01カスタムスクロールバー — 太さ・色・角丸を自分で決めるスクロールバー

<main class="stage">
<div class="wrap is-demo" id="wrap">
  <div class="head">
    <span class="modeLabel" id="modeLabel">標準サポート版</span>
    <button class="simToggle" id="simToggle" type="button" aria-pressed="false">フォールバックを再現</button>
  </div>
  <div class="body">
    <ol class="itemList" id="list">
      <li>設計ドキュメント</li>
      <li>カラートークン</li>
      <li>余白の規則</li>
      <li>ボタンの状態</li>
      <li>入力バリデーション</li>
      <li>アクセシビリティ点検</li>
      <li>パフォーマンス計測</li>
      <li>デプロイノート</li>
    </ol>
    <div class="track" id="track"><div class="trackDot" id="trackDot"></div></div>
  </div>
</div>
</main>

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

// 계산 겹은 index.html 의 target(실제 scrollTop 비율) → shown(lerp 추적) 에 있다 — 여기는 두 스크롤바
// 표기법(표준 속성 · 옛 의사요소)과 트랙 점의 모양만 맡는다.
// 표준: scrollbar-width·scrollbar-color. 옛 폴백: ::-webkit-scrollbar 3형제. 두 선언은 같은 목록에 둬도
// 충돌하지 않는다 — 표준을 아는 환경은 표준을, 모르는 환경은 옛 의사요소만 읽는다.
@include stage($stage-ink);

.wrap {
  width: 260px;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
}

.head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: $sp-2;
}

.modeLabel {
  color: $subject-cream;
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .02em;
}

.simToggle {
  appearance: none;
  border: none;
  border-radius: $r-pill;
  background: rgba($subject-cream, .14);
  color: $subject-cream;
  padding: $sp-1 $sp-3;
  font-size: 10px;
  font-weight: 600;
  cursor: pointer;
  transition: background $dur-instant $ease-out, transform $dur-instant $ease-out;

  &:hover { background: rgba($subject-cream, .22); }
  &:active { transform: scale(.96); }
  &[aria-pressed="true"] { background: $subject-orange-deep; }
  &:focus-visible { outline: 2px solid $subject-blue-light; outline-offset: 2px; }
}

.body {
  display: flex;
  align-items: stretch;
  gap: $sp-2;
}

.itemList {
  margin: 0;
  padding: $sp-2 $sp-3;
  list-style: none;
  width: 220px;
  height: 116px;
  overflow-y: scroll;
  border-radius: $r-sm;
  background: rgba($subject-cream, .06);

  // 폴백: 옛 브라우저용 의사요소 스크롤바 — 항상 선언돼 있고, 표준 속성을 아는 환경은 아래 @supports 가 덮는다
  &::-webkit-scrollbar { width: 10px; }
  &::-webkit-scrollbar-track { background: transparent; }
  &::-webkit-scrollbar-thumb { background: $subject-blue; border-radius: $r-pill; }
}

@supports (scrollbar-width: thin) {
  .itemList:not(.force-fallback) {
    scrollbar-width: thin;
    scrollbar-color: $subject-blue-light transparent;
  }
}

// 시뮬레이션 토글 — 실제 UA 감지가 아니라 표준 속성을 강제로 꺼서 옛 폴백 경로가 눈에 보이게 한다
.force-fallback .itemList {
  scrollbar-width: auto;
  scrollbar-color: unset;
}

.itemList li {
  padding: $sp-1 0;
  color: $subject-cream;
  font-size: 12px;
  border-bottom: 1px solid rgba($subject-cream, .1);

  &:last-child { border-bottom: none; }
}

.track {
  position: relative;
  width: 6px;
  border-radius: $r-pill;
  background: rgba($subject-cream, .1);
}

.trackDot {
  position: absolute;
  top: 0;
  left: 0;
  width: 6px;
  height: 28px;
  border-radius: $r-pill;
  background: $subject-blue-light;
  transform: translateY(0);
}

.force-fallback .trackDot { background: $subject-orange-deep; }

@media (max-width: 340px) {
  .wrap { width: 210px; }
  .itemList { width: 174px; height: 96px; font-size: 11px; }
  .modeLabel { font-size: 10px; }
}

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

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);
}

.wrap {
  width: 260px;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
}

.modeLabel {
  color: #fff7e6;
  font-size: 11px;
  font-weight: 600;
  letter-spacing: 0.02em;
}

