01Scroll inertia coast — the position value keeps carrying the last scroll speed and gradually shrinks to zero after scrolling stops

<main class="stage">
<div class="mic is-demo" id="mic">
  <div class="mic__viewport" id="micViewport" role="application" aria-label="Inertia scroll list">
    <ul class="mic__track" id="micTrack">
      <li class="mic__row">Item 01</li>
      <li class="mic__row">Item 02</li>
      <li class="mic__row">Item 03</li>
      <li class="mic__row">Item 04</li>
      <li class="mic__row">Item 05</li>
      <li class="mic__row">Item 06</li>
      <li class="mic__row">Item 07</li>
      <li class="mic__row">Item 08</li>
      <li class="mic__row">Item 09</li>
      <li class="mic__row">Item 10</li>
    </ul>
  </div>
  <p class="mic__hint">Scroll with the wheel or a drag, then release — it glides on with inertia.</p>
</div>
</main>

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

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

.stage { position: relative; }

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

.mic__viewport {
  position: relative;
  width: 220px;
  height: 150px;
  overflow: hidden;
  border-radius: $r-card;
  background: rgba($stage-paper, .08);
  border: 1px solid rgba($stage-paper, .18);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.mic__track {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  margin: 0;
  padding: $sp-2;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
  will-change: transform;
}

.mic__row {
  height: 34px;
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  padding-left: $sp-3;
  border-radius: $r-sm;
  background: $subject-mint;
  color: $subject-ink;
  font-size: 13px;
  font-weight: 600;
}

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

@media (max-width: 340px) {
  .mic { gap: $sp-2; }
  .mic__viewport { width: 168px; height: 82px; }
  .mic__row { height: 26px; font-size: 12px; }
  .mic__hint { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .mic__track { 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;
}

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

.mic__viewport {
  position: relative;
  width: 220px;
  height: 150px;
  overflow: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.08);
  border: 1px solid rgba(255, 255, 255, 0.18);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
}

.mic__track {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  margin: 0;
  padding: 8px;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 8px;
  will-change: transform;
}

.mic__row {
  height: 34px;
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  padding-left: 12px;
  border-radius: 8px;
  background: #4cd4a6;
  color: #17141a;
  font-size: 13px;
  font-weight: 600;
}

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

@media (max-width: 340px) {
  .mic {
    gap: 8px;
  }
  .mic__viewport {
    width: 168px;
    height: 82px;
  }
  .mic__row {
    height: 26px;
    font-size: 12px;
  }
  .mic__hint {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .mic__track {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('mic');
  var viewport = document.getElementById('micViewport');
  var track = document.getElementById('micTrack');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 손을 뗀 순간의 속도 값(vel)이 매 프레임 FRICTION 배씩 줄어들며 이동 값(pos)에
  // 더해진다 — vel *= FRICTION; pos += vel. MIN_VELOCITY 밑으로 떨어지면 코스팅을 멈춘다.
  var FRICTION = 0.93;
  var MIN_VELOCITY = 0.04;
  var WHEEL_SCALE = 0.55;

  var pos = 0, vel = 0, raf = null, maxScroll = 0;

  function measure() {
    maxScroll = Math.max(0, track.scrollHeight - viewport.clientHeight);
  }
  function clamp(v) { return Math.max(-maxScroll, Math.min(0, v)); }
  function paint() { track.style.transform = 'translateY(' + pos.toFixed(1) + 'px)'; }

  function stopCoast() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
  function coastTick() {
    vel *= FRICTION;
    pos = clamp(pos + vel);
    if (pos === 0 || pos === -maxScroll) { vel = 0; }
    paint();
    if (Math.abs(vel) < MIN_VELOCITY) { raf = null; return; }
    raf = requestAnimationFrame(coastTick);
  }
  function startCoast(v) {
    vel = v;
    if (reduce) { pos = clamp(pos + vel * 6); vel = 0; paint(); return; }
    stopCoast();
    raf = requestAnimationFrame(coastTick);
  }

  function wireReal() {
    viewport.addEventListener('wheel', function (e) {
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay();
      stopCoast();
      pos = clamp(pos - e.deltaY * WHEEL_SCALE);
      paint();
      startCoast(-e.deltaY * WHEEL_SCALE * 0.4);
    }, { passive: false });

    var dragging = false, startY = 0, startPos = 0, lastY = 0, lastT = 0, dragVel = 0;
    viewport.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay(); stopCoast();
      dragging = true; startY = lastY = e.clientY; startPos = pos; lastT = performance.now(); dragVel = 0;
      viewport.setPointerCapture(e.pointerId);
    });
    viewport.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      var t = performance.now();
      var dt = Math.max(1, t - lastT);
      dragVel = (e.clientY - lastY) / dt * 16.6;
      pos = clamp(startPos + (e.clientY - startY));
      paint();
      lastY = e.clientY; lastT = t;
    });
    viewport.addEventListener('pointerup', function () {
      if (!dragging) { return; }
      dragging = false;
      startCoast(dragVel);
    });
  }
  wireReal();
  measure();
  window.addEventListener('resize', measure);

