main>

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

// ── 변수 3개. 변형은 이것만 바꾼다 (값은 토큰에서 받는다) ──────
$shift: 24px;                                    // 배경 이동량 (스크롤의 0.4배 느낌)
$duration: $dur-loop;                            // 데모 왕복 주기
$color: $subject-cream;                          // 배경 점 색 (ink 무대라 cream)

@include stage($stage-ink);

.bgslow {
  position: relative;
  width: 100%;
  height: 100%;
  overflow: hidden;
  display: grid;
  place-items: center;

  &__bg {
    position: absolute;
    inset: -20% -10%;
    background:
      radial-gradient($color 0 46px, transparent 47px) 24% 30%,
      radial-gradient($color 0 30px, transparent 31px) 78% 62%,
      radial-gradient($color 0 20px, transparent 21px) 55% 82%;
    background-repeat: no-repeat;
    opacity: .5;

    // ── 실제 사용 코드: 배경 하나만 스크롤보다 느리게(0.4배) 움직인다 ──
    @supports (animation-timeline: scroll()) {
      animation: drift linear both;
      animation-timeline: scroll(root);
    }
    @supports not (animation-timeline: scroll()) {
      transform: none;
    }
  }

  &__title {
    position: relative;
    z-index: 1;
    margin: 0;
    padding: 14px 22px;
    border-radius: $r-pill;
    background: $subject-blue;
    color: #fff;
    font: 700 clamp(15px, 5vw, 19px)/1 -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  }
}

@keyframes drift { from { transform: translateY(0); } to { transform: translateY(-#{$shift}); } }

// ── 데모(.is-demo): 스크롤 대신 왕복 루프로 흉내 ────────
.stage.is-demo .bgslow__bg {
  animation: drift-loop $duration ease-in-out infinite;
}
@keyframes drift-loop {
  0%, 100% { transform: translateY($shift); }
  50%      { transform: translateY(-$shift); }
}

// ── 접근성: 움직임 줄이기 설정이면 배경을 그대로 고정 ────────────
@media (prefers-reduced-motion: reduce) {
  .bgslow__bg, .stage.is-demo .bgslow__bg { animation: none !important; transform: none; }
}

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);
}

.bgslow {
  position: relative;
  width: 100%;
  height: 100%;
  overflow: hidden;
  display: grid;
  place-items: center;
}
.bgslow__bg {
  position: absolute;
  inset: -20% -10%;
  background: radial-gradient(#fff7e6 0 46px, transparent 47px) 24% 30%, radial-gradient(#fff7e6 0 30px, transparent 31px) 78% 62%, radial-gradient(#fff7e6 0 20px, transparent 21px) 55% 82%;
  background-repeat: no-repeat;
  opacity: 0.5;
}
@supports (animation-timeline: scroll()) {
  .bgslow__bg {
    animation: drift linear both;
    animation-timeline: scroll(root);
  }
}
@supports not (animation-timeline: scroll()) {
  .bgslow__bg {
    transform: none;
  }
}
.bgslow__title {
  position: relative;
  z-index: 1;
  margin: 0;
  padding: 14px 22px;
  border-radius: 999px;
  background: #2f6df6;
  color: #fff;
  font: 700 clamp(15px, 5vw, 19px)/1 -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

@keyframes drift {
  from {
    transform: translateY(0);
  }
  to {
    transform: translateY(-24px);
  }
}
.stage.is-demo .bgslow__bg {
  animation: drift-loop 2s ease-in-out infinite;
}

@keyframes drift-loop {
  0%, 100% {
    transform: translateY(24px);
  }
  50% {
    transform: translateY(-24px);
  }
}
@media (prefers-reduced-motion: reduce) {
  .bgslow__bg, .stage.is-demo .bgslow__bg {
    animation: none !important;
    transform: none;
  }
}

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

export type SlowBackgroundProps = {
  children: ReactNode;
  /** 배경 이동량 (기본 24px, 스크롤의 0.4배 느낌) */
  shift?: number;
  /** 배경 점 색 */
  color?: string;
  className?: string;
};

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

export default function SlowBackground({
  children,
  shift = 24,
  color = "#fff7e6",
  className = "",
}: SlowBackgroundProps) {
  const vars: Vars = { "--shift": `${shift}px`, "--color": color };

  return (
    <div className={`${styles.wrap} ${className}`} style={vars}>
      <div className={styles.bg} aria-hidden="true" />
      <div className={styles.title}>{children}</div>
    </div>
  );
}