02Active-indicator slide — when the active-button value changes, the highlight background's position value glides to the new button along measured coordinates

<main class="stage">
<div class="asi-wrap">
<div class="asi is-demo" id="asi" role="group" aria-label="Sort by">
  <span class="asi__indicator" id="asiIndicator" aria-hidden="true"></span>
  <button class="asi__btn asi__btn--on" data-i="0" aria-pressed="true">Newest</button>
  <button class="asi__btn" data-i="1" aria-pressed="false">Popular</button>
  <button class="asi__btn" data-i="2" aria-pressed="false">Price</button>
  <button class="asi__btn" data-i="3" aria-pressed="false">Top rated</button>
</div>
</div>
</main>

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

// 계산 겹: index.html 이 getBoundingClientRect 로 활성 버튼의 실측 좌표·폭을 재고, 감쇠 스프링
// 방정식(x'' = k*(target-x) - c*x')을 매 프레임 적분해 표시자를 오버슛 후 정착시킨다.
// 폭 변화는 CSS width 가 아니라 transform: scaleX 비율로만 표현해 레이아웃을 다시 계산하지 않는다.
@include stage($stage-ink);

.asi-wrap {
  max-width: 100%;
  overflow-x: auto;
  overflow-y: hidden;
}

.asi {
  position: relative;
  display: inline-flex;
  gap: 0;
  padding: 4px;
  border-radius: $r-pill;
  background: rgba($subject-cream, .07);
}

.asi__indicator {
  position: absolute;
  top: 4px;
  left: 4px;
  height: calc(100% - #{$sp-2});
  border-radius: $r-pill;
  background: $subject-blue-deep;
  transform-origin: left center;
  will-change: transform;
  z-index: 0;
}

.asi__btn {
  position: relative;
  z-index: 1;
  appearance: none;
  border: none;
  background: transparent;
  color: rgba($subject-cream, .78);
  font-size: 13px;
  font-weight: 600;
  padding: $sp-2 $sp-4;
  white-space: nowrap;
  cursor: pointer;
  border-radius: $r-pill;
  transition: color $dur-quick $ease-out;
}

.asi__btn--on {
  color: $subject-cream;
}

.asi__btn:focus-visible {
  outline: 2px solid $subject-blue-light;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .asi__btn { font-size: 12px; padding: $sp-1 $sp-3; }
}

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

.asi-wrap {
  max-width: 100%;
  overflow-x: auto;
  overflow-y: hidden;
}

.asi {
  position: relative;
  display: inline-flex;
  gap: 0;
  padding: 4px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.07);
}

.asi__indicator {
  position: absolute;
  top: 4px;
  left: 4px;
  height: calc(100% - 8px);
  border-radius: 999px;
  background: #0a4fe8;
  transform-origin: left center;
  will-change: transform;
  z-index: 0;
}

.asi__btn {
  position: relative;
  z-index: 1;
  appearance: none;
  border: none;
  background: transparent;
  color: rgba(255, 247, 230, 0.78);
  font-size: 13px;
  font-weight: 600;
  padding: 8px 16px;
  white-space: nowrap;
  cursor: pointer;
  border-radius: 999px;
  transition: color 220ms ease-out;
}

.asi__btn--on {
  color: #fff7e6;
}

.asi__btn:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .asi__btn {
    font-size: 12px;
    padding: 4px 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .asi__btn {
    transition: none;
  }
}

(function () {
  var CYCLE_MS = 1100; // 자동 루프가 다음 버튼으로 넘어가는 간격
  var STIFF = 210; // 계산 겹 상수: 스프링 강성(k) — 클수록 목표에 빨리 붙는다
  var DAMP = 18; // 계산 겹 상수: 감쇠(c) — 작을수록 오버슛이 크게 남는다
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var root = document.getElementById('asi');
  var indicator = document.getElementById('asiIndicator');
  var btns = Array.prototype.slice.call(root.querySelectorAll('.asi__btn'));
  var isDemo = true;

  // 계산 겹: 목표(버튼 실측 좌표)를 향해 감쇠 스프링 방정식으로 위치·폭 비율을 매 프레임 적분한다.
  // x'' = STIFF*(target-x) - DAMP*x' — cubic-bezier 로는 못 만드는 오버슛 후 정착 곡선이다.
  // 폭은 CSS width 가 아니라 scaleX(비율) 로만 바꿔 레이아웃을 다시 계산하지 않는다(transform 전용 규칙).
  var x = 0, w = 0, vx = 0, vw = 0, targetX = 0, targetW = 0, last = 0, activeIdx = 0;
  var maxW = 0;

  function measure(idx) {
    var rootRect = root.getBoundingClientRect();
    var r = btns[idx].getBoundingClientRect();
    return { x: r.left - rootRect.left, w: r.width };
  }

  // 시작 시 모든 버튼 폭을 실측해 인디케이터의 고정 기준 폭(maxW)을 정한다 — scaleX 는 이 기준에 대한 비율이다
  btns.forEach(function (b) { maxW = Math.max(maxW, b.getBoundingClientRect().width); });
  indicator.style.width = maxW + 'px';

  function goTo(idx, ts) {
    activeIdx = idx;
    btns.forEach(function (b, i) {
      b.classList.toggle('asi__btn--on', i === idx);
      b.setAttribute('aria-pressed', String(i === idx));
    });
    var m = measure(idx);
    targetX = m.x; targetW = m.w;
    if (reduce) { x = targetX; w = targetW; vx = 0; vw = 0; }
  }

  function human() {
    if (isDemo) { root.classList.remove('is-demo'); isDemo = false; }
  }

  btns.forEach(function (b, i) {
    b.addEventListener('click', function () { human(); goTo(i, performance.now()); });
  });

  window.addEventListener('resize', function () { var m = measure(activeIdx); targetX = m.x; targetW = m.w; });

