03手を離しても慣性値の分だけ滑り続けるつまみ

<main class="stage">
<div class="sb is-demo" id="sb">
  <div class="sb__viewport" id="sbViewport">
    <div class="sb__content" id="sbContent"></div>
  </div>
  <div class="sb__track" id="sbTrack">
    <div class="sb__thumb" id="sbThumb" role="scrollbar" aria-orientation="vertical" aria-controls="sbViewport" aria-valuenow="0" aria-valuemin="0" aria-valuemax="100" tabindex="0"></div>
  </div>
</div>
</main>

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

// 핵심: 드래그를 놓는 순간의 속도 값이 SCROLLBAR_INERTIA_DECAY 만큼 매 프레임 줄며
// coastStep() 이 몇 프레임 더 스크롤을 미끄러뜨린 뒤 멈춘다.

$duration: $dur-quick;
$easing: $ease-out;
$color: $subject-ink;

@include stage($stage-orange);

.sb {
  display: flex;
  gap: $sp-2;
  width: min(88vw, 300px);
  height: min(56vw, 176px);
}

.sb__viewport {
  flex: 1;
  min-width: 0;
  height: 100%;
  overflow-y: auto;
  scrollbar-width: none;
  -ms-overflow-style: none;
  border-radius: $r-card;
  background: rgba(255, 255, 255, .16);
}

.sb__viewport::-webkit-scrollbar { display: none; }

.sb__content {
  display: flex;
  flex-direction: column;
  gap: $sp-2;
  padding: $sp-2;
}

.sb__card {
  border-radius: $r-sm;
  padding: $sp-2 $sp-3;
  background: rgba(255, 255, 255, .22);
}

.sb__cardTitle {
  margin: 0 0 $sp-1;
  color: $color;
  font-weight: 700;
  font-size: 12px;
}

.sb__cardLine {
  margin: 0;
  color: rgba(23, 20, 26, .68);
  font-size: 10px;
  line-height: 1.4;
}

.sb__track {
  position: relative;
  width: $sp-2;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .28);
}

.sb__thumb {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  border-radius: $r-pill;
  background: $subject-ink;
  transition: none;
  cursor: grab;
}

.sb__thumb:focus-visible {
  outline: 2px solid $subject-ink;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .sb { width: min(94vw, 260px); height: 148px; }
  .sb__cardTitle { font-size: 11px; }
  .sb__cardLine { font-size: 9px; }
}

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

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sb {
  display: flex;
  gap: 8px;
  width: min(88vw, 300px);
  height: min(56vw, 176px);
}

.sb__viewport {
  flex: 1;
  min-width: 0;
  height: 100%;
  overflow-y: auto;
  scrollbar-width: none;
  -ms-overflow-style: none;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.16);
}

.sb__viewport::-webkit-scrollbar {
  display: none;
}

.sb__content {
  display: flex;
  flex-direction: column;
  gap: 8px;
  padding: 8px;
}

.sb__card {
  border-radius: 8px;
  padding: 8px 12px;
  background: rgba(255, 255, 255, 0.22);
}

.sb__cardTitle {
  margin: 0 0 2px;
  color: #17141a;
  font-weight: 700;
  font-size: 12px;
}

.sb__cardLine {
  margin: 0;
  color: rgba(23, 20, 26, 0.68);
  font-size: 10px;
  line-height: 1.4;
}

.sb__track {
  position: relative;
  width: 8px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.28);
}

.sb__thumb {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  border-radius: 999px;
  background: #17141a;
  transition: none;
  cursor: grab;
}

.sb__thumb:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .sb {
    width: min(94vw, 260px);
    height: 148px;
  }
  .sb__cardTitle {
    font-size: 11px;
  }
  .sb__cardLine {
    font-size: 9px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sb__thumb {
    transition: none;
  }
}

(function () {
  var THUMB_MIN_PX = 26;
  var KEY_STEP_PX = 32;
  var SCROLLBAR_INERTIA_DECAY = 0.92;
  var VEL_EPS = 0.4;
  var SIM_KICK_PX = 30;

  var root = document.getElementById('sb');
  var viewport = document.getElementById('sbViewport');
  var content = document.getElementById('sbContent');
  var track = document.getElementById('sbTrack');
  var thumb = document.getElementById('sbThumb');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  for (var i = 1; i <= 8; i++) {
    var card = document.createElement('div');
    card.className = 'sb__card';
    var h = document.createElement('p');
    h.className = 'sb__cardTitle';
    h.textContent = 'カード ' + i;
    var p = document.createElement('p');
    p.className = 'sb__cardLine';
    p.textContent = '離した瞬間の速度値が減衰係数の分だけ毎フレーム縮みます。';
    card.appendChild(h);
    card.appendChild(p);
    content.appendChild(card);
  }

