css download button 9パターン — 沈み・満ち・ファイルが飛ぶボタン

<main class="stage">
<div class="sk is-demo" id="skRoot">
  <button class="sk__btn" id="skBtn" type="button">
    <svg class="sk__icon" id="skIcon" viewBox="0 0 24 24" aria-hidden="true"><path d="M12 4v11M6.5 10.5 12 16l5.5-5.5M5 20h14"/></svg>
    <span>ダウンロード</span>
  </button>
  <p class="sk__meta"><span class="sk__tag">Y</span> <b id="skY">0.0</b> <span class="sk__tag">TILT</span> <b id="skTilt">0.0</b></p>
</div>
</main>

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

// 핵심: 눌림 깊이는 CSS transition 이 아니라 JS 가 스프링 방정식으로 적분한 값이다.
// 그 값 하나(--p)가 버튼 색 · 그림자 길이 · 화살표 위치를 함께 끈다.

$sink-face: $subject-lilac;
$sink-deep: $subject-pink;

@include stage($stage-yellow);

.sk {
  width: min(100%, 280px);
  display: grid;
  gap: $sp-4;
  justify-items: center;
}

.sk__btn {
  --p: 0;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 64px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: $sp-3;
  border: 0;
  border-radius: $r-control;
  background: color-mix(in srgb, #{$sink-face}, #{$sink-deep} calc(var(--p) * 100%));
  box-shadow: 0 calc(14px - 8px * var(--p)) calc(24px - 12px * var(--p)) rgba($subject-ink, .32);
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.sk__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.sk__icon {
  width: 28px;
  height: 28px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.sk__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: $subject-ink;
}
.sk__tag { font-weight: 700; letter-spacing: .08em; }
.sk__meta b { min-width: 5ch; text-align: right; font-variant-numeric: tabular-nums; }

@media (prefers-reduced-motion: reduce) {
  .sk__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: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sk {
  width: min(100%, 280px);
  display: grid;
  gap: 16px;
  justify-items: center;
}

.sk__btn {
  --p: 0;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 64px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
  border: 0;
  border-radius: 12px;
  background: color-mix(in srgb, #b9a0ff, #ff9ebb calc(var(--p) * 100%));
  box-shadow: 0 calc(14px - 8px * var(--p)) calc(24px - 12px * var(--p)) rgba(23, 20, 26, 0.32);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.sk__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.sk__icon {
  width: 28px;
  height: 28px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.sk__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: #17141a;
}

.sk__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.sk__meta b {
  min-width: 5ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

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

(function () {
  // 기법: 눌림 깊이와 기울기를 스프링 방정식 x'' = -K (x - 목표) - D x' 로 매 프레임 적분한다.
  // 감쇠가 약해서 화살표는 제자리를 지나쳤다가 한 번 더 출렁이고 멈춘다 (CSS transition 으로는 못 만드는 곡선).
  var SINK_PX = 10;
  var SPRING_K = 120;
  var SPRING_DAMP = 9;
  var TILT_MAX = 14;
  var START_MS = 200;
  var HOLD_MS = 560;
  var CYCLE_MS = 2000;

  var root = document.getElementById('skRoot');
  var btn = document.getElementById('skBtn');
  var icon = document.getElementById('skIcon');
  var yEl = document.getElementById('skY');
  var tiltEl = document.getElementById('skTilt');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var y = 0;
  var vy = 0;
  var tilt = 0;
  var vt = 0;
  var pressed = false;
  var side = 0;
  var t0 = null;
  var lastTs = null;

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function stopDemo() { root.classList.remove('is-demo'); }

  function goal(now) {
    if (root.classList.contains('is-demo') && now >= START_MS) {
      var ph = (now - START_MS) % CYCLE_MS;
      return { depth: ph < HOLD_MS ? SINK_PX : 0, lean: ph < HOLD_MS ? -0.8 : (ph < 1100 ? 0.8 : 0) };
    }
    return { depth: pressed ? SINK_PX : 0, lean: side };
  }

  function integrate(dt, g) {
    var n = Math.ceil(dt / 0.008);
    var h = dt / n;
    for (var i = 0; i < n; i += 1) {
      vy += (-SPRING_K * (y - g.depth) - SPRING_DAMP * vy) * h;
      y += vy * h;
      vt += (-SPRING_K * (tilt - g.lean * TILT_MAX) - SPRING_DAMP * vt) * h;
      tilt += vt * h;
    }
  }

  function render() {
    var p = clamp(y / SINK_PX, -0.3, 1);
    icon.style.transform = 'translateY(' + y.toFixed(2) + 'px) rotate(' + tilt.toFixed(2) + 'deg)';
    btn.style.transform = 'translateY(' + (y * 0.3).toFixed(2) + 'px)';
    btn.style.setProperty('--p', p.toFixed(3));
    yEl.textContent = y.toFixed(1);
    tiltEl.textContent = tilt.toFixed(1);
  }

  function snap() {
    var g = goal(0);
    y = g.depth; vy = 0; tilt = g.lean * TILT_MAX; vt = 0;
    render();
  }

  function press() { stopDemo(); pressed = true; if (reduce) snap(); }
  function release() { pressed = false; if (reduce) snap(); }

  btn.addEventListener('pointerdown', function (e) {
    var r = btn.getBoundingClientRect();
    side = clamp((e.clientX - r.left - r.width / 2) / (r.width / 2), -1, 1);
    press();
  });
  btn.addEventListener('pointerup', release);
  btn.addEventListener('pointerleave', release);
  btn.addEventListener('pointercancel', release);
  btn.addEventListener('keydown', function (e) {
    if ((e.key === 'Enter' || e.key === ' ') && !e.repeat) { side = 0; press(); }
  });
  btn.addEventListener('keyup', function (e) { if (e.key === 'Enter' || e.key === ' ') release(); });
  btn.addEventListener('blur', release);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    integrate(dt, goal(now));
    render();
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    render();
  } else {
    requestAnimationFrame(tick);
  }
})();

// SinkArrowButton — vanilla 판(01-dlbtn-arrow-sink-spring) 의 React 포트.
// 눌림 깊이와 기울기를 스프링 방정식으로 매 프레임 적분한다. CSS transition 은 쓰지 않는다.
import { useEffect, useRef } from "react";
import styles from "./SinkArrowButton.module.scss";

const SINK_PX = 10;
const SPRING_K = 120;
const SPRING_DAMP = 9;
const TILT_MAX = 14;
const START_MS = 200;
const HOLD_MS = 560;
const CYCLE_MS = 2000;

type Goal = { depth: number; lean: number };

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

export default function SinkArrowButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const iconRef = useRef<SVGSVGElement | null>(null);
  const yRef = useRef<HTMLElement | null>(null);
  const tiltRef = useRef<HTMLElement | null>(null);

  useEffect(() => {
    const btn = btnRef.current;
    const icon = iconRef.current;
    const yEl = yRef.current;
    const tiltEl = tiltRef.current;
    if (!btn || !icon || !yEl || !tiltEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let y = 0;
    let vy = 0;
    let tilt = 0;
    let vt = 0;
    let pressed = false;
    let side = 0;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const goal = (now: number): Goal => {
      if (demo && now >= START_MS) {
        const ph = (now - START_MS) % CYCLE_MS;
        return { depth: ph < HOLD_MS ? SINK_PX : 0, lean: ph < HOLD_MS ? -0.8 : ph < 1100 ? 0.8 : 0 };
      }
      return { depth: pressed ? SINK_PX : 0, lean: side };
    };
    const integrate = (dt: number, g: Goal): void => {
      const n = Math.ceil(dt / 0.008);
      const h = dt / n;
      for (let i = 0; i < n; i += 1) {
        vy += (-SPRING_K * (y - g.depth) - SPRING_DAMP * vy) * h;
        y += vy * h;
        vt += (-SPRING_K * (tilt - g.lean * TILT_MAX) - SPRING_DAMP * vt) * h;
        tilt += vt * h;
      }
    };
    const render = (): void => {
      const p = clamp(y / SINK_PX, -0.3, 1);
      icon.style.transform = `translateY(${y.toFixed(2)}px) rotate(${tilt.toFixed(2)}deg)`;
      btn.style.transform = `translateY(${(y * 0.3).toFixed(2)}px)`;
      btn.style.setProperty("--p", p.toFixed(3));
      yEl.textContent = y.toFixed(1);
      tiltEl.textContent = tilt.toFixed(1);
    };
    const snap = (): void => {
      const g = goal(0);
      y = g.depth; vy = 0; tilt = g.lean * TILT_MAX; vt = 0;
      render();
    };
    const press = (): void => { demo = false; pressed = true; if (reduce) snap(); };
    const release = (): void => { pressed = false; if (reduce) snap(); };

