08Lag tracking — a trailing value doesn't match the real scroll value instantly, chasing it with a lag value instead

<main class="stage">
<div class="lst is-demo" id="lst">
  <div class="lst__viewport" id="lstViewport" role="application" aria-label="Lag scrollbar test panel" tabindex="0">
    <div class="lst__track" id="lstTrack">
      <div class="lst__row">Row 1</div>
      <div class="lst__row">Row 2</div>
      <div class="lst__row">Row 3</div>
      <div class="lst__row">Row 4</div>
      <div class="lst__row">Row 5</div>
      <div class="lst__row">Row 6</div>
      <div class="lst__row">Row 7</div>
      <div class="lst__row">Row 8</div>
    </div>
    <div class="lst__rail" id="lstRail">
      <div class="lst__thumb" id="lstThumb"></div>
    </div>
  </div>
  <p class="lst__hint">Scroll and the list follows instantly, while the scrollbar handle lags behind and slowly catches up.</p>
</div>
</main>

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

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

.stage { position: relative; }

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

.lst__viewport {
  position: relative;
  width: 220px;
  height: 150px;
  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;
}

.lst__track {
  position: absolute;
  inset: 0;
  will-change: transform;
}

.lst__row {
  height: 40px;
  margin: 0 $sp-6 0 $sp-3;
  border-radius: $r-control;
  background: rgba($stage-paper, .1);
  color: $stage-paper;
  display: flex;
  align-items: center;
  padding-left: $sp-2;
  font-size: 12px;
  box-sizing: border-box;
}

.lst__rail {
  position: absolute;
  top: $sp-2;
  right: $sp-2;
  bottom: $sp-2;
  width: 6px;
  border-radius: $r-pill;
  background: rgba($stage-paper, .12);
}

.lst__thumb {
  position: absolute;
  top: 0;
  left: 0;
  width: 6px;
  border-radius: $r-pill;
  background: $subject-sky;
  will-change: transform;
}

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

@media (max-width: 340px) {
  .lst { gap: $sp-2; }
  .lst__viewport { width: 176px; height: 90px; }
  .lst__row { height: 30px; }
  .lst__hint { font-size: 11px; }
}

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

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

.lst__viewport {
  position: relative;
  width: 220px;
  height: 150px;
  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;
}

.lst__track {
  position: absolute;
  inset: 0;
  will-change: transform;
}

.lst__row {
  height: 40px;
  margin: 0 24px 0 12px;
  border-radius: 12px;
  background: rgba(255, 255, 255, 0.1);
  color: #ffffff;
  display: flex;
  align-items: center;
  padding-left: 8px;
  font-size: 12px;
  box-sizing: border-box;
}

.lst__rail {
  position: absolute;
  top: 8px;
  right: 8px;
  bottom: 8px;
  width: 6px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.12);
}

.lst__thumb {
  position: absolute;
  top: 0;
  left: 0;
  width: 6px;
  border-radius: 999px;
  background: #6ec8ff;
  will-change: transform;
}

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

@media (max-width: 340px) {
  .lst {
    gap: 8px;
  }
  .lst__viewport {
    width: 176px;
    height: 90px;
  }
  .lst__row {
    height: 30px;
  }
  .lst__hint {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .lst__thumb {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('lst');
  var viewport = document.getElementById('lstViewport');
  var track = document.getElementById('lstTrack');
  var rail = document.getElementById('lstRail');
  var thumb = document.getElementById('lstThumb');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 목록 내용은 입력에 1:1 로 즉시 따라가지만(scrollPos), 손잡이 위치(thumbY)는
  // 목표값(targetThumbY)을 매 프레임 LAG 만큼만 따라잡는 지수 보간(lerp)이라 항상 한 박자 늦게 도착한다.
  var LAG = 0.15;
  var SNAP_EPS = 0.3;
  var rowH = 40, rowCount = 8;
  var maxScroll = Math.max(0, rowH * rowCount - viewport.clientHeight);
  var scrollPos = 0;
  var thumbY = 0;
  var raf = null;

  // BD19 — 막대·손잡이 실측 픽셀은 getBoundingClientRect 로 잰다(짐작값 금지).
  var railRect = rail.getBoundingClientRect();
  var railH = railRect.height;
  var thumbH = Math.max(28, railH * (viewport.clientHeight / (rowH * rowCount)));
  thumb.style.height = thumbH.toFixed(1) + 'px';

  function paintTrack() { track.style.transform = 'translateY(-' + scrollPos.toFixed(1) + 'px)'; }
  function targetThumbY() { return maxScroll <= 0 ? 0 : (scrollPos / maxScroll) * (railH - thumbH); }
  function paintThumb() { thumb.style.transform = 'translateY(' + thumbY.toFixed(1) + 'px)'; }
  function loopTick() {
    var target = targetThumbY();
    if (reduce) { thumbY = target; paintThumb(); raf = null; return; }
    thumbY += (target - thumbY) * LAG;
    if (Math.abs(target - thumbY) < SNAP_EPS) {
      thumbY = target; paintThumb(); raf = null; return;
    }
    paintThumb();
    raf = requestAnimationFrame(loopTick);
  }
  function ensureLoop() { if (raf === null) { raf = requestAnimationFrame(loopTick); } }
  function setScroll(v) {
    scrollPos = Math.max(0, Math.min(maxScroll, v));
    paintTrack();
    ensureLoop();
  }
  paintTrack(); paintThumb();

  function wireReal() {
    viewport.addEventListener('wheel', function (e) {
      e.preventDefault();
      root.classList.remove('is-demo'); stopAutoplay();
      setScroll(scrollPos + e.deltaY);
    }, { passive: false });

    var dragging = false, lastY = 0;
    viewport.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay();
      dragging = true; lastY = e.clientY;
      viewport.setPointerCapture(e.pointerId);
    });
    viewport.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      setScroll(scrollPos - (e.clientY - lastY));
      lastY = e.clientY;
    });
    viewport.addEventListener('pointerup', function () { dragging = false; });
    viewport.addEventListener('keydown', function (e) {
      if (e.key !== 'ArrowDown' && e.key !== 'ArrowUp') { return; }
      root.classList.remove('is-demo'); stopAutoplay();
      setScroll(scrollPos + (e.key === 'ArrowDown' ? 40 : -40));
    });
  }
  wireReal();