.simToggle {
  appearance: none;
  border: none;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.14);
  color: #fff7e6;
  padding: 4px 12px;
  font-size: 10px;
  font-weight: 600;
  cursor: pointer;
  transition: background 140ms ease-out, transform 140ms ease-out;
}
.simToggle:hover {
  background: rgba(255, 247, 230, 0.22);
}
.simToggle:active {
  transform: scale(0.96);
}
.simToggle[aria-pressed=true] {
  background: #c93000;
}
.simToggle:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

.body {
  display: flex;
  align-items: stretch;
  gap: 8px;
}

.itemList {
  margin: 0;
  padding: 8px 12px;
  list-style: none;
  width: 220px;
  height: 116px;
  overflow-y: scroll;
  border-radius: 8px;
  background: rgba(255, 247, 230, 0.06);
}
.itemList::-webkit-scrollbar {
  width: 10px;
}
.itemList::-webkit-scrollbar-track {
  background: transparent;
}
.itemList::-webkit-scrollbar-thumb {
  background: #2f6df6;
  border-radius: 999px;
}

@supports (scrollbar-width: thin) {
  .itemList:not(.force-fallback) {
    scrollbar-width: thin;
    scrollbar-color: #5b8cf8 transparent;
  }
}
.force-fallback .itemList {
  scrollbar-width: auto;
  scrollbar-color: unset;
}

.itemList li {
  padding: 4px 0;
  color: #fff7e6;
  font-size: 12px;
  border-bottom: 1px solid rgba(255, 247, 230, 0.1);
}
.itemList li:last-child {
  border-bottom: none;
}

.track {
  position: relative;
  width: 6px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.1);
}

.trackDot {
  position: absolute;
  top: 0;
  left: 0;
  width: 6px;
  height: 28px;
  border-radius: 999px;
  background: #5b8cf8;
  transform: translateY(0);
}

.force-fallback .trackDot {
  background: #c93000;
}

@media (max-width: 340px) {
  .wrap {
    width: 210px;
  }
  .itemList {
    width: 174px;
    height: 96px;
    font-size: 11px;
  }
  .modeLabel {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .trackDot {
    transition: none;
  }
  .simToggle {
    transition: none;
  }
}

(function () {
  var LERP_RATE = 0.22; // 트랙 점이 목표 위치를 따라붙는 비율 — 작을수록 관성이 크게 느껴진다
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var wrap = document.getElementById('wrap');
  var list = document.getElementById('list');
  var track = document.getElementById('track');
  var dot = document.getElementById('trackDot');
  var simToggle = document.getElementById('simToggle');
  var modeLabel = document.getElementById('modeLabel');
  var isDemo = true;

  var target = 0; // 목표 스크롤 비율 0~1 — 실제 scrollTop 에서 나온다
  var shown = 0;  // 화면에 그려지는 비율 — target 을 lerp 로 뒤따른다

  function ratio() {
    var max = list.scrollHeight - list.clientHeight;
    return max > 0 ? list.scrollTop / max : 0;
  }

  function onScroll() { target = ratio(); }
  list.addEventListener('scroll', onScroll);

  function human() { if (isDemo) { wrap.classList.remove('is-demo'); isDemo = false; } }
  list.addEventListener('pointerdown', human);
  list.addEventListener('keydown', human);

  function setSim(on) {
    wrap.classList.toggle('force-fallback', on);
    simToggle.setAttribute('aria-pressed', String(on));
    modeLabel.textContent = on ? 'フォールバックを再現' : '標準サポート版';
  }
  var simOn = false;
  simToggle.addEventListener('click', function () {
    human();
    simOn = !simOn;
    setSim(simOn);
  });

  var dir = 1;
  var last = 0;
  var STEP_MS = 520;

  function autoAdvance(ts) {
    if (ts - last < STEP_MS) return;
    last = ts;
    var max = list.scrollHeight - list.clientHeight;
    var next = list.scrollTop + dir * (max * 0.42);
    if (next >= max - 1) { next = max; dir = -1; }
    else if (next <= 1) { next = 0; dir = 1; }
    list.scrollTop = next; // 진짜 스크롤 — 위 scroll 리스너가 target 을 갱신한다
  }

  function loop(ts) {
    if (!last) last = ts;
    if (isDemo && !reduce) autoAdvance(ts);
    var trackH = track.clientHeight - dot.offsetHeight;
    if (reduce) { shown = target; } else { shown += (target - shown) * LERP_RATE; }
    dot.style.transform = 'translateY(' + Math.max(0, Math.min(trackH, shown * trackH)) + 'px)';
    requestAnimationFrame(loop);
  }
  requestAnimationFrame(loop);
})();