  // 초기 위치를 즉시 실측값으로 놓는다
  requestAnimationFrame(function () { var m = measure(0); x = targetX = m.x; w = targetW = m.w; render(); });

  function render() {
    var scale = maxW > 0 ? w / maxW : 1;
    indicator.style.transform = 'translateX(' + x.toFixed(2) + 'px) scaleX(' + scale.toFixed(4) + ')';
  }

  var cycleStart = 0, lastIdx = -1;
  function loop(ts) {
    if (!last) last = ts;
    var dt = Math.min(0.032, (ts - last) / 1000);
    last = ts;

    if (!cycleStart) cycleStart = ts;
    if (isDemo && !reduce) {
      var idx = Math.floor((ts - cycleStart) / CYCLE_MS) % btns.length;
      if (idx !== lastIdx) { lastIdx = idx; goTo(idx, ts); }
    }

    var ax = STIFF * (targetX - x) - DAMP * vx;
    var aw = STIFF * (targetW - w) - DAMP * vw;
    vx += ax * dt; x += vx * dt;
    vw += aw * dt; w += vw * dt;
    render();

    requestAnimationFrame(loop);
  }
  requestAnimationFrame(loop);
})();

// ActiveSlideIndicator — vanilla 판(run/369/demo/02-btn-group-active-slide-indicator/index.html) 의 React 포트.
// getBoundingClientRect 로 활성 버튼의 실측 좌표·폭을 재고, 감쇠 스프링 방정식을 매 프레임 적분해
// 표시자를 오버슛 후 정착시킨다. 폭 변화는 transform: scaleX 비율로만 표현한다.
import { useEffect, useRef } from "react";
import styles from "./ActiveSlideIndicator.module.scss";

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

const LABELS = ["최신순", "인기순", "가격순", "평점 높은순"];

export default function ActiveSlideIndicator() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const indicatorRef = useRef(null) as unknown as Box<HTMLSpanElement>;
  const btnRefs = useRef([]) as unknown as Box<HTMLButtonElement[]>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const CYCLE_MS = 1100;
    const STIFF = 210; // 계산 겹 상수: 스프링 강성(k)
    const DAMP = 18; // 계산 겹 상수: 감쇠(c)
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    const root = rootRef.current;
    const indicator = indicatorRef.current;
    const btns = btnRefs.current;
    if (!root || !indicator || btns.length === 0) return;

    let isDemo = true;
    let x = 0, w = 0, vx = 0, vw = 0, targetX = 0, targetW = 0, last = 0, activeIdx = 0;
    let maxW = 0;

    btns.forEach((b) => { maxW = Math.max(maxW, b.getBoundingClientRect().width); });
    indicator.style.width = maxW + "px";

    function measure(idx: number) {
      const rootRect = root!.getBoundingClientRect();
      const r = btns[idx].getBoundingClientRect();
      return { x: r.left - rootRect.left, w: r.width };
    }

    function goTo(idx: number) {
      activeIdx = idx;
      btns.forEach((b, i) => {
        b.classList.toggle(styles.btnOn, i === idx);
        b.setAttribute("aria-pressed", String(i === idx));
      });
      const m = measure(idx);
      targetX = m.x; targetW = m.w;
      if (reduce) { x = targetX; w = targetW; vx = 0; vw = 0; }
    }

    function human() { if (isDemo) { root!.classList.remove(styles.isDemo); isDemo = false; } }

    function onResize() { const m = measure(activeIdx); targetX = m.x; targetW = m.w; }
    window.addEventListener("resize", onResize);

    const clicks: (() => void)[] = [];
    btns.forEach((b, i) => {
      const fn = () => { human(); goTo(i); };
      b.addEventListener("click", fn);
      clicks.push(fn);
    });

    function render() {
      const scale = maxW > 0 ? w / maxW : 1;
      indicator!.style.transform = "translateX(" + x.toFixed(2) + "px) scaleX(" + scale.toFixed(4) + ")";
    }

    let cycleStart = 0, lastIdx = -1;
    function loop(ts: number) {
      if (!last) last = ts;
      const dt = Math.min(0.032, (ts - last) / 1000);
      last = ts;

      if (!cycleStart) cycleStart = ts;
      if (isDemo && !reduce) {
        const idx = Math.floor((ts - cycleStart) / CYCLE_MS) % btns.length;
        if (idx !== lastIdx) { lastIdx = idx; goTo(idx); }
      }

      const ax = STIFF * (targetX - x) - DAMP * vx;
      const aw = STIFF * (targetW - w) - DAMP * vw;
      vx += ax * dt; x += vx * dt;
      vw += aw * dt; w += vw * dt;
      render();

      rafRef.current = requestAnimationFrame(loop);
    }

    requestAnimationFrame(() => { const m = measure(0); x = targetX = m.x; w = targetW = m.w; render(); });
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      window.removeEventListener("resize", onResize);
      btns.forEach((b, i) => b.removeEventListener("click", clicks[i]));
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.asiWrap}>
        <div className={styles.asi + " " + styles.isDemo} ref={rootRef} role="group" aria-label="정렬 기준">
          <span className={styles.indicator} ref={indicatorRef} aria-hidden="true" />
          {LABELS.map((label, i) => (
            <button
              key={label}
              className={styles.btn + (i === 0 ? " " + styles.btnOn : "")}
              aria-pressed={i === 0}
              ref={(el: HTMLButtonElement | null) => { if (el) btnRefs.current[i] = el; }}
            >
              {label}
            </button>
          ))}
        </div>
      </div>
    </main>
  );
}