    const onDown = (e: PointerEvent): void => {
      const r = btn.getBoundingClientRect();
      side = clamp((e.clientX - r.left - r.width / 2) / (r.width / 2), -1, 1);
      press();
    };
    const onKeyDown = (e: KeyboardEvent): void => {
      if ((e.key === "Enter" || e.key === " ") && !e.repeat) { side = 0; press(); }
    };
    const onKeyUp = (e: KeyboardEvent): void => { if (e.key === "Enter" || e.key === " ") release(); };
    btn.addEventListener("pointerdown", onDown);
    btn.addEventListener("pointerup", release);
    btn.addEventListener("pointerleave", release);
    btn.addEventListener("pointercancel", release);
    btn.addEventListener("keydown", onKeyDown);
    btn.addEventListener("keyup", onKeyUp);
    btn.addEventListener("blur", release);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      integrate(dt, goal(now));
      render();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) render();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("pointerdown", onDown);
      btn.removeEventListener("pointerup", release);
      btn.removeEventListener("pointerleave", release);
      btn.removeEventListener("pointercancel", release);
      btn.removeEventListener("keydown", onKeyDown);
      btn.removeEventListener("keyup", onKeyUp);
      btn.removeEventListener("blur", release);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.sk}>
        <button className={styles.sk__btn} type="button" ref={btnRef}>
          <svg className={styles.sk__icon} viewBox="0 0 24 24" aria-hidden="true" ref={iconRef}>
            <path d="M12 4v11M6.5 10.5 12 16l5.5-5.5M5 20h14" />
          </svg>
          <span>내려받기</span>
        </button>
        <p className={styles.sk__meta}>
          <span className={styles.sk__tag}>Y</span> <b ref={yRef}>0.0</b> <span className={styles.sk__tag}>TILT</span> <b ref={tiltRef}>0.0</b>
        </p>
      </div>
    </main>
  );
}

<main class="stage">
<div class="rg is-demo" id="rgRoot">
  <div class="rg__wrap" id="rgBar" role="progressbar" aria-labelledby="rgLabel" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-valuetext="0%">
    <svg class="rg__svg" viewBox="0 0 232 96" aria-hidden="true">
      <rect class="rg__ghost" id="rgGhost" x="11" y="11" width="210" height="74" pathLength="100"/>
      <rect class="rg__ring" id="rgRing" x="3" y="3" width="226" height="90" pathLength="100"/>
    </svg>
    <button class="rg__btn" id="rgBtn" type="button"><span id="rgLabel">ダウンロード</span><b id="rgPct">0%</b></button>
  </div>
  <p class="rg__meta"><span class="rg__tag">POW</span> <b id="rgPow">2</b> <span class="rg__tag">LEAD</span> <b id="rgLead">0</b></p>
</div>
</main>

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

// 핵심: 링 길이는 SVG stroke-dashoffset, 값은 JS 가 곡선으로 계산한다. 버튼 안 채움도 같은 진행률(--f)을 쓴다.

$ring-face: $subject-sky;
$ring-fill: $subject-mint;

@include stage($stage-ink);

.rg {
  width: min(100%, 280px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.rg__wrap {
  position: relative;
  width: 232px;
  max-width: 100%;
  aspect-ratio: 232 / 96;
}
.rg__svg {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
}
.rg__ring, .rg__ghost {
  fill: none;
  stroke-dasharray: 100 100;
  stroke-dashoffset: 100;
  stroke-linecap: round;
}
.rg__ring { stroke: $subject-mint; stroke-width: 5; }
.rg__ghost { stroke: $stage-paper; stroke-width: 2; }

.rg__btn {
  --f: 0;
  position: absolute;
  left: 7%;
  right: 7%;
  top: 21%;
  bottom: 21%;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 $sp-4;
  border: 0;
  border-radius: $r-control;
  background: linear-gradient(90deg, #{$ring-fill} calc(var(--f) * 100%), #{$ring-face} calc(var(--f) * 100%));
  color: $subject-ink;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.rg__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 4px; }
.rg__btn b { font-variant-numeric: tabular-nums; }

.rg__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: $stage-paper;
}
.rg__tag { font-weight: 700; letter-spacing: .08em; }
.rg__meta b { min-width: 3ch; text-align: right; font-variant-numeric: tabular-nums; }

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

.rg {
  width: min(100%, 280px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.rg__wrap {
  position: relative;
  width: 232px;
  max-width: 100%;
  aspect-ratio: 232/96;
}

.rg__svg {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
}

.rg__ring, .rg__ghost {
  fill: none;
  stroke-dasharray: 100 100;
  stroke-dashoffset: 100;
  stroke-linecap: round;
}

.rg__ring {
  stroke: #4cd4a6;
  stroke-width: 5;
}

.rg__ghost {
  stroke: #ffffff;
  stroke-width: 2;
}

.rg__btn {
  --f: 0;
  position: absolute;
  left: 7%;
  right: 7%;
  top: 21%;
  bottom: 21%;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 16px;
  border: 0;
  border-radius: 12px;
  background: linear-gradient(90deg, #4cd4a6 calc(var(--f) * 100%), #6ec8ff calc(var(--f) * 100%));
  color: #17141a;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.rg__btn:focus-visible {
  outline: 3px solid #ffffff;
  outline-offset: 4px;
}

.rg__btn b {
  font-variant-numeric: tabular-nums;
}

.rg__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: #ffffff;
}

.rg__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.rg__meta b {
  min-width: 3ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

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

(function () {
  // 기법: 진행률을 시간에 비례시키지 않고 곡선 p = 1 - (1 - u)^CURVE_POW 로 계산한다 (u 는 시간 비율).
  // 굵은 링 = 곡선 진행률, 가는 안쪽 링 = 같은 시간의 균일 진행률. 둘의 차이(LEAD)가 곡선이 앞서는 만큼이다.
  var CURVE_POW = 2;
  var RING_R = 16;
  var RUN_MS = 1400;
  var START_MS = 200;
  var CYCLE_MS = 2000;

  var root = document.getElementById('rgRoot');
  var bar = document.getElementById('rgBar');
  var btn = document.getElementById('rgBtn');
  var ring = document.getElementById('rgRing');
  var ghost = document.getElementById('rgGhost');
  var labelEl = document.getElementById('rgLabel');
  var pctEl = document.getElementById('rgPct');
  var powEl = document.getElementById('rgPow');
  var leadEl = document.getElementById('rgLead');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var clock = 0;
  var runStart = null;
  var t0 = null;
  var lastTs = null;

  function ringCurve(u) { return 1 - Math.pow(1 - u, CURVE_POW); }
  function stopDemo() { root.classList.remove('is-demo'); }

  ring.setAttribute('rx', String(RING_R));
  ghost.setAttribute('rx', String(RING_R - 6));
  powEl.textContent = String(CURVE_POW);

  function startOf(now) {
    if (root.classList.contains('is-demo')) {
      return now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS;
    }
    return runStart === 'done' ? null : runStart;
  }

  function render(now) {
    var s = startOf(now);
    var u = s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
    if (reduce && runStart === 'done') u = 1;
    var p = ringCurve(u);
    var pct = Math.round(p * 100);
    ring.style.strokeDashoffset = String((100 - p * 100).toFixed(2));
    ghost.style.strokeDashoffset = String((100 - u * 100).toFixed(2));
    ring.style.opacity = p < 0.004 ? '0' : '1';
    ghost.style.opacity = u < 0.004 ? '0' : '1';
    btn.style.setProperty('--f', p.toFixed(3));
    pctEl.textContent = pct + '%';
    leadEl.textContent = String(Math.round((p - u) * 100));
    labelEl.textContent = u >= 1 ? '完了' : (u > 0 ? '受信中' : 'ダウンロード');
    bar.setAttribute('aria-valuenow', String(pct));
    bar.setAttribute('aria-valuetext', pct + '%');
  }

  function activate() {
    stopDemo();
    if (reduce) { runStart = 'done'; render(clock); return; }
    if (runStart !== null && runStart !== 'done' && clock - runStart < RUN_MS) return;
    runStart = clock;
  }
  btn.addEventListener('click', activate);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    clock = ts - t0;
    lastTs = ts;
    render(clock);
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    render(0);
  } else {
    requestAnimationFrame(tick);
  }
})();