  // 자동 루프: 바닥까지 내렸다 다시 위로 올려 손잡이가 뒤늦게 따라붙는 지연을 반복 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    phase += 1;
    setScroll(phase % 2 === 0 ? 0 : maxScroll);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 1500); autoTick(); }
})();

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

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

export default function ScrollMomentumLagScrollbarThumb({
  color = "#6ec8ff",
}: ScrollMomentumLagScrollbarThumbProps) {
  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}`);
    const rail = root.querySelector<HTMLDivElement>(`.${styles.rail}`);
    const thumb = root.querySelector<HTMLDivElement>(`.${styles.thumb}`);
    if (!viewport || !track || !rail || !thumb) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹: 목록 내용은 입력에 1:1 로 즉시 따라가지만(scrollPos), 손잡이 위치(thumbY)는
    // 목표값(targetThumbY)을 매 프레임 LAG 만큼만 따라잡는 지수 보간(lerp)이라 항상 한 박자 늦게 도착한다.
    const LAG = 0.15;
    const SNAP_EPS = 0.3;
    const rowH = 40, rowCount = 8;
    const maxScroll = Math.max(0, rowH * rowCount - viewport.clientHeight);
    let scrollPos = 0;
    let thumbY = 0;
    let raf: number | null = null;

    // BD19 — 막대·손잡이 실측 픽셀은 getBoundingClientRect 로 잰다(짐작값 금지).
    const railRect = rail.getBoundingClientRect();
    const railH = railRect.height;
    const thumbH = Math.max(28, railH * (viewport.clientHeight / (rowH * rowCount)));
    thumb.style.height = thumbH.toFixed(1) + "px";

    function paintTrack() { track!.style.transform = "translateY(-" + scrollPos.toFixed(1) + "px)"; }
    function targetThumbY() { return maxScroll <= 0 ? 0 : (scrollPos / maxScroll) * (railH - thumbH); }
    function paintThumb() { thumb!.style.transform = "translateY(" + thumbY.toFixed(1) + "px)"; }
    function loopTick() {
      const target = targetThumbY();
      if (reduce) { thumbY = target; paintThumb(); raf = null; return; }
      thumbY += (target - thumbY) * LAG;
      if (Math.abs(target - thumbY) < SNAP_EPS) { thumbY = target; paintThumb(); raf = null; return; }
      paintThumb();
      raf = requestAnimationFrame(loopTick);
    }
    function ensureLoop() { if (raf === null) raf = requestAnimationFrame(loopTick); }
    function setScroll(v: number) {
      scrollPos = Math.max(0, Math.min(maxScroll, v));
      paintTrack();
      ensureLoop();
    }
    paintTrack(); paintThumb();

    function onWheel(e: WheelEvent) {
      e.preventDefault();
      stopAutoplay();
      setScroll(scrollPos + e.deltaY);
    }
    let dragging = false, lastY = 0;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true; lastY = e.clientY;
      viewport!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      setScroll(scrollPos - (e.clientY - lastY));
      lastY = e.clientY;
    }
    function onPointerUp() { dragging = false; }
    function onKeydown(e: KeyboardEvent) {
      if (e.key !== "ArrowDown" && e.key !== "ArrowUp") return;
      stopAutoplay();
      setScroll(scrollPos + (e.key === "ArrowDown" ? 40 : -40));
    }
    viewport.addEventListener("wheel", onWheel, { passive: false });
    viewport.addEventListener("pointerdown", onPointerDown);
    viewport.addEventListener("pointermove", onPointerMove);
    viewport.addEventListener("pointerup", onPointerUp);
    viewport.addEventListener("keydown", onKeydown);

    let autoTimer: number | null = null, phase = 0;
    function autoTick() { phase += 1; setScroll(phase % 2 === 0 ? 0 : maxScroll); }
    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("wheel", onWheel);
      viewport.removeEventListener("pointerdown", onPointerDown);
      viewport.removeEventListener("pointermove", onPointerMove);
      viewport.removeEventListener("pointerup", onPointerUp);
      viewport.removeEventListener("keydown", onKeydown);
    };
  }, []);

  const rows = [1, 2, 3, 4, 5, 6, 7, 8];

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.lst}>
        <div className={styles.viewport} role="application" aria-label="지연 스크롤바 실험판" tabIndex={0}>
          <div className={styles.track}>
            {rows.map((n) => (
              <div key={n} className={styles.row}>행 {n}</div>
            ))}
          </div>
          <div className={styles.rail}>
            <div className={styles.thumb} style={{ background: color }} />
          </div>
        </div>
        <p className={styles.hint}>스크롤하면 목록은 바로 따라가고, 막대 손잡이는 뒤처졌다 서서히 따라붙습니다</p>
      </div>
    </div>
  );
}