08스크롤 값만큼 그림자가 자라나는 글자

<main class="stage">
<div class="sc is-demo" id="sc">
  <p class="sc__badge"><span class="sc__badgeLabel">스크롤 진행</span> <span class="sc__badgeValue" id="scValue">0%</span></p>
  <div class="sc__viewport" id="scViewport" tabindex="0">
    <div class="sc__track" id="scTrack">
      <h1 class="sc__word" id="scWord">깊이</h1>
      <div class="sc__spacer" aria-hidden="true"></div>
    </div>
  </div>
  <p class="sc__hint">휠·방향키로 내려 본다</p>
</div>
</main>

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

// 핵심: 스크롤 진행 값(ratio 0~1) 하나를 그림자 거리 값에 그대로 이어 붙인다.
// 컨테이너를 내릴수록 진행 값이 커지고 그림자가 그만큼 자란다.

$duration: $dur-loop;
$easing: linear;
$color: $subject-ink;

@include stage($stage-paper);

.sc {
  display: grid;
  gap: $sp-3;
  justify-items: center;
  text-align: center;
  width: 100%;
}

.sc__badge {
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(23, 20, 26, .06);
  color: $subject-ink;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.sc__viewport {
  width: min(100%, 320px);
  height: 150px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba(23, 20, 26, .04);
  scrollbar-width: thin;
}

.sc__viewport:focus-visible {
  outline: 2px solid $subject-blue-deep;
  outline-offset: 2px;
}

.sc__track {
  display: grid;
  justify-items: center;
  padding-top: $sp-6;
}

.sc__word {
  position: sticky;
  top: $sp-4;
  margin: 0;
  font-weight: 800;
  font-size: clamp(48px, 16vw, 80px);
  line-height: 1;
  white-space: nowrap;
  color: $color;
  will-change: text-shadow;
}

.sc__spacer {
  height: 260px;
  width: 1px;
}

.sc__hint {
  margin: 0;
  color: rgba(23, 20, 26, .55);
  font-size: 11px;
}

@media (max-width: 340px) {
  .sc { gap: $sp-1; }
  .sc__word { font-size: clamp(32px, 18vw, 48px); }
  .sc__viewport { height: 84px; }
  .sc__hint { font-size: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  .sc__word { transition: none; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sc {
  display: grid;
  gap: 12px;
  justify-items: center;
  text-align: center;
  width: 100%;
}

.sc__badge {
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.06);
  color: #17141a;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.sc__viewport {
  width: min(100%, 320px);
  height: 150px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.04);
  scrollbar-width: thin;
}

.sc__viewport:focus-visible {
  outline: 2px solid #0a4fe8;
  outline-offset: 2px;
}

.sc__track {
  display: grid;
  justify-items: center;
  padding-top: 24px;
}

.sc__word {
  position: sticky;
  top: 16px;
  margin: 0;
  font-weight: 800;
  font-size: clamp(48px, 16vw, 80px);
  line-height: 1;
  white-space: nowrap;
  color: #17141a;
  will-change: text-shadow;
}

.sc__spacer {
  height: 260px;
  width: 1px;
}

.sc__hint {
  margin: 0;
  color: rgba(23, 20, 26, 0.55);
  font-size: 11px;
}

@media (max-width: 340px) {
  .sc {
    gap: 4px;
  }
  .sc__word {
    font-size: clamp(32px, 18vw, 48px);
  }
  .sc__viewport {
    height: 84px;
  }
  .sc__hint {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sc__word {
    transition: none;
  }
}

(function () {
  var SCROLL_MAX_PX = 26;
  var PERIOD_MS = 1000;

  var viewport = document.getElementById('scViewport');
  var word = document.getElementById('scWord');
  var value = document.getElementById('scValue');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var hasUser = false;

  // 스크롤 진행 값(ratio 0~1) 하나를 그림자 거리 값에 그대로 이어, 내려갈수록 그림자가 자란다
  function buildScrollShadow(ratio) {
    var d = (ratio * SCROLL_MAX_PX).toFixed(2);
    return [
      (d * 0.4).toFixed(2) + 'px ' + d + 'px 0 rgba(10,79,232,.5)',
      (d * 0.4).toFixed(2) + 'px ' + (d * 2).toFixed(2) + 'px ' + (4 + ratio * 10).toFixed(1) + 'px rgba(10,79,232,' + (0.28 + ratio * 0.3).toFixed(2) + ')'
    ].join(', ');
  }

  function renderScroll(ratio) {
    word.style.textShadow = buildScrollShadow(ratio);
    value.textContent = Math.round(ratio * 100) + '%';
  }

  function ratioFromScroll() {
    var max = viewport.scrollHeight - viewport.clientHeight;
    return max > 0 ? Math.max(0, Math.min(1, viewport.scrollTop / max)) : 0;
  }

  viewport.addEventListener('scroll', function () {
    renderScroll(ratioFromScroll());
  });
  ['wheel', 'touchstart', 'keydown'].forEach(function (ev) {
    viewport.addEventListener(ev, function () { hasUser = true; }, { passive: true });
  });

  var startTime = null;
  function loop(now) {
    if (startTime === null) startTime = now;
    if (!hasUser) {
      var t = (now - startTime) % PERIOD_MS;
      var phase = (Math.sin((t / PERIOD_MS) * Math.PI * 2 - Math.PI / 2) + 1) / 2;
      var max = viewport.scrollHeight - viewport.clientHeight;
      viewport.scrollTop = phase * max;
    }
    requestAnimationFrame(loop);
  }

  renderScroll(0);
  if (!reduce) requestAnimationFrame(loop);
})();