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function maxScroll() { return Math.max(0, viewport.scrollHeight - viewport.clientHeight); }

  function syncThumb() {
    var trackLen = track.clientHeight;
    var total = viewport.scrollHeight;
    var visible = viewport.clientHeight;
    var thumbLen = clamp(trackLen * (visible / total), THUMB_MIN_PX, trackLen);
    var ms = maxScroll();
    var ratio = ms > 0 ? viewport.scrollTop / ms : 0;
    var thumbPos = (trackLen - thumbLen) * ratio;
    thumb.style.height = thumbLen.toFixed(1) + 'px';
    thumb.style.transform = 'translateY(' + thumbPos.toFixed(1) + 'px)';
    thumb.setAttribute('aria-valuenow', String(Math.round(ratio * 100)));
  }

  viewport.addEventListener('scroll', syncThumb);

  var dragging = false;
  var dragStartY = 0;
  var dragStartScroll = 0;
  var lastMoveY = 0;
  var vel = 0;

  thumb.addEventListener('pointerdown', function (e) {
    dragging = true;
    vel = 0;
    root.classList.remove('is-demo');
    dragStartY = e.clientY;
    lastMoveY = e.clientY;
    dragStartScroll = viewport.scrollTop;
    thumb.setPointerCapture(e.pointerId);
  });
  thumb.addEventListener('pointermove', function (e) {
    if (!dragging) return;
    var trackLen = track.clientHeight;
    var thumbLen = thumb.clientHeight;
    var moveRange = Math.max(1, trackLen - thumbLen);
    var scale = maxScroll() / moveRange;
    var frameDeltaPx = (e.clientY - lastMoveY) * scale;
    vel = frameDeltaPx;
    lastMoveY = e.clientY;
    var totalDeltaPx = (e.clientY - dragStartY) * scale;
    viewport.scrollTop = clamp(dragStartScroll + totalDeltaPx, 0, maxScroll());
  });
  function coastStep() {
    if (Math.abs(vel) <= VEL_EPS) { vel = 0; return; }
    vel = vel * SCROLLBAR_INERTIA_DECAY;
    viewport.scrollTop = clamp(viewport.scrollTop + vel, 0, maxScroll());
  }
  thumb.addEventListener('pointerup', function () { dragging = false; });
  thumb.addEventListener('pointercancel', function () { dragging = false; vel = 0; });

  track.addEventListener('pointerdown', function (e) {
    if (e.target !== track) return;
    root.classList.remove('is-demo');
    var rect = track.getBoundingClientRect();
    var trackLen = track.clientHeight;
    var thumbLen = thumb.clientHeight;
    var clickPos = clamp(e.clientY - rect.top - thumbLen / 2, 0, trackLen - thumbLen);
    var ratio = (trackLen - thumbLen) > 0 ? clickPos / (trackLen - thumbLen) : 0;
    viewport.scrollTop = ratio * maxScroll();
  });

  thumb.addEventListener('keydown', function (e) {
    var ms = maxScroll();
    if (e.key === 'ArrowDown') { viewport.scrollTop = clamp(viewport.scrollTop + KEY_STEP_PX, 0, ms); e.preventDefault(); }
    else if (e.key === 'ArrowUp') { viewport.scrollTop = clamp(viewport.scrollTop - KEY_STEP_PX, 0, ms); e.preventDefault(); }
    else if (e.key === 'PageDown') { viewport.scrollTop = clamp(viewport.scrollTop + viewport.clientHeight, 0, ms); e.preventDefault(); }
    else if (e.key === 'PageUp') { viewport.scrollTop = clamp(viewport.scrollTop - viewport.clientHeight, 0, ms); e.preventDefault(); }
    else if (e.key === 'Home') { viewport.scrollTop = 0; e.preventDefault(); }
    else if (e.key === 'End') { viewport.scrollTop = ms; e.preventDefault(); }
    if (e.key.indexOf('Arrow') === 0 || e.key === 'PageDown' || e.key === 'PageUp' || e.key === 'Home' || e.key === 'End') {
      root.classList.remove('is-demo');
      vel = 0;
    }
  });

  var startTime = null;
  var nextKickAt = 0;
  var kickSign = 1;
  function demoTick(now) {
    if (startTime === null) { startTime = now; }
    var t = (now - startTime) / 1000;
    if (root.classList.contains('is-demo') && !dragging) {
      if (t >= nextKickAt) {
        vel = SIM_KICK_PX * kickSign;
        kickSign = -kickSign;
        nextKickAt = t + 0.9;
      }
    }
    if (!dragging && vel !== 0) { coastStep(); }
    requestAnimationFrame(demoTick);
  }

  syncThumb();
  if (!reduce) { requestAnimationFrame(demoTick); }
})();

