05Per-layer decay parallax — after scrolling stops, each layer's position value keeps shrinking at its own decay rate

<main class="stage">
<div class="dpl is-demo" id="dpl">
  <div class="dpl__viewport" id="dplViewport" role="application" aria-label="Layer parallax test panel">
    <div class="dpl__layer dpl__layer--far" id="dplFar"></div>
    <div class="dpl__layer dpl__layer--mid" id="dplMid"></div>
    <div class="dpl__layer dpl__layer--near" id="dplNear"></div>
  </div>
  <p class="dpl__hint">Drag sideways and release — each layer slows down at its own speed.</p>
</div>
</main>

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

// 계산 겹(레이어별 감쇠 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-ink);

.stage { position: relative; }

.dpl {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
}

.dpl__viewport {
  position: relative;
  width: 230px;
  height: 130px;
  overflow: hidden;
  border-radius: $r-card;
  background: rgba($stage-paper, .06);
  border: 1px solid rgba($stage-paper, .16);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.dpl__layer {
  position: absolute;
  left: 0;
  width: 640px;
  height: 32px;
  will-change: transform;
  background-repeat: repeat-x;
  background-size: 80px 100%;
}

.dpl__layer--far {
  top: $sp-3;
  opacity: .35;
  background-image: radial-gradient(circle, $subject-sky 0 8px, transparent 9px);
}

.dpl__layer--mid {
  top: 50%;
  margin-top: -16px;
  opacity: .6;
  background-image: radial-gradient(circle, $subject-lilac 0 10px, transparent 11px);
}

.dpl__layer--near {
  bottom: $sp-3;
  opacity: 1;
  background-image: radial-gradient(circle, $subject-mint 0 13px, transparent 14px);
}

.dpl__hint {
  color: rgba($stage-paper, .72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .dpl__viewport { width: 176px; height: 96px; }
}

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

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);
}

.stage {
  position: relative;
}

.dpl {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.dpl__viewport {
  position: relative;
  width: 230px;
  height: 130px;
  overflow: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
  border: 1px solid rgba(255, 255, 255, 0.16);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.dpl__layer {
  position: absolute;
  left: 0;
  width: 640px;
  height: 32px;
  will-change: transform;
  background-repeat: repeat-x;
  background-size: 80px 100%;
}

.dpl__layer--far {
  top: 12px;
  opacity: 0.35;
  background-image: radial-gradient(circle, #6ec8ff 0 8px, transparent 9px);
}

.dpl__layer--mid {
  top: 50%;
  margin-top: -16px;
  opacity: 0.6;
  background-image: radial-gradient(circle, #b9a0ff 0 10px, transparent 11px);
}

.dpl__layer--near {
  bottom: 12px;
  opacity: 1;
  background-image: radial-gradient(circle, #4cd4a6 0 13px, transparent 14px);
}

.dpl__hint {
  color: rgba(255, 255, 255, 0.72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .dpl__viewport {
    width: 176px;
    height: 96px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .dpl__layer {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('dpl');
  var viewport = document.getElementById('dplViewport');
  var layers = [
    { el: document.getElementById('dplFar'),  speed: 0.35, decay: 0.988, pos: 0, vel: 0 },
    { el: document.getElementById('dplMid'),  speed: 0.65, decay: 0.95,  pos: 0, vel: 0 },
    { el: document.getElementById('dplNear'), speed: 1.0,  decay: 0.88,  pos: 0, vel: 0 }
  ];
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 입력 값(inputVel)에 레이어마다 다른 SPEED 를 곱해 각자의 속도(layer.vel)를 만들고,
  // 입력이 끊기면 레이어마다 다른 DECAY 값으로 각자 감쇠한다 — 감쇠율이 클수록(1에 가까울수록)
  // 더 오래 움직인다. 그래서 손을 떼도 레이어들이 서로 다른 시점에 멈춘다.
  var WRAP = 320;
  var raf = null;

  function clampWrap(v) { var m = v % WRAP; if (m > 0) { m -= WRAP; } return m; }
  function paint() {
    for (var i = 0; i < layers.length; i++) {
      layers[i].el.style.transform = 'translateX(' + layers[i].pos.toFixed(1) + 'px)';
    }
  }
  function stopLoop() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
  var inputActive = false;
  function loopTick() {
    var moving = false;
    for (var i = 0; i < layers.length; i++) {
      var L = layers[i];
      if (!inputActive) { L.vel *= L.decay; if (reduce) { L.vel = 0; } }
      L.pos = clampWrap(L.pos + L.vel);
      if (Math.abs(L.vel) > 0.02) { moving = true; }
    }
    paint();
    if (reduce || (!moving && !inputActive)) { raf = null; return; }
    raf = requestAnimationFrame(loopTick);
  }
  function ensureLoop() { if (raf === null) { raf = requestAnimationFrame(loopTick); } }
  function setInput(v) {
    for (var i = 0; i < layers.length; i++) { layers[i].vel = v * layers[i].speed; }
    ensureLoop();
  }

  function wireReal() {
    viewport.addEventListener('wheel', function (e) {
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay();
      inputActive = true;
      setInput(-e.deltaY * 0.5 - e.deltaX * 0.5);
      clearTimeout(viewport._wt);
      viewport._wt = setTimeout(function () { inputActive = false; }, 90);
    }, { passive: false });

    var dragging = false, lastX = 0, lastT = 0;
    viewport.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay();
      dragging = true; inputActive = true; lastX = e.clientX; lastT = performance.now();
      viewport.setPointerCapture(e.pointerId);
    });
    viewport.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      var t = performance.now();
      var dt = Math.max(1, t - lastT);
      var v = (e.clientX - lastX) / dt * 16.6;
      setInput(v);
      lastX = e.clientX; lastT = t;
    });
    viewport.addEventListener('pointerup', function () {
      if (!dragging) { return; }
      dragging = false; inputActive = false;
      ensureLoop();
    });
  }
  wireReal();
  paint();

