03Spring-pop toggle — the instant the checked value flips, the box's size value overshoots its target then settles with a bounce

<main class="stage">
<label class="cbs is-demo" id="cbs">
  <input type="checkbox" class="cbs__input" id="cbsInput">
  <span class="cbs__box" id="cbsBox" aria-hidden="true">
    <svg class="cbs__svg" viewBox="0 0 20 20" width="14" height="14">
      <path d="M4 10.5l3.5 3.5L16 5.5" fill="none" stroke="currentColor" stroke-width="2.6" stroke-linecap="round" stroke-linejoin="round"/>
    </svg>
  </span>
  <span class="cbs__text">Mission complete</span>
</label>
</main>

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

// 계산 겹: index.html popScale() 이 감쇠 코사인 진동(SPRING_ZETA·SCALE_OVERSHOOT)으로
// 매 프레임 배율 값을 계산해 박스의 transform: scale() 에 직접 적용한다 — CSS transition
// 이 아니라 물리 적분이 튐과 잦아듦을 만든다.
@include stage($stage-orange);

.cbs {
  display: flex;
  align-items: center;
  gap: $sp-3;
  cursor: pointer;
  user-select: none;
}

.cbs__input {
  position: absolute;
  width: 1px;
  height: 1px;
  opacity: 0;
  pointer-events: none;
}

.cbs__box {
  flex: 0 0 auto;
  width: 28px;
  height: 28px;
  border-radius: $r-control;
  background: $subject-cream;
  border: 2px solid rgba($subject-ink, .3);
  color: transparent;
  display: grid;
  place-items: center;
  box-sizing: border-box;
  transform-origin: 50% 50%;
  transition: background-color $dur-quick $ease-out, border-color $dur-quick $ease-out, color $dur-quick $ease-out;
}

.cbs__box--on {
  background: $subject-blue;
  border-color: $subject-blue;
  color: $subject-cream;
}

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

.cbs__text {
  color: $subject-ink;
  font-size: 14px;
  font-weight: 700;
}

@media (max-width: 340px) {
  .cbs__text { font-size: 13px; }
}

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

.cbs {
  display: flex;
  align-items: center;
  gap: 12px;
  cursor: pointer;
  user-select: none;
}

.cbs__input {
  position: absolute;
  width: 1px;
  height: 1px;
  opacity: 0;
  pointer-events: none;
}

.cbs__box {
  flex: 0 0 auto;
  width: 28px;
  height: 28px;
  border-radius: 12px;
  background: #fff7e6;
  border: 2px solid rgba(23, 20, 26, 0.3);
  color: transparent;
  display: grid;
  place-items: center;
  box-sizing: border-box;
  transform-origin: 50% 50%;
  transition: background-color 220ms ease-out, border-color 220ms ease-out, color 220ms ease-out;
}

.cbs__box--on {
  background: #2f6df6;
  border-color: #2f6df6;
  color: #fff7e6;
}

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

.cbs__text {
  color: #17141a;
  font-size: 14px;
  font-weight: 700;
}

@media (max-width: 340px) {
  .cbs__text {
    font-size: 13px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .cbs__box {
    transition: none;
  }
}

(function () {
  var POP_MS = 480; // 계산 겹 상수: 스프링 튐이 잦아드는 데 걸리는 실제 시간(ms)
  var SPRING_ZETA = 3.4; // 계산 겹: 감쇠 상수 — 낮을수록 더 오래 튕긴다
  var SCALE_OVERSHOOT = .34; // 계산 겹: 목표(scale 1)를 넘어서는 최대 배율
  var POP_FREQ = 2 * Math.PI * 2.1 / POP_MS; // 계산 겹: 튐 각속도
  var CYCLE_MS = 900; // 자동 루프가 체크·해제를 오가는 간격
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var root = document.getElementById('cbs');
  var input = document.getElementById('cbsInput');
  var box = document.getElementById('cbsBox');

  var isDemo = true;
  var popStart = -99999;

