02固定背景を通るキャプション

<main class="stage">
  <div class="cb is-demo">
    <div class="cb__scroller">
      <div class="cb__bg">
        <div class="cb__scene" aria-hidden="true">
          <span class="cb__sun"></span>
          <svg class="cb__peaks" viewBox="0 0 256 256" width="120" height="120" aria-hidden="true"><g fill="currentColor"><path d="M144 52a20 20 0 1 1 20 20a20 20 0 0 1-20-20m49.44 55.92a8 8 0 0 0-13.77 0l-33 55.75l-21-35.67H50.35L8 200h240Z" opacity=".2"/><path d="M164 80a28 28 0 1 0-28-28a28 28 0 0 0 28 28m0-40a12 12 0 1 1-12 12a12 12 0 0 1 12-12m90.88 155.92l-54.56-92.08A15.87 15.87 0 0 0 186.55 96a15.85 15.85 0 0 0-13.76 7.84L146.63 148l-44.84-76.1a16 16 0 0 0-27.58 0L1.11 195.94A8 8 0 0 0 8 208h240a8 8 0 0 0 6.88-12.08M88 80l23.57 40H64.43ZM22 192l33-56h66l33 56Zm150.57 0l-16.66-28.28L186.55 112L234 192Z"/></g></svg>
        </div>
        <div class="cb__dim" aria-hidden="true"></div>
      </div>
      <div class="cb__caps">
        <p class="cb__cap">最初の場面は一歩も動きません。</p>
        <p class="cb__cap">カードだけが上がって上へ抜けていきます。</p>
        <p class="cb__cap">覆われるほど背景が少しずつ暗くなります。</p>
      </div>
    </div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: 1s;          // alternate 라 한 바퀴는 2s
$easing: linear;
$color: $subject-ink;

@include stage($stage-yellow);

.cb {
  --h: clamp(148px, 58vh, 176px);
  width: min(360px, 92vw);
}

// ⚠ 타임라인에 이름을 붙여 둔다. scroll(nearest) 는 `overflow: hidden` 인 상자도
// 스크롤 컨테이너로 쳐서, 배경 상자 안의 요소가 **절대 안 움직이는** 타임라인에 묶인다 (실측)
.cb__scroller {
  height: var(--h);
  overflow-y: auto;
  scroll-timeline-name: --cb-scroll;
  scroll-timeline-axis: block;
  border-radius: $r-card;
  scrollbar-width: none;
  &::-webkit-scrollbar { display: none; }
}

// ── 실제 동작 ① 배경 장면은 스크롤 상자 안에 붙어 제자리에 남는다 ──
.cb__bg {
  position: sticky;
  top: 0;
  height: var(--h);
  overflow: hidden;
}

.cb__scene {
  display: grid;
  place-items: center;
  height: 100%;
  background: linear-gradient(160deg, $subject-pink 0%, $stage-orange 58%, #aa6f54 100%);
  color: rgba(255, 247, 230, .92);
}

.cb__sun {
  grid-area: 1 / 1;
  width: 72px;
  height: 72px;
  border-radius: 50%;
  background: rgba($stage-yellow, .85);
  transform: translate(64px, -32px);
}

.cb__peaks { grid-area: 1 / 1; transform: translateY(12px); }

// ── 실제 동작 ② 가려진 만큼 어두워진다 — 스크롤 진행이 곧 불투명도 ──
.cb__dim {
  position: absolute;
  inset: 0;
  background: $color;
  opacity: 0;
  animation-name: cb-dim;
  animation-duration: 1s;
  animation-timing-function: linear;
  animation-fill-mode: both;
  animation-timeline: --cb-scroll;
}

@keyframes cb-dim {
  from { opacity: 0; }
  to   { opacity: .46; }
}

// 캡션 줄은 배경 위로 올라탄다 — 음수 margin 이 배경 한 판을 통째로 덮는다
.cb__caps {
  position: relative;
  display: grid;
  gap: calc(var(--h) * .62);
  margin-top: calc(var(--h) * -1);
  padding: calc(var(--h) * .44) $sp-4 calc(var(--h) * .5);
}

.cb__cap {
  margin: 0;
  padding: $sp-3;
  border: 1px solid rgba(255, 255, 255, .72);
  border-radius: $r-card;
  // ⚠ 반투명 흰 판을 어두워지는 배경 위에 올리면 합성색이 같이 어두워져 먹색 글자 대비가 떨어진다.
  // .34 로 뒀더니 마지막 카드가 읽히지 않아 .70 까지 올렸다 (실측)
  background: rgba(255, 255, 255, .7);
  backdrop-filter: blur(10px) saturate(150%);
  -webkit-backdrop-filter: blur(10px) saturate(150%);
  color: $color;
  font-size: 13px;
  line-height: 19px;
  font-weight: 700;
}

// ── 격자 미리보기 루프 ────────────────────────────────
// 트랙(.cb__caps)만 민다. 스크롤 상자 자체에 transform 을 걸면 내용이 날아간다(051 실측)
.cb.is-demo .cb__caps {
  animation-name: cb-scrub;
  animation-duration: $duration;
  animation-timing-function: $easing;
  animation-direction: alternate;
  animation-iteration-count: infinite;
  animation-fill-mode: both;
}
.cb.is-demo .cb__dim {
  animation-duration: $duration;
  animation-direction: alternate;
  animation-iteration-count: infinite;
  animation-timeline: auto;
}

@keyframes cb-scrub {
  from { transform: translateY(0); }
  to   { transform: translateY(-328px); }
}