// RingProgressButton — vanilla 판(02-dlbtn-ring-progress-curve) 의 React 포트.
// 진행률은 곡선 p = 1 - (1 - u)^CURVE_POW 로 계산한다. 굵은 링 = 곡선, 가는 링 = 같은 시간의 균일 진행률.
import { useEffect, useRef } from "react";
import styles from "./RingProgressButton.module.scss";

const CURVE_POW = 2;
const RING_R = 16;
const RUN_MS = 1400;
const START_MS = 200;
const CYCLE_MS = 2000;

type Start = number | "done" | null;

function ringCurve(u: number): number {
  return 1 - Math.pow(1 - u, CURVE_POW);
}

export default function RingProgressButton() {
  const barRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const ringRef = useRef<SVGRectElement | null>(null);
  const ghostRef = useRef<SVGRectElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const pctRef = useRef<HTMLElement | null>(null);
  const leadRef = useRef<HTMLElement | null>(null);

  useEffect(() => {
    const bar = barRef.current;
    const btn = btnRef.current;
    const ring = ringRef.current;
    const ghost = ghostRef.current;
    const labelEl = labelRef.current;
    const pctEl = pctRef.current;
    const leadEl = leadRef.current;
    if (!bar || !btn || !ring || !ghost || !labelEl || !pctEl || !leadEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let clock = 0;
    let runStart: Start = null;
    let t0: number | null = null;
    let raf = 0;

    ring.setAttribute("rx", String(RING_R));
    ghost.setAttribute("rx", String(RING_R - 6));

    const startOf = (now: number): number | null => {
      if (demo) return now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS;
      return runStart === "done" ? null : runStart;
    };
    const render = (now: number): void => {
      const s = startOf(now);
      let u = s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
      if (reduce && runStart === "done") u = 1;
      const p = ringCurve(u);
      const pct = Math.round(p * 100);
      ring.style.strokeDashoffset = (100 - p * 100).toFixed(2);
      ghost.style.strokeDashoffset = (100 - u * 100).toFixed(2);
      ring.style.opacity = p < 0.004 ? "0" : "1";
      ghost.style.opacity = u < 0.004 ? "0" : "1";
      btn.style.setProperty("--f", p.toFixed(3));
      pctEl.textContent = pct + "%";
      leadEl.textContent = String(Math.round((p - u) * 100));
      labelEl.textContent = u >= 1 ? "완료" : u > 0 ? "받는 중" : "내려받기";
      bar.setAttribute("aria-valuenow", String(pct));
      bar.setAttribute("aria-valuetext", pct + "%");
    };
    const activate = (): void => {
      demo = false;
      if (reduce) { runStart = "done"; render(clock); return; }
      if (typeof runStart === "number" && clock - runStart < RUN_MS) return;
      runStart = clock;
    };
    btn.addEventListener("click", activate);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      clock = ts - t0;
      render(clock);
      raf = requestAnimationFrame(tick);
    }

    if (reduce) render(0);
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", activate);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.rg}>
        <div className={styles.rg__wrap} ref={barRef} role="progressbar" aria-labelledby="rgLabel" aria-valuemin={0} aria-valuemax={100} aria-valuenow={0} aria-valuetext="0%">
          <svg className={styles.rg__svg} viewBox="0 0 232 96" aria-hidden="true">
            <rect className={styles.rg__ghost} ref={ghostRef} x="11" y="11" width="210" height="74" pathLength="100" />
            <rect className={styles.rg__ring} ref={ringRef} x="3" y="3" width="226" height="90" pathLength="100" />
          </svg>
          <button className={styles.rg__btn} type="button" ref={btnRef}>
            <span id="rgLabel" ref={labelRef}>내려받기</span>
            <b ref={pctRef}>0%</b>
          </button>
        </div>
        <p className={styles.rg__meta}>
          <span className={styles.rg__tag}>POW</span> <b>{CURVE_POW}</b> <span className={styles.rg__tag}>LEAD</span> <b ref={leadRef}>0</b>
        </p>
      </div>
    </main>
  );
}

<main class="stage">
<div class="fl is-demo" id="flRoot">
  <button class="fl__btn" id="flBtn" type="button">
    <svg class="fl__icon" viewBox="0 0 24 24" aria-hidden="true"><path d="M12 4v11M6.5 10.5 12 16l5.5-5.5M5 20h14"/></svg>
    <span>ダウンロード</span>
  </button>
  <svg class="fl__file" id="flFile" viewBox="0 0 24 24" aria-hidden="true"><path d="M7 3h7l5 5v13H7zM14 3v5h5"/></svg>
  <div class="fl__tray" id="flTray" role="status"><span>受信済み</span><b id="flCount">0</b></div>
</div>
</main>

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

// 핵심: 파일은 JS 가 베지어 곡선 위 좌표를 계산해 transform 으로 옮긴다. 도착한 순간 트레이가 스프링으로 튄다.

$fly-face: $subject-lilac;
$fly-tray: $subject-sky;
$fly-land: $subject-mint;

@include stage($stage-paper);

.fl {
  position: relative;
  width: 240px;
  max-width: 100%;
  height: 164px;
}

.fl__btn {
  position: absolute;
  left: 20px;
  right: 20px;
  top: 0;
  box-sizing: border-box;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: $sp-3;
  border: 0;
  border-radius: $r-control;
  background: $fly-face;
  box-shadow: $shadow-press;
  color: $subject-ink;
  font-family: inherit;
  font-size: 18px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.fl__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }
.fl__icon {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.fl__file {
  position: absolute;
  left: 0;
  top: 0;
  width: 28px;
  height: 28px;
  opacity: 0;
  fill: $stage-paper;
  stroke: $subject-ink;
  stroke-width: 2;
  stroke-linejoin: round;
  pointer-events: none;
}

.fl__tray {
  --h: 0;
  position: absolute;
  left: 0;
  right: 0;
  top: 92px;
  box-sizing: border-box;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 $sp-6;
  border: 3px solid $subject-ink;
  border-top-width: 0;
  border-radius: 0 0 $r-card $r-card;
  background: color-mix(in srgb, #{$fly-tray}, #{$fly-land} calc(var(--h) * 100%));
  color: $subject-ink;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
}
.fl__tray b { min-width: 3ch; text-align: right; font-variant-numeric: tabular-nums; }

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

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

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

.fl {
  position: relative;
  width: 240px;
  max-width: 100%;
  height: 164px;
}

.fl__btn {
  position: absolute;
  left: 20px;
  right: 20px;
  top: 0;
  box-sizing: border-box;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
  border: 0;
  border-radius: 12px;
  background: #b9a0ff;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  color: #17141a;
  font-family: inherit;
  font-size: 18px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.fl__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.fl__icon {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.fl__file {
  position: absolute;
  left: 0;
  top: 0;
  width: 28px;
  height: 28px;
  opacity: 0;
  fill: #ffffff;
  stroke: #17141a;
  stroke-width: 2;
  stroke-linejoin: round;
  pointer-events: none;
}

.fl__tray {
  --h: 0;
  position: absolute;
  left: 0;
  right: 0;
  top: 92px;
  box-sizing: border-box;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 24px;
  border: 3px solid #17141a;
  border-top-width: 0;
  border-radius: 0 0 16px 16px;
  background: color-mix(in srgb, #6ec8ff, #4cd4a6 calc(var(--h) * 100%));
  color: #17141a;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
}

.fl__tray b {
  min-width: 3ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