  // 자동 루프: 강하게 밀었다가 손을 떼 레이어별 감쇠 경로를 반복 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    phase += 1;
    var dir = (phase % 2 === 0) ? -1 : 1;
    inputActive = true;
    setInput(dir * 6.5);
    setTimeout(function () { inputActive = false; ensureLoop(); }, 160);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 1500); autoTick(); }
})();

import { useEffect, useRef } from "react";
import styles from "./ScrollMomentumDecayParallaxLayers.module.scss";

export type ScrollMomentumDecayParallaxLayersProps = {
  color?: string;
};

type Layer = { el: HTMLDivElement; speed: number; decay: number; pos: number; vel: number };

export default function ScrollMomentumDecayParallaxLayers({
  color = "#4cd4a6",
}: ScrollMomentumDecayParallaxLayersProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const viewport = root.querySelector<HTMLDivElement>(`.${styles.viewport}`);
    const farEl = root.querySelector<HTMLDivElement>(`.${styles.far}`);
    const midEl = root.querySelector<HTMLDivElement>(`.${styles.mid}`);
    const nearEl = root.querySelector<HTMLDivElement>(`.${styles.near}`);
    if (!viewport || !farEl || !midEl || !nearEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹: 입력 값(inputVel)에 레이어마다 다른 SPEED 를 곱해 각자의 속도(layer.vel)를 만들고,
    // 입력이 끊기면 레이어마다 다른 DECAY 값으로 각자 감쇠한다 — 감쇠율이 클수록(1에 가까울수록)
    // 더 오래 움직인다. 그래서 손을 떼도 레이어들이 서로 다른 시점에 멈춘다.
    const WRAP = 320;
    const layers: Layer[] = [
      { el: farEl, speed: 0.35, decay: 0.988, pos: 0, vel: 0 },
      { el: midEl, speed: 0.65, decay: 0.95, pos: 0, vel: 0 },
      { el: nearEl, speed: 1.0, decay: 0.88, pos: 0, vel: 0 },
    ];
    let raf: number | null = null;
    let inputActive = false;

    function clampWrap(v: number) { let m = v % WRAP; if (m > 0) m -= WRAP; return m; }
    function paint() {
      for (const L of layers) { L.el.style.transform = "translateX(" + L.pos.toFixed(1) + "px)"; }
    }
    function stopLoop() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
    function loopTick() {
      let moving = false;
      for (const L of layers) {
        if (!inputActive) { L.vel *= L.decay; if (reduce) L.vel = 0; }
        L.pos = clampWrap(L.pos + L.vel);
        if (Math.abs(L.vel) > 0.02) moving = true;
      }
      paint();
      if (reduce || (!moving && !inputActive)) { raf = null; return; }
      raf = requestAnimationFrame(loopTick);
    }
    function ensureLoop() { if (raf === null) raf = requestAnimationFrame(loopTick); }
    function setInput(v: number) {
      for (const L of layers) { L.vel = v * L.speed; }
      ensureLoop();
    }

    let wheelTimer: number | null = null;
    function onWheel(e: WheelEvent) {
      e.preventDefault();
      stopAutoplay();
      inputActive = true;
      setInput(-e.deltaY * 0.5 - e.deltaX * 0.5);
      if (wheelTimer !== null) window.clearTimeout(wheelTimer);
      wheelTimer = window.setTimeout(() => { inputActive = false; }, 90);
    }

    let dragging = false, lastX = 0, lastT = 0;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true; inputActive = true; lastX = e.clientX; lastT = performance.now();
      viewport!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const t = performance.now();
      const dt = Math.max(1, t - lastT);
      const v = (e.clientX - lastX) / dt * 16.6;
      setInput(v);
      lastX = e.clientX; lastT = t;
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false; inputActive = false;
      ensureLoop();
    }

    viewport.addEventListener("wheel", onWheel, { passive: false });
    viewport.addEventListener("pointerdown", onPointerDown);
    viewport.addEventListener("pointermove", onPointerMove);
    viewport.addEventListener("pointerup", onPointerUp);
    paint();

    let autoTimer: number | null = null, phase = 0;
    let autoReleaseTimer: number | null = null;
    function autoTick() {
      phase += 1;
      const dir = phase % 2 === 0 ? -1 : 1;
      inputActive = true;
      setInput(dir * 6.5);
      autoReleaseTimer = window.setTimeout(() => { inputActive = false; ensureLoop(); }, 160);
    }
    function stopAutoplay() {
      if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; }
      if (autoReleaseTimer !== null) { window.clearTimeout(autoReleaseTimer); autoReleaseTimer = null; }
    }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 1500); autoTick(); }

    return () => {
      stopAutoplay();
      stopLoop();
      viewport.removeEventListener("wheel", onWheel);
      viewport.removeEventListener("pointerdown", onPointerDown);
      viewport.removeEventListener("pointermove", onPointerMove);
      viewport.removeEventListener("pointerup", onPointerUp);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.dpl}>
        <div className={styles.viewport} role="application" aria-label="레이어 패럴랙스 실험판">
          <div className={`${styles.layer} ${styles.far}`} />
          <div className={`${styles.layer} ${styles.mid}`} />
          <div
            className={`${styles.layer} ${styles.near}`}
            style={{ backgroundImage: `radial-gradient(circle, ${color} 0 13px, transparent 14px)` }}
          />
        </div>
        <p className={styles.hint}>가로로 드래그한 뒤 놓으면 레이어마다 다른 속도로 잦아듭니다</p>
      </div>
    </div>
  );
}