// ScrollbarStyling — vanilla 판(run/375/demo/01-compat-scrollbar-styling/index.html) 의 React 포트.
// 표준 scrollbar-width/scrollbar-color 와 옛 ::-webkit-scrollbar 의사요소를 함께 선언하고,
// 트랙 점은 실제 scroll 이벤트에서 나온 비율(target)을 lerp 로 뒤따른다.
import { useEffect, useRef } from "react";
import styles from "./ScrollbarStyling.module.scss";

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

const LERP_RATE = 0.22; // 트랙 점이 목표 위치를 따라붙는 비율 — 작을수록 관성이 크게 느껴진다
const STEP_MS = 520;

const ITEMS = ["설계 문서", "색상 토큰", "여백 규칙", "버튼 상태", "입력 검증", "접근성 점검", "성능 측정", "배포 노트"];

export default function ScrollbarStyling() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const listRef = useRef(null) as unknown as Box<HTMLOListElement>;
  const trackRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const dotRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const simToggleRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const modeLabelRef = useRef(null) as unknown as Box<HTMLSpanElement>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const wrap = rootRef.current;
    const list = listRef.current;
    const track = trackRef.current;
    const dot = dotRef.current;
    const simToggle = simToggleRef.current;
    const modeLabel = modeLabelRef.current;
    if (!wrap || !list || !track || !dot || !simToggle || !modeLabel) return;

    let isDemo = true;
    let target = 0;
    let shown = 0;

    function ratio() {
      const max = list!.scrollHeight - list!.clientHeight;
      return max > 0 ? list!.scrollTop / max : 0;
    }
    function onScroll() { target = ratio(); }
    list.addEventListener("scroll", onScroll);

    function human() { if (isDemo) { wrap!.classList.remove(styles.isDemo); isDemo = false; } }
    function onPointerDown() { human(); }
    function onKeyDown() { human(); }
    list.addEventListener("pointerdown", onPointerDown);
    list.addEventListener("keydown", onKeyDown);

    let simOn = false;
    function setSim(on: boolean) {
      simOn = on;
      wrap!.classList.toggle(styles.forceFallback, on);
      simToggle!.setAttribute("aria-pressed", String(on));
      modeLabel!.textContent = on ? "폴백 흉내내기" : "표준 지원판";
    }
    function onSimClick() { human(); simOn = !simOn; setSim(simOn); }
    simToggle.addEventListener("click", onSimClick);

    let dir = 1;
    let last = 0;
    function autoAdvance(ts: number) {
      if (ts - last < STEP_MS) return;
      last = ts;
      const max = list!.scrollHeight - list!.clientHeight;
      let next = list!.scrollTop + dir * (max * 0.42);
      if (next >= max - 1) { next = max; dir = -1; }
      else if (next <= 1) { next = 0; dir = 1; }
      list!.scrollTop = next;
    }

    function loop(ts: number) {
      if (!last) last = ts;
      if (isDemo && !reduce) autoAdvance(ts);
      const trackH = track!.clientHeight - dot!.offsetHeight;
      if (reduce) { shown = target; } else { shown += (target - shown) * LERP_RATE; }
      dot!.style.transform = "translateY(" + Math.max(0, Math.min(trackH, shown * trackH)) + "px)";
      rafRef.current = requestAnimationFrame(loop);
    }
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      list!.removeEventListener("scroll", onScroll);
      list!.removeEventListener("pointerdown", onPointerDown);
      list!.removeEventListener("keydown", onKeyDown);
      simToggle!.removeEventListener("click", onSimClick);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.wrap + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.head}>
          <span className={styles.modeLabel} ref={modeLabelRef}>표준 지원판</span>
          <button className={styles.simToggle} ref={simToggleRef} type="button" aria-pressed="false">폴백 흉내내기</button>
        </div>
        <div className={styles.body}>
          <ol className={styles.itemList} ref={listRef}>
            {ITEMS.map(function (s) { return <li key={s}>{s}</li>; })}
          </ol>
          <div className={styles.track} ref={trackRef}>
            <div className={styles.trackDot} ref={dotRef}></div>
          </div>
        </div>
      </div>
    </main>
  );
}