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

(function () {
  // 기법: 파일 아이콘은 2차 베지어 곡선 B(t) = (1-t)^2 P0 + 2(1-t)t P1 + t^2 P2 위를 날아간다.
  // 제어점 P1 을 옆으로 ARC_HEIGHT 만큼 밀어 활처럼 휘게 하고, 도착하면 트레이를 TRAY_BOUNCE 만큼 눌러 스프링으로 되돌린다.
  var FLY_MS = 700;
  var ARC_HEIGHT = 56;
  var TRAY_BOUNCE = 14;
  var TRAY_K = 180;
  var TRAY_DAMP = 10;
  var START_MS = 200;
  var CYCLE_MS = 2000;

  var root = document.getElementById('flRoot');
  var btn = document.getElementById('flBtn');
  var file = document.getElementById('flFile');
  var tray = document.getElementById('flTray');
  var countEl = document.getElementById('flCount');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var clock = 0;
  var flightStart = null;
  var count = 0;
  var ty = 0;
  var tv = 0;
  var lastCycle = -1;
  var t0 = null;
  var lastTs = null;

  function stopDemo() { root.classList.remove('is-demo'); }
  function ease(t) { return t < 0.5 ? 4 * t * t * t : 1 - Math.pow(-2 * t + 2, 3) / 2; }

  function geometry() {
    var w = root.clientWidth;
    var b = btn.offsetTop + btn.offsetHeight / 2;
    var tr = tray.offsetTop + tray.offsetHeight / 2;
    return { x: w / 2, y0: b, y2: tr };
  }

  function land() {
    count += 1;
    ty = TRAY_BOUNCE;
    tv = 0;
    flightStart = null;
  }

  function renderFile(now) {
    if (flightStart === null) { file.style.opacity = '0'; return; }
    var t = Math.min(1, (now - flightStart) / FLY_MS);
    var e = ease(t);
    var g = geometry();
    var cx = g.x + 2 * ARC_HEIGHT;
    var cy = (g.y0 + g.y2) / 2;
    var x = (1 - e) * (1 - e) * g.x + 2 * (1 - e) * e * cx + e * e * g.x;
    var y = (1 - e) * (1 - e) * g.y0 + 2 * (1 - e) * e * cy + e * e * g.y2;
    var rot = 28 * Math.sin(Math.PI * e);
    var sc = 1 - 0.45 * e;
    file.style.opacity = t > 0.9 ? String((1 - t) * 10) : '1';
    file.style.transform = 'translate(' + (x - 14).toFixed(1) + 'px,' + (y - 14).toFixed(1) + 'px) rotate(' + rot.toFixed(1) + 'deg) scale(' + sc.toFixed(3) + ')';
  }

  function renderTray() {
    tray.style.transform = 'translateY(' + ty.toFixed(2) + 'px)';
    tray.style.setProperty('--h', Math.max(0, Math.min(1, ty / TRAY_BOUNCE)).toFixed(3));
    countEl.textContent = String(count);
  }

  function activate() {
    stopDemo();
    if (reduce) { count += 1; renderTray(); return; }
    if (flightStart === null) flightStart = clock;
  }
  btn.addEventListener('click', activate);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    clock = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    if (root.classList.contains('is-demo') && clock >= START_MS) {
      var cyc = Math.floor((clock - START_MS) / CYCLE_MS);
      if (cyc > lastCycle) { lastCycle = cyc; count = 0; flightStart = START_MS + cyc * CYCLE_MS; }
    }
    if (flightStart !== null && clock - flightStart >= FLY_MS) land();
    var n = Math.ceil(dt / 0.008);
    var h = dt / n;
    for (var i = 0; i < n; i += 1) {
      tv += (-TRAY_K * ty - TRAY_DAMP * tv) * h;
      ty += tv * h;
    }
    renderFile(clock);
    renderTray();
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    renderTray();
  } else {
    requestAnimationFrame(tick);
  }
})();

// FlyTrayButton — vanilla 판(03-dlbtn-file-fly-tray) 의 React 포트.
// 파일 아이콘은 2차 베지어 곡선 위를 날고, 도착하면 트레이가 TRAY_BOUNCE 만큼 눌렸다가 스프링으로 돌아온다.
import { useEffect, useRef } from "react";
import styles from "./FlyTrayButton.module.scss";

const FLY_MS = 700;
const ARC_HEIGHT = 56;
const TRAY_BOUNCE = 14;
const TRAY_K = 180;
const TRAY_DAMP = 10;
const START_MS = 200;
const CYCLE_MS = 2000;

function ease(t: number): number {
  return t < 0.5 ? 4 * t * t * t : 1 - Math.pow(-2 * t + 2, 3) / 2;
}

export default function FlyTrayButton() {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const fileRef = useRef<SVGSVGElement | null>(null);
  const trayRef = useRef<HTMLDivElement | null>(null);
  const countRef = useRef<HTMLElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    const btn = btnRef.current;
    const file = fileRef.current;
    const tray = trayRef.current;
    const countEl = countRef.current;
    if (!root || !btn || !file || !tray || !countEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let clock = 0;
    let flightStart: number | null = null;
    let count = 0;
    let ty = 0;
    let tv = 0;
    let lastCycle = -1;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const land = (): void => { count += 1; ty = TRAY_BOUNCE; tv = 0; flightStart = null; };
    const renderFile = (now: number): void => {
      if (flightStart === null) { file.style.opacity = "0"; return; }
      const t = Math.min(1, (now - flightStart) / FLY_MS);
      const e = ease(t);
      const gx = root.clientWidth / 2;
      const y0 = btn.offsetTop + btn.offsetHeight / 2;
      const y2 = tray.offsetTop + tray.offsetHeight / 2;
      const cx = gx + 2 * ARC_HEIGHT;
      const cy = (y0 + y2) / 2;
      const x = (1 - e) * (1 - e) * gx + 2 * (1 - e) * e * cx + e * e * gx;
      const y = (1 - e) * (1 - e) * y0 + 2 * (1 - e) * e * cy + e * e * y2;
      const rot = 28 * Math.sin(Math.PI * e);
      const sc = 1 - 0.45 * e;
      file.style.opacity = t > 0.9 ? String((1 - t) * 10) : "1";
      file.style.transform = `translate(${(x - 14).toFixed(1)}px,${(y - 14).toFixed(1)}px) rotate(${rot.toFixed(1)}deg) scale(${sc.toFixed(3)})`;
    };
    const renderTray = (): void => {
      tray.style.transform = `translateY(${ty.toFixed(2)}px)`;
      tray.style.setProperty("--h", Math.max(0, Math.min(1, ty / TRAY_BOUNCE)).toFixed(3));
      countEl.textContent = String(count);
    };
    const activate = (): void => {
      demo = false;
      if (reduce) { count += 1; renderTray(); return; }
      if (flightStart === null) flightStart = clock;
    };
    btn.addEventListener("click", activate);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      clock = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      if (demo && clock >= START_MS) {
        const cyc = Math.floor((clock - START_MS) / CYCLE_MS);
        if (cyc > lastCycle) { lastCycle = cyc; count = 0; flightStart = START_MS + cyc * CYCLE_MS; }
      }
      if (flightStart !== null && clock - flightStart >= FLY_MS) land();
      const n = Math.ceil(dt / 0.008);
      const h = dt / n;
      for (let i = 0; i < n; i += 1) {
        tv += (-TRAY_K * ty - TRAY_DAMP * tv) * h;
        ty += tv * h;
      }
      renderFile(clock);
      renderTray();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) renderTray();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", activate);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.fl} ref={rootRef}>
        <button className={styles.fl__btn} type="button" ref={btnRef}>
          <svg className={styles.fl__icon} viewBox="0 0 24 24" aria-hidden="true">
            <path d="M12 4v11M6.5 10.5 12 16l5.5-5.5M5 20h14" />
          </svg>
          <span>내려받기</span>
        </button>
        <svg className={styles.fl__file} viewBox="0 0 24 24" aria-hidden="true" ref={fileRef}>
          <path d="M7 3h7l5 5v13H7zM14 3v5h5" />
        </svg>
        <div className={styles.fl__tray} role="status" ref={trayRef}>
          <span>받은 파일</span>
          <b ref={countRef}>0</b>
        </div>
      </div>
    </main>
  );
}

<main class="stage">
<div class="sz is-demo" id="szRoot">
  <button class="sz__btn" id="szBtn" type="button"><span id="szLabel">ダウンロード</span></button>
  <div class="sz__chips">
    <p class="sz__chip"><span class="sz__tag">MB</span> <b id="szGot">0.0</b><span class="sz__of">/ 48</span></p>
    <p class="sz__chip"><span class="sz__tag">ETA</span> <b id="szEta">--:--</b></p>
  </div>
  <p class="sz__sr" id="szStatus" role="status"></p>
</div>
</main>

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

// 핵심: 숫자 두 개(받은 크기 · 남은 시간)와 버튼 안 채움이 같은 계산값 하나에서 나온다.