@media (prefers-reduced-motion: reduce) {
  .cb__dim { animation: none !important; opacity: .3; }
  .cb.is-demo .cb__caps { 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: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.cb {
  --h: clamp(148px, 58vh, 176px);
  width: min(360px, 92vw);
}

.cb__scroller {
  height: var(--h);
  overflow-y: auto;
  scroll-timeline-name: --cb-scroll;
  scroll-timeline-axis: block;
  border-radius: 16px;
  scrollbar-width: none;
}
.cb__scroller::-webkit-scrollbar {
  display: none;
}

.cb__bg {
  position: sticky;
  top: 0;
  height: var(--h);
  overflow: hidden;
}

.cb__scene {
  display: grid;
  place-items: center;
  height: 100%;
  background: linear-gradient(160deg, #ff9ebb 0%, #ff4d1f 58%, #aa6f54 100%);
  color: rgba(255, 247, 230, 0.92);
}

.cb__sun {
  grid-area: 1/1;
  width: 72px;
  height: 72px;
  border-radius: 50%;
  background: rgba(255, 210, 63, 0.85);
  transform: translate(64px, -32px);
}

.cb__peaks {
  grid-area: 1/1;
  transform: translateY(12px);
}

.cb__dim {
  position: absolute;
  inset: 0;
  background: #17141a;
  opacity: 0;
  animation-name: cb-dim;
  animation-duration: 1s;
  animation-timing-function: linear;
  animation-fill-mode: both;
  animation-timeline: --cb-scroll;
}

@keyframes cb-dim {
  from {
    opacity: 0;
  }
  to {
    opacity: 0.46;
  }
}
.cb__caps {
  position: relative;
  display: grid;
  gap: calc(var(--h) * 0.62);
  margin-top: calc(var(--h) * -1);
  padding: calc(var(--h) * 0.44) 16px calc(var(--h) * 0.5);
}

.cb__cap {
  margin: 0;
  padding: 12px;
  border: 1px solid rgba(255, 255, 255, 0.72);
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.7);
  backdrop-filter: blur(10px) saturate(150%);
  -webkit-backdrop-filter: blur(10px) saturate(150%);
  color: #17141a;
  font-size: 13px;
  line-height: 19px;
  font-weight: 700;
}

.cb.is-demo .cb__caps {
  animation-name: cb-scrub;
  animation-duration: 1s;
  animation-timing-function: linear;
  animation-direction: alternate;
  animation-iteration-count: infinite;
  animation-fill-mode: both;
}

.cb.is-demo .cb__dim {
  animation-duration: 1s;
  animation-direction: alternate;
  animation-iteration-count: infinite;
  animation-timeline: auto;
}

@keyframes cb-scrub {
  from {
    transform: translateY(0);
  }
  to {
    transform: translateY(-328px);
  }
}
@media (prefers-reduced-motion: reduce) {
  .cb__dim {
    animation: none !important;
    opacity: 0.3;
  }
  .cb.is-demo .cb__caps {
    animation: none !important;
    transform: none;
  }
}

// 실제 동작은 CSS 만: 배경은 position: sticky, 어두워지는 정도는 animation-timeline: scroll().
var r = document.querySelector('.cb');
var s = r.querySelector('.cb__scroller');
function stopDemo(){ r.classList.remove('is-demo'); }
r.addEventListener('pointerdown', stopDemo);
s.addEventListener('scroll', stopDemo, { passive: true });

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

export type CaptionOverPinnedBgProps = {
  captions?: string[];
  preview?: boolean;
  duration?: number;
  easing?: string;
  color?: string;
};

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

const IconPeaks = (): ReactNode => (
  <svg className={styles.peaks} viewBox="0 0 256 256" width="120" height="120" aria-hidden="true"><g fill="currentColor"><path d="M144 52a20 20 0 1 1 20 20a20 20 0 0 1-20-20m49.44 55.92a8 8 0 0 0-13.77 0l-33 55.75l-21-35.67H50.35L8 200h240Z" opacity=".2"/><path d="M164 80a28 28 0 1 0-28-28a28 28 0 0 0 28 28m0-40a12 12 0 1 1-12 12a12 12 0 0 1 12-12m90.88 155.92l-54.56-92.08A15.87 15.87 0 0 0 186.55 96a15.85 15.85 0 0 0-13.76 7.84L146.63 148l-44.84-76.1a16 16 0 0 0-27.58 0L1.11 195.94A8 8 0 0 0 8 208h240a8 8 0 0 0 6.88-12.08M88 80l23.57 40H64.43ZM22 192l33-56h66l33 56Zm150.57 0l-16.66-28.28L186.55 112L234 192Z"/></g></svg>
);

export default function CaptionOverPinnedBg({
  captions = [
    "The first scene never moves an inch.",
    "Only the cards rise and leave through the top.",
    "The more it is covered, the darker the backdrop gets.",
  ],
  preview = true,
  duration = 1000,
  easing = "linear",
  color = "#17141a",
}: CaptionOverPinnedBgProps): ReactNode {
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };
  const cls = preview ? `${styles.cb} ${styles.isDemo}` : styles.cb;

  return (
    <div className={cls} style={vars}>
      <div className={styles.scroller}>
        <div className={styles.bg}>
          <div className={styles.scene} aria-hidden="true">
            <span className={styles.sun} />
            <IconPeaks />
          </div>
          <div className={styles.dim} aria-hidden="true" />
        </div>
        <div className={styles.caps}>
          {captions.map((c: string): ReactNode => (
            <p className={styles.cap} key={c}>{c}</p>
          ))}
        </div>
      </div>
    </div>
  );
}