09Cường độ glow theo scroll

<main class="stage is-demo">
  <div class="row" aria-hidden="true">
    <span class="track"><span class="dot"></span></span>
    <div class="card">
      <span class="bar bar--lg"></span>
      <span class="bar bar--sm"></span>
    </div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;   // 2s — 오르내림 한 번
$easing: ease-in-out;
$color: #3ec9ff;        // 네온 블루

@include stage($stage-ink);

.row { display: flex; align-items: center; gap: $sp-5; }

$track-h: 96px;

.track {
  position: relative;
  width: 3px; height: $track-h;
  border-radius: $r-xs;
  background: rgba($stage-paper, .15);
}

.dot {
  position: absolute; top: 0; left: 50%;
  width: $sp-2; height: $sp-2; margin-left: -$sp-1;
  border-radius: 50%;
  background: $color;
  box-shadow: 0 0 $sp-2 $color;
}

// @property 로 등록해야 숫자로 보간된다 — box-shadow·opacity 계산에 그대로 쓴다
@property --intensity { syntax: '<number>'; inherits: false; initial-value: 0; }

.card {
  box-sizing: border-box;
  width: clamp(150px, 44vmin, 200px);
  padding: $sp-5 $sp-4;
  border-radius: $r-card;
  background: rgba($stage-paper, .04);
  border: 1px solid rgba($stage-paper, .1);
  display: flex; flex-direction: column; gap: $sp-3;
  opacity: calc(.45 + var(--intensity) * .55);
  box-shadow: 0 0 calc(var(--intensity) * #{$sp-3}) $color,
              0 0 calc(var(--intensity) * #{$sp-10}) rgba($color, .55);
}

.bar { display: block; height: $sp-2; border-radius: $r-xs; background: rgba($stage-paper, .18); }
.bar--lg { width: 75%; }
.bar--sm { width: 48%; }

// translateY(점) · --intensity(카드) 만 움직인다
.stage.is-demo .dot {
  animation: move $duration $easing infinite;
}
.stage.is-demo .card:not(.is-live) {
  animation: rise $duration $easing infinite;
}

@keyframes move {
  0%, 100% { transform: translateY(0); }
  50%      { transform: translateY(#{$track-h - 8px}); }
}

@keyframes rise {
  0%, 100% { --intensity: 0; }
  50%      { --intensity: 1; }
}

@media (prefers-reduced-motion: reduce) {
  .stage.is-demo .dot,
  .stage.is-demo .card:not(.is-live) { animation: none; }
  .card { --intensity: 1; }
}

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);
}

.row {
  display: flex;
  align-items: center;
  gap: 20px;
}

.track {
  position: relative;
  width: 3px;
  height: 96px;
  border-radius: 4px;
  background: rgba(255, 255, 255, 0.15);
}

.dot {
  position: absolute;
  top: 0;
  left: 50%;
  width: 8px;
  height: 8px;
  margin-left: -4px;
  border-radius: 50%;
  background: #3ec9ff;
  box-shadow: 0 0 8px #3ec9ff;
}

@property --intensity {
  syntax: "<number>";
  inherits: false;
  initial-value: 0;
}
.card {
  box-sizing: border-box;
  width: clamp(150px, 44vmin, 200px);
  padding: 20px 16px;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.04);
  border: 1px solid rgba(255, 255, 255, 0.1);
  display: flex;
  flex-direction: column;
  gap: 12px;
  opacity: calc(0.45 + var(--intensity) * 0.55);
  box-shadow: 0 0 calc(var(--intensity) * 12px) #3ec9ff, 0 0 calc(var(--intensity) * 40px) rgba(62, 201, 255, 0.55);
}

.bar {
  display: block;
  height: 8px;
  border-radius: 4px;
  background: rgba(255, 255, 255, 0.18);
}

.bar--lg {
  width: 75%;
}

.bar--sm {
  width: 48%;
}

.stage.is-demo .dot {
  animation: move 2s ease-in-out infinite;
}

.stage.is-demo .card:not(.is-live) {
  animation: rise 2s ease-in-out infinite;
}

@keyframes move {
  0%, 100% {
    transform: translateY(0);
  }
  50% {
    transform: translateY(88px);
  }
}
@keyframes rise {
  0%, 100% {
    --intensity: 0;
  }
  50% {
    --intensity: 1;
  }
}
@media (prefers-reduced-motion: reduce) {
  .stage.is-demo .dot,
  .stage.is-demo .card:not(.is-live) {
    animation: none;
  }
  .card {
    --intensity: 1;
  }
}

// 실제 스크롤에서는 IntersectionObserver 가 뷰포트 교차 비율로 --intensity 를 정한다(8줄).
// 이 아이프레임은 너무 작아 실스크롤이 안 나므로, 데모 루프(.is-demo)가 같은 변수를 대신 돌린다.
(function () {
  var card = document.querySelector('.card');
  var io = new IntersectionObserver(function (entries) {
    entries.forEach(function (en) {
      card.style.setProperty('--intensity', en.intersectionRatio.toFixed(2));
      card.classList.add('is-live');
    });
  }, { threshold: [0, .25, .5, .75, 1] });
  io.observe(card);
})();

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

export type ScrollIntensityGlowProps = {
  /** intensity 값이 부드러워지는 시간 (기본 300ms) */
  duration?: number;
  color?: string;
};

export default function ScrollIntensityGlow({ duration = 300, color = "#3ec9ff" }: ScrollIntensityGlowProps) {
  const cardRef = useRef<HTMLDivElement | null>(null);

  // 뷰포트 교차 비율(0~1)을 그대로 --intensity 로 쓴다 — 실제 스크롤에서만 값이 바뀐다
  useEffect(() => {
    const el = cardRef.current;
    if (!el) return;
    const io = new IntersectionObserver(
      (entries) => {
        entries.forEach((en) => el.style.setProperty("--intensity", en.intersectionRatio.toFixed(2)));
      },
      { threshold: [0, .25, .5, .75, 1] }
    );
    io.observe(el);
    return () => io.disconnect();
  }, []);

  const vars: CSSProperties = { "--duration": `${duration}ms`, "--color": color } as CSSProperties;

  return (
    <div ref={cardRef} className={styles.card} style={vars}>
      <span className={`${styles.bar} ${styles.barLg}`} />
      <span className={`${styles.bar} ${styles.barSm}`} />
    </div>
  );
}