$sz-face: $subject-lilac;
$sz-fill: $subject-mint;

@include stage($stage-orange);

.sz {
  --f: 0;
  width: min(100%, 280px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.sz__btn {
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 64px;
  border: 0;
  border-radius: $r-control;
  background: linear-gradient(90deg, #{$sz-fill} calc(var(--f) * 100%), #{$sz-face} calc(var(--f) * 100%));
  box-shadow: $shadow-press;
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.sz__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.sz__chips {
  width: 100%;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: $sp-2;
}
.sz__chip {
  margin: 0;
  box-sizing: border-box;
  display: flex;
  align-items: baseline;
  gap: $sp-1;
  padding: $sp-2;
  border-radius: $r-sm;
  background: color-mix(in srgb, #{$subject-cream}, #{$sz-fill} calc(var(--f) * 100%));
  color: $subject-ink;
  font-size: 14px;
  line-height: 24px;
}
.sz__tag { font-size: 12px; font-weight: 700; letter-spacing: .08em; }
.sz__chip b { min-width: 4ch; font-size: 20px; text-align: right; font-variant-numeric: tabular-nums; }
.sz__of { font-size: 12px; }

.sz__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

@media (prefers-reduced-motion: reduce) {
  .sz__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: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sz {
  --f: 0;
  width: min(100%, 280px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.sz__btn {
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 64px;
  border: 0;
  border-radius: 12px;
  background: linear-gradient(90deg, #4cd4a6 calc(var(--f) * 100%), #b9a0ff calc(var(--f) * 100%));
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.sz__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.sz__chips {
  width: 100%;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 8px;
}

.sz__chip {
  margin: 0;
  box-sizing: border-box;
  display: flex;
  align-items: baseline;
  gap: 4px;
  padding: 8px;
  border-radius: 8px;
  background: color-mix(in srgb, #fff7e6, #4cd4a6 calc(var(--f) * 100%));
  color: #17141a;
  font-size: 14px;
  line-height: 24px;
}

.sz__tag {
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.08em;
}

.sz__chip b {
  min-width: 4ch;
  font-size: 20px;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

.sz__of {
  font-size: 12px;
}

.sz__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

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

(function () {
  // 기법: 받은 크기 = TARGET_MB x (1 - (1 - u)^COUNT_EASE) 로 감속하며 올라간다 (u 는 시간 비율).
  // 남은 시간 = 남은 양 / 지금까지의 평균 속도. 평균 속도가 시간에 따라 변하므로 ETA 는 시작 때 가장 크고 끝에서 0 이 된다.
  var TARGET_MB = 48;
  var ETA_MIN = 2;
  var COUNT_EASE = 3;
  var RUN_MS = 1500;
  var START_MS = 200;
  var CYCLE_MS = 2000;

  var root = document.getElementById('szRoot');
  var btn = document.getElementById('szBtn');
  var labelEl = document.getElementById('szLabel');
  var gotEl = document.getElementById('szGot');
  var etaEl = document.getElementById('szEta');
  var statusEl = document.getElementById('szStatus');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var clock = 0;
  var runStart = null;
  var finished = false;
  var announced = false;
  var t0 = null;

  function stopDemo() { root.classList.remove('is-demo'); }
  function two(n) { return n < 10 ? '0' + n : String(n); }

  function startOf(now) {
    if (root.classList.contains('is-demo')) {
      return now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS;
    }
    return runStart;
  }

  function render(now) {
    var s = startOf(now);
    var u = s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
    if (reduce && finished) u = 1;
    var got = TARGET_MB * (1 - Math.pow(1 - u, COUNT_EASE));
    var eta = '--:--';
    if (u > 0) {
      var elapsedSec = Math.max(u, 0.004) * ETA_MIN * 60;
      var avg = Math.max(got, 0.01) / elapsedSec;
      var left = Math.round((TARGET_MB - got) / avg);
      eta = Math.floor(left / 60) + ':' + two(left % 60);
    }
    root.style.setProperty('--f', (got / TARGET_MB).toFixed(3));
    gotEl.textContent = got.toFixed(1);
    etaEl.textContent = eta;
    labelEl.textContent = u >= 1 ? '完了' : (u > 0 ? '受信中' : 'ダウンロード');
    if (u >= 1 && !announced) { announced = true; statusEl.textContent = 'ダウンロード完了'; }
    if (u < 1 && announced) { announced = false; statusEl.textContent = ''; }
  }

  function activate() {
    stopDemo();
    if (reduce) { finished = true; render(clock); return; }
    if (runStart !== null && clock - runStart < RUN_MS) return;
    runStart = clock;
  }
  btn.addEventListener('click', activate);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    clock = ts - t0;
    render(clock);
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    render(0);
  } else {
    requestAnimationFrame(tick);
  }
})();

// SizeCountButton — vanilla 판(04-dlbtn-size-count-estimate) 의 React 포트.
// 받은 크기는 TARGET_MB x (1 - (1 - u)^COUNT_EASE) 로 감속하며 오르고, 남은 시간은 남은 양 / 평균 속도 로 계산한다.
import { useEffect, useRef } from "react";
import styles from "./SizeCountButton.module.scss";

const TARGET_MB = 48;
const ETA_MIN = 2;
const COUNT_EASE = 3;
const RUN_MS = 1500;
const START_MS = 200;
const CYCLE_MS = 2000;

function two(n: number): string {
  return n < 10 ? "0" + n : String(n);
}

export default function SizeCountButton() {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const gotRef = useRef<HTMLElement | null>(null);
  const etaRef = useRef<HTMLElement | null>(null);
  const statusRef = useRef<HTMLParagraphElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    const btn = btnRef.current;
    const labelEl = labelRef.current;
    const gotEl = gotRef.current;
    const etaEl = etaRef.current;
    const statusEl = statusRef.current;
    if (!root || !btn || !labelEl || !gotEl || !etaEl || !statusEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let clock = 0;
    let runStart: number | null = null;
    let finished = false;
    let announced = false;
    let t0: number | null = null;
    let raf = 0;

    const startOf = (now: number): number | null => {
      if (demo) return now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS;
      return runStart;
    };
    const render = (now: number): void => {
      const s = startOf(now);
      let u = s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
      if (reduce && finished) u = 1;
      const got = TARGET_MB * (1 - Math.pow(1 - u, COUNT_EASE));
      let eta = "--:--";
      if (u > 0) {
        const elapsedSec = Math.max(u, 0.004) * ETA_MIN * 60;
        const avg = Math.max(got, 0.01) / elapsedSec;
        const left = Math.round((TARGET_MB - got) / avg);
        eta = Math.floor(left / 60) + ":" + two(left % 60);
      }
      root.style.setProperty("--f", (got / TARGET_MB).toFixed(3));
      gotEl.textContent = got.toFixed(1);
      etaEl.textContent = eta;
      labelEl.textContent = u >= 1 ? "완료" : u > 0 ? "받는 중" : "내려받기";
      if (u >= 1 && !announced) { announced = true; statusEl.textContent = "받기 완료"; }
      if (u < 1 && announced) { announced = false; statusEl.textContent = ""; }
    };
    const activate = (): void => {
      demo = false;
      if (reduce) { finished = true; render(clock); return; }
      if (runStart !== null && clock - runStart < RUN_MS) return;
      runStart = clock;
    };
    btn.addEventListener("click", activate);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      clock = ts - t0;
      render(clock);
      raf = requestAnimationFrame(tick);
    }

    if (reduce) render(0);
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", activate);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.sz} ref={rootRef}>
        <button className={styles.sz__btn} type="button" ref={btnRef}>
          <span ref={labelRef}>내려받기</span>
        </button>
        <div className={styles.sz__chips}>
          <p className={styles.sz__chip}>
            <span className={styles.sz__tag}>MB</span> <b ref={gotRef}>0.0</b>
            <span className={styles.sz__of}>/ {TARGET_MB}</span>
          </p>
          <p className={styles.sz__chip}>
            <span className={styles.sz__tag}>ETA</span> <b ref={etaRef}>--:--</b>
          </p>
        </div>
        <p className={styles.sz__sr} role="status" ref={statusRef} />
      </div>
    </main>
  );
}

<main class="stage">
<div class="lq is-demo" id="lqRoot">
  <button class="lq__btn" id="lqBtn" type="button">
    <svg class="lq__wave" viewBox="0 0 240 72" preserveAspectRatio="none" aria-hidden="true"><path id="lqPath" d="M0 72H240V72Z"/></svg>
    <span class="lq__text"><span id="lqLabel">ダウンロード</span> <b id="lqPct">0%</b></span>
  </button>
  <p class="lq__meta"><span class="lq__tag">LEVEL</span> <b id="lqLevel">0.00</b> <span class="lq__tag">AMP</span> <b id="lqAmp">0.0</b></p>
</div>
</main>

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

// 핵심: 수면은 SVG path 의 d 를 JS 가 매 프레임 다시 계산한다. 속성 트윈이 아니라 파동 합성이다.