// TextShadowScrollDepthGrow — vanilla 판(run/412/demo/08-text-shadow-scroll-depth-grow/index.html) 의 React 포트.
// 스크롤 진행 값(ratio 0~1) 하나를 그림자 거리 값에 그대로 이어, 내려갈수록 그림자가 자란다.
import { useEffect, useRef, useState } from "react";
import styles from "./TextShadowScrollDepthGrow.module.scss";

type Box<T> = { current: T };

const SCROLL_MAX_PX = 26;
const PERIOD_MS = 1000;

function buildScrollShadow(ratio: number): string {
  const d = (ratio * SCROLL_MAX_PX).toFixed(2);
  return [
    (Number(d) * 0.4).toFixed(2) + "px " + d + "px 0 rgba(10,79,232,.5)",
    (Number(d) * 0.4).toFixed(2) + "px " + (Number(d) * 2).toFixed(2) + "px " + (4 + ratio * 10).toFixed(1) + "px rgba(10,79,232," + (0.28 + ratio * 0.3).toFixed(2) + ")"
  ].join(", ");
}

export default function TextShadowScrollDepthGrow() {
  const viewportRef = useRef<HTMLDivElement | null>(null);
  const wordRef = useRef<HTMLHeadingElement | null>(null);
  const valueRef = useRef<HTMLSpanElement | null>(null);
  const t0Ref = useRef<number>(0) as Box<number>;
  const frameRef = useRef<number>(0) as Box<number>;
  const hasUserRef = useRef<boolean>(false) as Box<boolean>;
  const [demo] = useState(true);
  const [reduce, setReduce] = useState(false);

  function render(ratio: number): void {
    const word = wordRef.current;
    const value = valueRef.current;
    if (!word || !value) return;
    word.style.textShadow = buildScrollShadow(ratio);
    value.textContent = Math.round(ratio * 100) + "%";
  }

  function ratioFromScroll(): number {
    const viewport = viewportRef.current;
    if (!viewport) return 0;
    const max = viewport.scrollHeight - viewport.clientHeight;
    return max > 0 ? Math.max(0, Math.min(1, viewport.scrollTop / max)) : 0;
  }

  useEffect(() => {
    setReduce(window.matchMedia("(prefers-reduced-motion: reduce)").matches);
    render(0);
  }, []);

  useEffect(() => {
    const viewport = viewportRef.current;
    if (!viewport) return;
    function onScroll(): void {
      render(ratioFromScroll());
    }
    function onUser(): void {
      hasUserRef.current = true;
    }
    viewport.addEventListener("scroll", onScroll);
    viewport.addEventListener("wheel", onUser, { passive: true });
    viewport.addEventListener("touchstart", onUser, { passive: true });
    viewport.addEventListener("keydown", onUser);
    return () => {
      viewport.removeEventListener("scroll", onScroll);
      viewport.removeEventListener("wheel", onUser);
      viewport.removeEventListener("touchstart", onUser);
      viewport.removeEventListener("keydown", onUser);
    };
  }, []);

  useEffect(() => {
    if (reduce) return;
    function loop(now: number): void {
      if (t0Ref.current === 0) t0Ref.current = now;
      const viewport = viewportRef.current;
      if (viewport && !hasUserRef.current) {
        const t = (now - t0Ref.current) % PERIOD_MS;
        const phase = (Math.sin((t / PERIOD_MS) * Math.PI * 2 - Math.PI / 2) + 1) / 2;
        const max = viewport.scrollHeight - viewport.clientHeight;
        viewport.scrollTop = phase * max;
      }
      frameRef.current = requestAnimationFrame(loop);
    }
    frameRef.current = requestAnimationFrame(loop);
    return () => cancelAnimationFrame(frameRef.current);
  }, [reduce]);

  const cls = [styles.sc, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls}>
        <p className={styles.badge}><span>스크롤 진행</span> <span ref={valueRef}>0%</span></p>
        <div className={styles.viewport} ref={viewportRef} tabIndex={0}>
          <div className={styles.track}>
            <h1 className={styles.word} ref={wordRef}>깊이</h1>
            <div className={styles.spacer} aria-hidden="true" />
          </div>
        </div>
        <p className={styles.hint}>휠·방향키로 내려 본다</p>
      </div>
    </main>
  );
}