  // 자동 루프: 번갈아 방향을 바꿔 세게 튕겨 같은 관성 코스팅 경로를 반복 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    measure();
    if (maxScroll <= 0) { return; }
    phase += 1;
    var dir = (phase % 2 === 0) ? -1 : 1;
    pos = clamp(dir > 0 ? 0 : -maxScroll);
    paint();
    startCoast(dir * -9.5);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 1500); autoTick(); }
})();

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

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

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

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

    // 계산 겹: 손을 뗀 순간의 속도 값(vel)이 매 프레임 FRICTION 배씩 줄어들며 이동 값(pos)에
    // 더해진다 — vel *= FRICTION; pos += vel. MIN_VELOCITY 밑으로 떨어지면 코스팅을 멈춘다.
    const FRICTION = 0.93;
    const MIN_VELOCITY = 0.04;
    const WHEEL_SCALE = 0.55;

    let pos = 0, vel = 0, raf: number | null = null, maxScroll = 0;

    function measure() {
      maxScroll = Math.max(0, track!.scrollHeight - viewport!.clientHeight);
    }
    function clamp(v: number) { return Math.max(-maxScroll, Math.min(0, v)); }
    function paint() { track!.style.transform = "translateY(" + pos.toFixed(1) + "px)"; }

    function stopCoast() { if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
    function coastTick() {
      vel *= FRICTION;
      pos = clamp(pos + vel);
      if (pos === 0 || pos === -maxScroll) { vel = 0; }
      paint();
      if (Math.abs(vel) < MIN_VELOCITY) { raf = null; return; }
      raf = requestAnimationFrame(coastTick);
    }
    function startCoast(v: number) {
      vel = v;
      if (reduce) { pos = clamp(pos + vel * 6); vel = 0; paint(); return; }
      stopCoast();
      raf = requestAnimationFrame(coastTick);
    }

    function onWheel(e: WheelEvent) {
      e.preventDefault();
      stopAutoplay();
      stopCoast();
      pos = clamp(pos - e.deltaY * WHEEL_SCALE);
      paint();
      startCoast(-e.deltaY * WHEEL_SCALE * 0.4);
    }

    let dragging = false, startY = 0, startPos = 0, lastY = 0, lastT = 0, dragVel = 0;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay(); stopCoast();
      dragging = true; startY = lastY = e.clientY; startPos = pos; lastT = performance.now(); dragVel = 0;
      viewport!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const t = performance.now();
      const dt = Math.max(1, t - lastT);
      dragVel = (e.clientY - lastY) / dt * 16.6;
      pos = clamp(startPos + (e.clientY - startY));
      paint();
      lastY = e.clientY; lastT = t;
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      startCoast(dragVel);
    }

    viewport.addEventListener("wheel", onWheel, { passive: false });
    viewport.addEventListener("pointerdown", onPointerDown);
    viewport.addEventListener("pointermove", onPointerMove);
    viewport.addEventListener("pointerup", onPointerUp);
    measure();
    const onResize = () => measure();
    window.addEventListener("resize", onResize);

    let autoTimer: number | null = null, phase = 0;
    function autoTick() {
      measure();
      if (maxScroll <= 0) return;
      phase += 1;
      const dir = phase % 2 === 0 ? -1 : 1;
      pos = clamp(dir > 0 ? 0 : -maxScroll);
      paint();
      startCoast(dir * -9.5);
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 1500); autoTick(); }

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

  const rows = Array.from({ length: 10 }, (_, i) => i + 1);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.mic}>
        <div className={styles.viewport} role="application" aria-label="관성 스크롤 목록">
          <ul className={styles.track}>
            {rows.map((n) => (
              <li key={n} className={styles.row} style={{ background: color }}>
                항목 {String(n).padStart(2, "0")}
              </li>
            ))}
          </ul>
        </div>
        <p className={styles.hint}>휠이나 드래그로 스크롤한 뒤 놓으면 관성으로 미끄러집니다</p>
      </div>
    </div>
  );
}