$lq-face: $subject-cream;
$lq-water: $subject-sky;

@include stage($stage-ink);

.lq {
  width: min(100%, 280px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.lq__btn {
  position: relative;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 72px;
  overflow: hidden;
  border: 0;
  border-radius: $r-card;
  background: $lq-face;
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.lq__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }
.lq__wave {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  fill: $lq-water;
}
.lq__text { position: relative; }
.lq__text b { font-variant-numeric: tabular-nums; }

.lq__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: $stage-paper;
}
.lq__tag { font-weight: 700; letter-spacing: .08em; }
.lq__meta b { min-width: 4ch; text-align: right; font-variant-numeric: tabular-nums; }

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

.lq {
  width: min(100%, 280px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.lq__btn {
  position: relative;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 72px;
  overflow: hidden;
  border: 0;
  border-radius: 16px;
  background: #fff7e6;
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.lq__btn:focus-visible {
  outline: 3px solid #ffffff;
  outline-offset: 3px;
}

.lq__wave {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  fill: #6ec8ff;
}

.lq__text {
  position: relative;
}

.lq__text b {
  font-variant-numeric: tabular-nums;
}

.lq__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: #ffffff;
}

.lq__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.lq__meta b {
  min-width: 4ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

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

(function () {
  // 기법: 수위는 목표 진행률을 향해 지수 보간(LEVEL_LERP)으로 따라가고, 수면은 사인 두 개를 합성한 곡선이다.
  // 파동 진폭에 (1 - 0.7 x 수위) 를 곱해서 물이 찰수록 수면이 가라앉는다.
  var WAVE_AMP = 6;
  var WAVE_FREQ = 2;
  var LEVEL_LERP = 6;
  var RUN_MS = 1500;
  var START_MS = 200;
  var CYCLE_MS = 2000;
  var W = 240;
  var H = 72;
  var POINTS = 48;

  var root = document.getElementById('lqRoot');
  var btn = document.getElementById('lqBtn');
  var pathEl = document.getElementById('lqPath');
  var labelEl = document.getElementById('lqLabel');
  var pctEl = document.getElementById('lqPct');
  var levelEl = document.getElementById('lqLevel');
  var ampEl = document.getElementById('lqAmp');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var clock = 0;
  var runStart = null;
  var level = 0;
  var target = 0;
  var t0 = null;
  var lastTs = null;

  function stopDemo() { root.classList.remove('is-demo'); }

  function progressAt(now) {
    var s = root.classList.contains('is-demo')
      ? (now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS)
      : runStart;
    return s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
  }

  function draw(phase) {
    var reach = WAVE_AMP * 1.5;
    var base = -reach + (H + 2 * reach) * (1 - level);
    var amp = WAVE_AMP * (1 - 0.7 * level);
    var d = 'M0 ' + H;
    for (var i = 0; i <= POINTS; i += 1) {
      var x = (i / POINTS) * W;
      var a = 2 * Math.PI * WAVE_FREQ * (x / W);
      var y = base + amp * (Math.sin(a + phase) + 0.5 * Math.sin(2 * a - 1.3 * phase));
      d += 'L' + x.toFixed(1) + ' ' + y.toFixed(2);
    }
    d += 'L' + W + ' ' + H + 'Z';
    pathEl.setAttribute('d', d);
    var pct = Math.round(level * 100);
    pctEl.textContent = pct + '%';
    labelEl.textContent = level > 0.995 ? '完了' : (level > 0.01 ? '受信中' : 'ダウンロード');
    levelEl.textContent = level.toFixed(2);
    ampEl.textContent = amp.toFixed(1);
  }

  function activate() {
    stopDemo();
    if (reduce) { level = 1; draw(0); return; }
    if (runStart !== null && clock - runStart < RUN_MS) return;
    runStart = clock;
  }
  btn.addEventListener('click', activate);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    clock = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    target = progressAt(clock);
    level += (target - level) * (1 - Math.exp(-dt * LEVEL_LERP));
    draw(clock * 0.006);
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    draw(0);
  } else {
    requestAnimationFrame(tick);
  }
})();

// LiquidLevelButton — vanilla 판(05-dlbtn-liquid-level-wave) 의 React 포트.
// 수위는 목표 진행률을 지수 보간(LEVEL_LERP)으로 따라가고, 수면은 사인 두 개의 합성이며 물이 찰수록 진폭이 줄어든다.
import { useEffect, useRef } from "react";
import styles from "./LiquidLevelButton.module.scss";

const WAVE_AMP = 6;
const WAVE_FREQ = 2;
const LEVEL_LERP = 6;
const RUN_MS = 1500;
const START_MS = 200;
const CYCLE_MS = 2000;
const W = 240;
const H = 72;
const POINTS = 48;

export default function LiquidLevelButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const pathRef = useRef<SVGPathElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const pctRef = useRef<HTMLElement | null>(null);
  const levelRef = useRef<HTMLElement | null>(null);
  const ampRef = useRef<HTMLElement | null>(null);

  useEffect(() => {
    const btn = btnRef.current;
    const pathEl = pathRef.current;
    const labelEl = labelRef.current;
    const pctEl = pctRef.current;
    const levelEl = levelRef.current;
    const ampEl = ampRef.current;
    if (!btn || !pathEl || !labelEl || !pctEl || !levelEl || !ampEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let clock = 0;
    let runStart: number | null = null;
    let level = 0;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const progressAt = (now: number): number => {
      const s = demo
        ? now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS
        : runStart;
      return s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
    };
    const draw = (phase: number): void => {
      const reach = WAVE_AMP * 1.5;
      const base = -reach + (H + 2 * reach) * (1 - level);
      const amp = WAVE_AMP * (1 - 0.7 * level);
      let d = "M0 " + H;
      for (let i = 0; i <= POINTS; i += 1) {
        const x = (i / POINTS) * W;
        const a = 2 * Math.PI * WAVE_FREQ * (x / W);
        const y = base + amp * (Math.sin(a + phase) + 0.5 * Math.sin(2 * a - 1.3 * phase));
        d += "L" + x.toFixed(1) + " " + y.toFixed(2);
      }
      d += "L" + W + " " + H + "Z";
      pathEl.setAttribute("d", d);
      pctEl.textContent = Math.round(level * 100) + "%";
      labelEl.textContent = level > 0.995 ? "완료" : level > 0.01 ? "받는 중" : "내려받기";
      levelEl.textContent = level.toFixed(2);
      ampEl.textContent = amp.toFixed(1);
    };
    const activate = (): void => {
      demo = false;
      if (reduce) { level = 1; draw(0); return; }
      if (runStart !== null && clock - runStart < RUN_MS) return;
      runStart = clock;
    };
    btn.addEventListener("click", activate);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      clock = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      level += (progressAt(clock) - level) * (1 - Math.exp(-dt * LEVEL_LERP));
      draw(clock * 0.006);
      raf = requestAnimationFrame(tick);
    }

    if (reduce) draw(0);
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", activate);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.lq}>
        <button className={styles.lq__btn} type="button" ref={btnRef}>
          <svg className={styles.lq__wave} viewBox="0 0 240 72" preserveAspectRatio="none" aria-hidden="true">
            <path d="M0 72H240V72Z" ref={pathRef} />
          </svg>
          <span className={styles.lq__text}>
            <span ref={labelRef}>내려받기</span> <b ref={pctRef}>0%</b>
          </span>
        </button>
        <p className={styles.lq__meta}>
          <span className={styles.lq__tag}>LEVEL</span> <b ref={levelRef}>0.00</b> <span className={styles.lq__tag}>AMP</span> <b ref={ampRef}>0.0</b>
        </p>
      </div>
    </main>
  );
}

<main class="stage">
<div class="ck is-demo" id="ckRoot">
  <button class="ck__btn" id="ckBtn" type="button">
    <svg class="ck__icon" viewBox="0 0 24 24" aria-hidden="true">
      <path id="ckArrow" d="M12 4v11M6.5 10.5 12 16l5.5-5.5"/>
      <path id="ckCheck" d="M5 13l4.5 4.5L19 7"/>
    </svg>
    <span class="ck__label" id="ckLabel">ダウンロード</span>
  </button>
  <span class="ck__ruler" id="ckRulerA" aria-hidden="true">ダウンロード</span>
  <span class="ck__ruler" id="ckRulerB" aria-hidden="true">完了</span>
  <p class="ck__meta"><span class="ck__tag">P</span> <b id="ckP">0.00</b> <span class="ck__tag">W</span> <b id="ckW">0</b></p>
  <p class="ck__sr" id="ckStatus" role="status"></p>
</div>
</main>

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

// 핵심: 두 선(화살표·체크)의 stroke-dashoffset 이 같은 진행값 하나로 움직이고, 폭은 라벨 실측 너비를 따라간다.