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

  function popScale(t) {
    // 계산 겹: 감쇠 코사인 진동 — t=0 에서 SCALE_OVERSHOOT 만큼 튀어 오른 뒤 1로 잦아든다
    if (t < 0 || t > POP_MS) return 1;
    var amp = Math.exp(-SPRING_ZETA * (t / POP_MS));
    return 1 + amp * SCALE_OVERSHOOT * Math.cos(POP_FREQ * t);
  }

  function setChecked(next, ts) {
    input.checked = next;
    box.classList.toggle('cbs__box--on', next);
    popStart = ts;
  }

  input.addEventListener('change', function () {
    human();
    setChecked(input.checked, performance.now());
  });

  var cycleStart = 0;
  var lastState = false;

  function loop(ts) {
    if (!cycleStart) cycleStart = ts;

    if (isDemo) {
      var step = Math.floor((ts - cycleStart) / CYCLE_MS) % 2 === 1;
      if (step !== lastState) { lastState = step; setChecked(step, ts); }
    }

    var s = popScale(ts - popStart);
    box.style.transform = 'scale(' + s.toFixed(3) + ')';

    requestAnimationFrame(loop);
  }

  if (reduce) { POP_MS = 1; }
  requestAnimationFrame(loop);
})();

// SpringPop — vanilla 판(run/368/demo/03-custom-checkbox-spring-pop/index.html) 의 React 포트.
// 감쇠 코사인 진동(exp(-SPRING_ZETA·t)·cos(POP_FREQ·t))을 매 프레임 계산해 scale() 값을 만든다.
import { useEffect, useRef } from "react";
import styles from "./SpringPop.module.scss";

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

export default function SpringPop() {
  const rootRef = useRef(null) as unknown as Box<HTMLLabelElement>;
  const inputRef = useRef(null) as unknown as Box<HTMLInputElement>;
  const boxRef = useRef(null) as unknown as Box<HTMLSpanElement>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    let POP_MS = 480; // 계산 겹 상수: 스프링 튐이 잦아드는 데 걸리는 실제 시간(ms)
    const SPRING_ZETA = 3.4; // 계산 겹: 감쇠 상수 — 낮을수록 더 오래 튕긴다
    const SCALE_OVERSHOOT = 0.34; // 계산 겹: 목표(scale 1)를 넘어서는 최대 배율
    const POP_FREQ = (2 * Math.PI * 2.1) / POP_MS; // 계산 겹: 튐 각속도
    const CYCLE_MS = 900; // 자동 루프가 체크·해제를 오가는 간격
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    const root = rootRef.current;
    const input = inputRef.current;
    const box = boxRef.current;
    if (!root || !input || !box) return;

    let isDemo = true;
    let popStart = -99999;

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

    function popScale(t: number) {
      // 계산 겹: 감쇠 코사인 진동 — t=0 에서 SCALE_OVERSHOOT 만큼 튀어 오른 뒤 1로 잦아든다
      if (t < 0 || t > POP_MS) return 1;
      const amp = Math.exp(-SPRING_ZETA * (t / POP_MS));
      return 1 + amp * SCALE_OVERSHOOT * Math.cos(POP_FREQ * t);
    }

    function setChecked(next: boolean, ts: number) {
      input!.checked = next;
      box!.classList.toggle(styles.boxOn, next);
      popStart = ts;
    }

    function onChange() { human(); setChecked(input!.checked, performance.now()); }
    input.addEventListener("change", onChange);

    let cycleStart = 0;
    let lastState = false;

    function loop(ts: number) {
      if (!cycleStart) cycleStart = ts;

      if (isDemo) {
        const step = Math.floor((ts - cycleStart) / CYCLE_MS) % 2 === 1;
        if (step !== lastState) { lastState = step; setChecked(step, ts); }
      }

      const s = popScale(ts - popStart);
      box!.style.transform = "scale(" + s.toFixed(3) + ")";

      rafRef.current = requestAnimationFrame(loop);
    }

    if (reduce) { POP_MS = 1; }
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      input.removeEventListener("change", onChange);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <label className={styles.cbs + " " + styles.isDemo} ref={rootRef}>
        <input type="checkbox" className={styles.cbsInput} ref={inputRef} />
        <span className={styles.cbsBox} ref={boxRef} aria-hidden="true">
          <svg className={styles.cbsSvg} viewBox="0 0 20 20" width={14} height={14}>
            <path d="M4 10.5l3.5 3.5L16 5.5" fill="none" stroke="currentColor" strokeWidth={2.6} strokeLinecap="round" strokeLinejoin="round" />
          </svg>
        </span>
        <span className={styles.cbsText}>미션 완료</span>
      </label>
    </main>
  );
}