// ScrollbarThumbDragInertiaCoast — vanilla 판(run/411/demo/03-scrollbar-thumb-drag-inertia-coast/index.html) 의 React 포트.
// 핵심: 드래그를 놓는 순간의 속도 값이 SCROLLBAR_INERTIA_DECAY 만큼 매 프레임 줄며 coastStep() 이 더 미끄러뜨린다.
import { useEffect, useRef } from "react";
import styles from "./ScrollbarThumbDragInertiaCoast.module.scss";

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

const THUMB_MIN_PX = 26;
const KEY_STEP_PX = 32;
const SCROLLBAR_INERTIA_DECAY = 0.92;
const VEL_EPS = 0.4;
const SIM_KICK_PX = 30;

function clamp(v: number, lo: number, hi: number): number {
  return Math.max(lo, Math.min(hi, v));
}

export default function ScrollbarThumbDragInertiaCoast() {
  const viewportRef = useRef<HTMLDivElement | null>(null);
  const trackRef = useRef<HTMLDivElement | null>(null);
  const thumbRef = useRef<HTMLDivElement | null>(null);
  const demoRef = useRef<boolean>(true) as Box<boolean>;
  const draggingRef = useRef<boolean>(false) as Box<boolean>;
  const dragStartYRef = useRef<number>(0) as Box<number>;
  const dragStartScrollRef = useRef<number>(0) as Box<number>;
  const lastMoveYRef = useRef<number>(0) as Box<number>;
  const velRef = useRef<number>(0) as Box<number>;

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const viewport = viewportRef.current!;
    const track = trackRef.current!;
    const thumb = thumbRef.current!;
    if (!viewport || !track || !thumb) return;

    function maxScroll(): number {
      return Math.max(0, viewport.scrollHeight - viewport.clientHeight);
    }

    function syncThumb(): void {
      const trackLen = track.clientHeight;
      const total = viewport.scrollHeight;
      const visible = viewport.clientHeight;
      const thumbLen = clamp(trackLen * (visible / total), THUMB_MIN_PX, trackLen);
      const ms = maxScroll();
      const ratio = ms > 0 ? viewport.scrollTop / ms : 0;
      const thumbPos = (trackLen - thumbLen) * ratio;
      thumb.style.height = thumbLen.toFixed(1) + "px";
      thumb.style.transform = "translateY(" + thumbPos.toFixed(1) + "px)";
      thumb.setAttribute("aria-valuenow", String(Math.round(ratio * 100)));
    }

    function onScroll(): void { syncThumb(); }

    function onThumbPointerDown(e: PointerEvent): void {
      draggingRef.current = true;
      velRef.current = 0;
      demoRef.current = false;
      dragStartYRef.current = e.clientY;
      lastMoveYRef.current = e.clientY;
      dragStartScrollRef.current = viewport.scrollTop;
      thumb.setPointerCapture(e.pointerId);
    }
    function onThumbPointerMove(e: PointerEvent): void {
      if (!draggingRef.current) return;
      const trackLen = track.clientHeight;
      const thumbLen = thumb.clientHeight;
      const moveRange = Math.max(1, trackLen - thumbLen);
      const scale = maxScroll() / moveRange;
      const frameDeltaPx = (e.clientY - lastMoveYRef.current) * scale;
      velRef.current = frameDeltaPx;
      lastMoveYRef.current = e.clientY;
      const totalDeltaPx = (e.clientY - dragStartYRef.current) * scale;
      viewport.scrollTop = clamp(dragStartScrollRef.current + totalDeltaPx, 0, maxScroll());
    }
    function coastStep(): void {
      if (Math.abs(velRef.current) <= VEL_EPS) { velRef.current = 0; return; }
      velRef.current = velRef.current * SCROLLBAR_INERTIA_DECAY;
      viewport.scrollTop = clamp(viewport.scrollTop + velRef.current, 0, maxScroll());
    }
    function onThumbPointerUp(): void { draggingRef.current = false; }
    function onThumbPointerCancel(): void { draggingRef.current = false; velRef.current = 0; }