$ck-face: $subject-lilac;
$ck-done: $subject-mint;

@include stage($stage-yellow);

.ck {
  position: relative;
  width: min(100%, 280px);
  display: grid;
  gap: $sp-4;
  justify-items: center;
}

.ck__btn {
  --p: 0;
  box-sizing: border-box;
  height: 80px;
  max-width: 100%;
  display: inline-flex;
  align-items: center;
  justify-content: flex-start;
  gap: $sp-4;
  padding: 0 $sp-6;
  overflow: hidden;
  white-space: nowrap;
  border: 0;
  border-radius: $r-control;
  background: color-mix(in srgb, #{$ck-face}, #{$ck-done} calc(var(--p) * 100%));
  box-shadow: $shadow-press;
  color: $subject-ink;
  font-family: inherit;
  font-size: 24px;
  font-weight: 700;
  line-height: 32px;
  cursor: pointer;
}
.ck__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.ck__icon {
  flex: none;
  width: 36px;
  height: 36px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.ck__ruler {
  position: absolute;
  visibility: hidden;
  white-space: nowrap;
  font-size: 24px;
  font-weight: 700;
}

.ck__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: $subject-ink;
}
.ck__tag { font-weight: 700; letter-spacing: .08em; }
.ck__meta b { min-width: 4ch; text-align: right; font-variant-numeric: tabular-nums; }

.ck__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

@media (prefers-reduced-motion: reduce) {
  .ck__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: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.ck {
  position: relative;
  width: min(100%, 280px);
  display: grid;
  gap: 16px;
  justify-items: center;
}

.ck__btn {
  --p: 0;
  box-sizing: border-box;
  height: 80px;
  max-width: 100%;
  display: inline-flex;
  align-items: center;
  justify-content: flex-start;
  gap: 16px;
  padding: 0 24px;
  overflow: hidden;
  white-space: nowrap;
  border: 0;
  border-radius: 12px;
  background: color-mix(in srgb, #b9a0ff, #4cd4a6 calc(var(--p) * 100%));
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  color: #17141a;
  font-family: inherit;
  font-size: 24px;
  font-weight: 700;
  line-height: 32px;
  cursor: pointer;
}

.ck__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.ck__icon {
  flex: none;
  width: 36px;
  height: 36px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.ck__ruler {
  position: absolute;
  visibility: hidden;
  white-space: nowrap;
  font-size: 24px;
  font-weight: 700;
}

.ck__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: #17141a;
}

.ck__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.ck__meta b {
  min-width: 4ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

.ck__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

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

(function () {
  // 기법: 화살표와 체크 두 경로의 stroke-dashoffset 을 같은 진행값 p 에서 계산한다.
  // p 0 to 0.5 에서 화살표 선이 줄어들고, p 0.4 to 1 에서 체크 선이 그어진다. 버튼 폭은 라벨 실측 너비로 지수 보간한다.
  var STROKE_LEN = 100;
  var CHECK_MS = 360;
  var WIDTH_LERP = 10;
  var PAD_X = 24;
  var ICON_GAP = 16;
  var ICON_W = 36;
  var START_MS = 200;
  var HOLD_MS = 1300;
  var CYCLE_MS = 2000;

  var root = document.getElementById('ckRoot');
  var btn = document.getElementById('ckBtn');
  var arrow = document.getElementById('ckArrow');
  var check = document.getElementById('ckCheck');
  var label = document.getElementById('ckLabel');
  var rulerA = document.getElementById('ckRulerA');
  var rulerB = document.getElementById('ckRulerB');
  var pEl = document.getElementById('ckP');
  var wEl = document.getElementById('ckW');
  var statusEl = document.getElementById('ckStatus');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var p = 0;
  var width = 0;
  var done = false;
  var announced = false;
  var t0 = null;
  var lastTs = null;

  arrow.setAttribute('pathLength', String(STROKE_LEN));
  check.setAttribute('pathLength', String(STROKE_LEN));
  arrow.style.strokeDasharray = STROKE_LEN + ' ' + STROKE_LEN;
  check.style.strokeDasharray = STROKE_LEN + ' ' + STROKE_LEN;

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function stopDemo() { root.classList.remove('is-demo'); }
  function widthFor(isDone) { return 2 * PAD_X + ICON_W + ICON_GAP + (isDone ? rulerB : rulerA).offsetWidth; }

  function goal(now) {
    if (root.classList.contains('is-demo')) {
      return now >= START_MS && (now - START_MS) % CYCLE_MS < HOLD_MS;
    }
    return done;
  }

  function render(want) {
    var shrink = clamp(p / 0.5, 0, 1);
    var draw = clamp((p - 0.4) / 0.6, 0, 1);
    arrow.style.strokeDashoffset = String((STROKE_LEN * shrink).toFixed(2));
    check.style.strokeDashoffset = String((STROKE_LEN * (1 - draw)).toFixed(2));
    label.textContent = p >= 0.5 ? '完了' : 'ダウンロード';
    label.style.opacity = String(Math.abs(2 * p - 1).toFixed(2));
    btn.style.setProperty('--p', p.toFixed(3));
    btn.style.width = width.toFixed(1) + 'px';
    pEl.textContent = p.toFixed(2);
    wEl.textContent = String(Math.round(width));
    if (p >= 1 && !announced) { announced = true; statusEl.textContent = 'ダウンロード完了'; }
    if (p <= 0 && announced) { announced = false; statusEl.textContent = ''; }
  }

  function toggle() {
    stopDemo();
    done = !done;
    if (reduce) { p = done ? 1 : 0; width = widthFor(done); render(done); }
  }
  btn.addEventListener('click', toggle);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    var want = goal(now);
    var step = dt / (CHECK_MS / 1000);
    p = clamp(p + (want ? step : -step), 0, 1);
    width += (widthFor(want) - width) * (1 - Math.exp(-dt * WIDTH_LERP));
    render(want);
    requestAnimationFrame(tick);
  }

  width = widthFor(false);
  if (reduce) {
    stopDemo();
    render(false);
  } else {
    requestAnimationFrame(tick);
  }
})();

// CheckMorphButton — vanilla 판(06-dlbtn-check-stroke-morph) 의 React 포트.
// 화살표와 체크 두 경로의 stroke-dashoffset 을 같은 진행값 p 에서 계산하고, 버튼 폭은 라벨 실측 너비로 보간한다.
import { useEffect, useRef } from "react";
import styles from "./CheckMorphButton.module.scss";

const STROKE_LEN = 100;
const CHECK_MS = 360;
const WIDTH_LERP = 10;
const PAD_X = 24;
const ICON_GAP = 16;
const ICON_W = 36;
const START_MS = 200;
const HOLD_MS = 1300;
const CYCLE_MS = 2000;

