07Friction decel — once the input value stops, a speed value slows gradually by a friction value

<main class="stage">
<div class="fmd is-demo" id="fmd">
  <div class="fmd__viewport" id="fmdViewport" role="application" aria-label="Marquee drag test panel" tabindex="0">
    <div class="fmd__track" id="fmdTrack">
      <span class="fmd__chip fmd__chip--a">01</span>
      <span class="fmd__chip fmd__chip--b">02</span>
      <span class="fmd__chip fmd__chip--c">03</span>
      <span class="fmd__chip fmd__chip--a">04</span>
      <span class="fmd__chip fmd__chip--b">05</span>
      <span class="fmd__chip fmd__chip--c">06</span>
      <span class="fmd__chip fmd__chip--a">01</span>
      <span class="fmd__chip fmd__chip--b">02</span>
      <span class="fmd__chip fmd__chip--c">03</span>
      <span class="fmd__chip fmd__chip--a">04</span>
      <span class="fmd__chip fmd__chip--b">05</span>
      <span class="fmd__chip fmd__chip--c">06</span>
    </div>
  </div>
  <p class="fmd__hint">Drag it or spin the wheel — the speed shifts, then friction eases it back to the base speed.</p>
</div>
</main>

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

// 계산 겹(마찰 복귀 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-paper);

.stage { position: relative; }

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

.fmd__viewport {
  position: relative;
  width: 245px;
  height: 64px;
  overflow: hidden;
  border-radius: $r-card;
  background: rgba($subject-ink, .05);
  border: 1px solid rgba($subject-ink, .12);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
  display: flex;
  align-items: center;
}

.fmd__track {
  display: flex;
  align-items: center;
  gap: $sp-2;
  will-change: transform;
  padding-left: $sp-3;
}

.fmd__chip {
  flex: 0 0 auto;
  width: 56px;
  height: 32px;
  border-radius: $r-pill;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 12px;
  font-weight: 700;
  color: $subject-ink;
}

.fmd__chip--a { background: $subject-mint; }
.fmd__chip--b { background: $subject-sky; }
.fmd__chip--c { background: $subject-lilac; }

.fmd__hint {
  color: rgba($subject-ink, .72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .fmd__viewport { width: 196px; height: 52px; }
  .fmd__chip { width: 44px; height: 26px; }
}

@media (prefers-reduced-motion: reduce) {
  .fmd__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: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

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

.fmd__viewport {
  position: relative;
  width: 245px;
  height: 64px;
  overflow: hidden;
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.05);
  border: 1px solid rgba(23, 20, 26, 0.12);
  touch-action: none;
  overscroll-behavior: contain;
  cursor: grab;
  display: flex;
  align-items: center;
}

.fmd__track {
  display: flex;
  align-items: center;
  gap: 8px;
  will-change: transform;
  padding-left: 12px;
}

.fmd__chip {
  flex: 0 0 auto;
  width: 56px;
  height: 32px;
  border-radius: 999px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 12px;
  font-weight: 700;
  color: #17141a;
}

.fmd__chip--a {
  background: #4cd4a6;
}

.fmd__chip--b {
  background: #6ec8ff;
}

.fmd__chip--c {
  background: #b9a0ff;
}

.fmd__hint {
  color: rgba(23, 20, 26, 0.72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .fmd__viewport {
    width: 196px;
    height: 52px;
  }
  .fmd__chip {
    width: 44px;
    height: 26px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fmd__track {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('fmd');
  var viewport = document.getElementById('fmdViewport');
  var track = document.getElementById('fmdTrack');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 평소엔 BASE_SPEED 로 계속 흐르다가, 드래그·휠로 속도(vel)가 그 값에서 벗어나면
  // 매 프레임 FRICTION_PULL 만큼만 BASE_SPEED 쪽으로 되돌아온다 — 순간 정지가 아니라 서서히 복귀.
  var CHIP_W = 56, GAP = 10, SET_COUNT = 6;
  var WRAP = SET_COUNT * (CHIP_W + GAP);
  var BASE_SPEED = -0.9;
  var FRICTION_PULL = 0.06;
  var pos = 0, vel = BASE_SPEED;
  var inputActive = false;
  var raf = null;

  function clampWrap(v) { var m = v % WRAP; if (m > 0) { m -= WRAP; } return m; }
  function paint() { track.style.transform = 'translateX(' + pos.toFixed(1) + 'px)'; }
  function loopTick() {
    if (reduce) { raf = null; return; }
    if (!inputActive) { vel += (BASE_SPEED - vel) * FRICTION_PULL; }
    pos = clampWrap(pos + vel);
    paint();
    raf = requestAnimationFrame(loopTick);
  }
  function ensureLoop() { if (raf === null && !reduce) { raf = requestAnimationFrame(loopTick); } }
  ensureLoop();
  paint();