    function onTrackPointerDown(e: PointerEvent): void {
      if (e.target !== track) return;
      demoRef.current = false;
      const rect = track.getBoundingClientRect();
      const trackLen = track.clientHeight;
      const thumbLen = thumb.clientHeight;
      const clickPos = clamp(e.clientY - rect.top - thumbLen / 2, 0, trackLen - thumbLen);
      const ratio = (trackLen - thumbLen) > 0 ? clickPos / (trackLen - thumbLen) : 0;
      viewport.scrollTop = ratio * maxScroll();
    }

    function onThumbKeyDown(e: KeyboardEvent): void {
      const ms = maxScroll();
      if (e.key === "ArrowDown") { viewport.scrollTop = clamp(viewport.scrollTop + KEY_STEP_PX, 0, ms); e.preventDefault(); }
      else if (e.key === "ArrowUp") { viewport.scrollTop = clamp(viewport.scrollTop - KEY_STEP_PX, 0, ms); e.preventDefault(); }
      else if (e.key === "PageDown") { viewport.scrollTop = clamp(viewport.scrollTop + viewport.clientHeight, 0, ms); e.preventDefault(); }
      else if (e.key === "PageUp") { viewport.scrollTop = clamp(viewport.scrollTop - viewport.clientHeight, 0, ms); e.preventDefault(); }
      else if (e.key === "Home") { viewport.scrollTop = 0; e.preventDefault(); }
      else if (e.key === "End") { viewport.scrollTop = ms; e.preventDefault(); }
      if (e.key.indexOf("Arrow") === 0 || e.key === "PageDown" || e.key === "PageUp" || e.key === "Home" || e.key === "End") {
        demoRef.current = false;
        velRef.current = 0;
      }
    }

    let startTime: number | null = null;
    let nextKickAt = 0;
    let kickSign = 1;
    let frame = 0;
    function demoTick(now: number): void {
      if (startTime === null) startTime = now;
      const t = (now - startTime) / 1000;
      if (demoRef.current && !draggingRef.current) {
        if (t >= nextKickAt) {
          velRef.current = SIM_KICK_PX * kickSign;
          kickSign = -kickSign;
          nextKickAt = t + 0.9;
        }
      }
      if (!draggingRef.current && velRef.current !== 0) { coastStep(); }
      frame = requestAnimationFrame(demoTick);
    }

    viewport.addEventListener("scroll", onScroll);
    thumb.addEventListener("pointerdown", onThumbPointerDown);
    thumb.addEventListener("pointermove", onThumbPointerMove);
    thumb.addEventListener("pointerup", onThumbPointerUp);
    thumb.addEventListener("pointercancel", onThumbPointerCancel);
    track.addEventListener("pointerdown", onTrackPointerDown);
    thumb.addEventListener("keydown", onThumbKeyDown);

    syncThumb();
    if (!reduce) { frame = requestAnimationFrame(demoTick); }

    return () => {
      viewport.removeEventListener("scroll", onScroll);
      thumb.removeEventListener("pointerdown", onThumbPointerDown);
      thumb.removeEventListener("pointermove", onThumbPointerMove);
      thumb.removeEventListener("pointerup", onThumbPointerUp);
      thumb.removeEventListener("pointercancel", onThumbPointerCancel);
      track.removeEventListener("pointerdown", onTrackPointerDown);
      thumb.removeEventListener("keydown", onThumbKeyDown);
      if (frame) cancelAnimationFrame(frame);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.sb}>
        <div className={styles.sb__viewport} ref={viewportRef}>
          <div className={styles.sb__content}>
            {Array.from({ length: 8 }, (_, idx) => idx + 1).map((i) => (
              <div className={styles.sb__card} key={i}>
                <p className={styles.sb__cardTitle}>{"카드 " + i}</p>
                <p className={styles.sb__cardLine}>놓는 순간의 속도 값이 감쇠 계수만큼 매 프레임 줄어듭니다.</p>
              </div>
            ))}
          </div>
        </div>
        <div className={styles.sb__track} ref={trackRef}>
          <div
            className={styles.sb__thumb}
            ref={thumbRef}
            role="scrollbar"
            aria-orientation="vertical"
            aria-valuenow={0}
            aria-valuemin={0}
            aria-valuemax={100}
            tabIndex={0}
          />
        </div>
      </div>
    </main>
  );
}