  function wireReal() {
    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 dx = e.clientX - lastX;
      vel = dx / dt * 16.6;
      pos = clampWrap(pos + dx);
      paint();
      lastX = e.clientX; lastT = t;
    });
    viewport.addEventListener('pointerup', function () {
      if (!dragging) { return; }
      dragging = false; inputActive = false;
      ensureLoop();
    });
    viewport.addEventListener('wheel', function (e) {
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay();
      inputActive = true;
      vel += -e.deltaY * 0.4 - e.deltaX * 0.4;
      ensureLoop();
      clearTimeout(viewport._wt);
      viewport._wt = setTimeout(function () { inputActive = false; }, 80);
    }, { passive: false });
  }
  wireReal();

  // 자동 루프: 주기적으로 강하게 튕겨 마찰이 기준 속도로 되돌리는 과정을 반복 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    phase += 1;
    var dir = (phase % 2 === 0) ? 1 : -1;
    inputActive = true;
    vel = dir * 7 + BASE_SPEED;
    ensureLoop();
    setTimeout(function () { inputActive = false; }, 140);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 1500); autoTick(); }
})();

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

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

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

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

    // 계산 겹: 평소엔 BASE_SPEED 로 계속 흐르다가, 드래그·휠로 속도(vel)가 그 값에서 벗어나면
    // 매 프레임 FRICTION_PULL 만큼만 BASE_SPEED 쪽으로 되돌아온다 — 순간 정지가 아니라 서서히 복귀.
    const CHIP_W = 56, GAP = 10, SET_COUNT = 6;
    const WRAP = SET_COUNT * (CHIP_W + GAP);
    const BASE_SPEED = -0.9;
    const FRICTION_PULL = 0.06;
    let pos = 0, vel = BASE_SPEED;
    let inputActive = false;
    let raf: number | null = null;

    function clampWrap(v: number) { let m = v % WRAP; if (m > 0) m -= WRAP; return m; }
    function paint() { track!.style.transform = "translateX(" + pos.toFixed(1) + "px)"; }
    function loopTick() {
      if (reduce) { raf = null; return; }
      if (!inputActive) { vel += (BASE_SPEED - vel) * FRICTION_PULL; }
      pos = clampWrap(pos + vel);
      paint();
      raf = requestAnimationFrame(loopTick);
    }
    function ensureLoop() { if (raf === null && !reduce) raf = requestAnimationFrame(loopTick); }
    ensureLoop();
    paint();

    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 dx = e.clientX - lastX;
      vel = (dx / dt) * 16.6;
      pos = clampWrap(pos + dx);
      paint();
      lastX = e.clientX; lastT = t;
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false; inputActive = false;
      ensureLoop();
    }
    function onWheel(e: WheelEvent) {
      e.preventDefault();
      stopAutoplay();
      inputActive = true;
      vel += -e.deltaY * 0.4 - e.deltaX * 0.4;
      ensureLoop();
      if (wheelTimer !== null) window.clearTimeout(wheelTimer);
      wheelTimer = window.setTimeout(() => { inputActive = false; }, 80);
    }
    let wheelTimer: number | null = null;

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

    let autoTimer: number | null = null, phase = 0;
    function autoTick() {
      phase += 1;
      const dir = phase % 2 === 0 ? 1 : -1;
      inputActive = true;
      vel = dir * 7 + BASE_SPEED;
      ensureLoop();
      window.setTimeout(() => { inputActive = false; }, 140);
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 1500); autoTick(); }

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

  const labels = ["01", "02", "03", "04", "05", "06", "01", "02", "03", "04", "05", "06"];
  const cls = ["a", "b", "c"];

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.fmd}>
        <div className={styles.viewport} role="application" aria-label="마퀴 드래그 실험판" tabIndex={0}>
          <div className={styles.track}>
            {labels.map((label, i) => (
              <span
                key={i}
                className={`${styles.chip} ${styles[cls[i % 3]]}`}
                style={i === 0 ? { background: color } : undefined}
              >
                {label}
              </span>
            ))}
          </div>
        </div>
        <p className={styles.hint}>드래그하거나 휠을 굴리면 속도가 바뀌었다가 마찰로 기준 속도로 되돌아옵니다</p>
      </div>
    </div>
  );
}