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

export default function CheckMorphButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const arrowRef = useRef<SVGPathElement | null>(null);
  const checkRef = useRef<SVGPathElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const rulerARef = useRef<HTMLSpanElement | null>(null);
  const rulerBRef = useRef<HTMLSpanElement | null>(null);
  const pRef = useRef<HTMLElement | null>(null);
  const wRef = useRef<HTMLElement | null>(null);
  const statusRef = useRef<HTMLParagraphElement | null>(null);

  useEffect(() => {
    const btn = btnRef.current;
    const arrow = arrowRef.current;
    const check = checkRef.current;
    const label = labelRef.current;
    const rulerA = rulerARef.current;
    const rulerB = rulerBRef.current;
    const pEl = pRef.current;
    const wEl = wRef.current;
    const statusEl = statusRef.current;
    if (!btn || !arrow || !check || !label || !rulerA || !rulerB || !pEl || !wEl || !statusEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let p = 0;
    let width = 0;
    let done = false;
    let announced = false;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    arrow.setAttribute("pathLength", String(STROKE_LEN));
    check.setAttribute("pathLength", String(STROKE_LEN));
    arrow.style.strokeDasharray = STROKE_LEN + " " + STROKE_LEN;
    check.style.strokeDasharray = STROKE_LEN + " " + STROKE_LEN;

    const widthFor = (isDone: boolean): number =>
      2 * PAD_X + ICON_W + ICON_GAP + (isDone ? rulerB : rulerA).offsetWidth;
    const goal = (now: number): boolean => {
      if (demo) return now >= START_MS && (now - START_MS) % CYCLE_MS < HOLD_MS;
      return done;
    };
    const render = (): void => {
      const shrink = clamp(p / 0.5, 0, 1);
      const draw = clamp((p - 0.4) / 0.6, 0, 1);
      arrow.style.strokeDashoffset = (STROKE_LEN * shrink).toFixed(2);
      check.style.strokeDashoffset = (STROKE_LEN * (1 - draw)).toFixed(2);
      label.textContent = p >= 0.5 ? "완료" : "내려받기";
      label.style.opacity = Math.abs(2 * p - 1).toFixed(2);
      btn.style.setProperty("--p", p.toFixed(3));
      btn.style.width = width.toFixed(1) + "px";
      pEl.textContent = p.toFixed(2);
      wEl.textContent = String(Math.round(width));
      if (p >= 1 && !announced) { announced = true; statusEl.textContent = "받기 완료"; }
      if (p <= 0 && announced) { announced = false; statusEl.textContent = ""; }
    };
    const toggle = (): void => {
      demo = false;
      done = !done;
      if (reduce) { p = done ? 1 : 0; width = widthFor(done); render(); }
    };
    btn.addEventListener("click", toggle);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      const want = goal(now);
      const step = dt / (CHECK_MS / 1000);
      p = clamp(p + (want ? step : -step), 0, 1);
      width += (widthFor(want) - width) * (1 - Math.exp(-dt * WIDTH_LERP));
      render();
      raf = requestAnimationFrame(tick);
    }

    width = widthFor(false);
    if (reduce) render();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", toggle);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.ck}>
        <button className={styles.ck__btn} type="button" ref={btnRef}>
          <svg className={styles.ck__icon} viewBox="0 0 24 24" aria-hidden="true">
            <path d="M12 4v11M6.5 10.5 12 16l5.5-5.5" ref={arrowRef} />
            <path d="M5 13l4.5 4.5L19 7" ref={checkRef} />
          </svg>
          <span className={styles.ck__label} ref={labelRef}>내려받기</span>
        </button>
        <span className={styles.ck__ruler} ref={rulerARef} aria-hidden="true">내려받기</span>
        <span className={styles.ck__ruler} ref={rulerBRef} aria-hidden="true">완료</span>
        <p className={styles.ck__meta}>
          <span className={styles.ck__tag}>P</span> <b ref={pRef}>0.00</b> <span className={styles.ck__tag}>W</span> <b ref={wRef}>0</b>
        </p>
        <p className={styles.ck__sr} role="status" ref={statusRef} />
      </div>
    </main>
  );
}

<main class="stage">
<div class="sp is-demo" id="spRoot">
  <div class="sp__row">
    <button class="sp__main" id="spMain" type="button">
      <span class="sp__win" aria-hidden="true"><span class="sp__fmt" id="spOld"></span><span class="sp__fmt" id="spNew">PDF</span></span>
      <span class="sp__sr" id="spFmtText">PDF</span>
      <span>ダウンロード</span>
    </button>
    <button class="sp__toggle" id="spToggle" type="button" aria-haspopup="menu" aria-expanded="false" aria-controls="spMenu" aria-label="形式を選択">
      <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M7 10l5 5 5-5"/></svg>
    </button>
  </div>
  <div class="sp__menu" id="spMenu" role="menu" aria-label="形式を選択">
    <button class="sp__item" type="button" role="menuitem" data-i="0">PDF</button>
    <button class="sp__item" type="button" role="menuitem" data-i="1">PNG</button>
    <button class="sp__item" type="button" role="menuitem" data-i="2">SVG</button>
  </div>
</div>
</main>

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

// 핵심: 메뉴 칸마다 진행값이 따로 있다(JS 계산). 라벨은 창 안에서 위로 밀려 나가고 새 글자가 아래에서 올라온다.

$sp-face: $subject-lilac;
$sp-side: $subject-pink;
$sp-picked: $subject-mint;

@include stage($stage-paper);

.sp {
  width: 260px;
  max-width: 100%;
  display: grid;
  gap: $sp-2;
}

.sp__row {
  display: grid;
  grid-template-columns: 1fr 64px;
  gap: $sp-1;
}

.sp__main, .sp__toggle {
  box-sizing: border-box;
  height: 64px;
  border: 0;
  color: $subject-ink;
  font-family: inherit;
  font-size: 18px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.sp__main {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: $sp-2;
  border-radius: $r-control 0 0 $r-control;
  background: color-mix(in srgb, #{$sp-face}, #{$sp-picked} calc(var(--k, 0) * 100%));
}
.sp__toggle {
  --o: 0;
  display: grid;
  place-items: center;
  border-radius: 0 $r-control $r-control 0;
  background: $sp-side;
}
.sp__toggle svg {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
  transform: rotate(calc(var(--o) * 180deg));
}
.sp__main:focus-visible, .sp__toggle:focus-visible, .sp__item:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.sp__win {
  position: relative;
  display: block;
  width: 44px;
  height: 24px;
  overflow: hidden;
}
.sp__fmt { position: absolute; inset: 0; text-align: center; }

.sp__menu {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: $sp-2;
}
.sp__item {
  box-sizing: border-box;
  height: 48px;
  border: 2px solid $subject-ink;
  border-radius: $r-sm;
  background: $subject-cream;
  color: $subject-ink;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
  visibility: hidden;
}
.sp__item.is-picked { background: $sp-picked; }

.sp__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

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

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

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

.sp {
  width: 260px;
  max-width: 100%;
  display: grid;
  gap: 8px;
}

.sp__row {
  display: grid;
  grid-template-columns: 1fr 64px;
  gap: 4px;
}

.sp__main, .sp__toggle {
  box-sizing: border-box;
  height: 64px;
  border: 0;
  color: #17141a;
  font-family: inherit;
  font-size: 18px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.sp__main {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  border-radius: 12px 0 0 12px;
  background: color-mix(in srgb, #b9a0ff, #4cd4a6 calc(var(--k, 0) * 100%));
}

.sp__toggle {
  --o: 0;
  display: grid;
  place-items: center;
  border-radius: 0 12px 12px 0;
  background: #ff9ebb;
}

.sp__toggle svg {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
  transform: rotate(calc(var(--o) * 180deg));
}

.sp__main:focus-visible, .sp__toggle:focus-visible, .sp__item:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.sp__win {
  position: relative;
  display: block;
  width: 44px;
  height: 24px;
  overflow: hidden;
}

.sp__fmt {
  position: absolute;
  inset: 0;
  text-align: center;
}

.sp__menu {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 8px;
}

.sp__item {
  box-sizing: border-box;
  height: 48px;
  border: 2px solid #17141a;
  border-radius: 8px;
  background: #fff7e6;
  color: #17141a;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
  visibility: hidden;
}

.sp__item.is-picked {
  background: #4cd4a6;
}

.sp__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

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

(function () {
  // 기법: 메뉴 칸마다 시작 시각을 MENU_STAGGER 씩 늦춰 easeOutBack 으로 펼친다 (칸마다 따로 계산한 진행값).
  // 고른 형식은 라벨 창 안에서 옛 글자가 위로 나가고 새 글자가 LABEL_RISE 만큼 아래에서 올라온다.
  var MENU_STAGGER = 70;
  var FORMATS = ['PDF', 'PNG', 'SVG'];
  var LABEL_RISE = 24;
  var ITEM_MS = 280;
  var OPEN_MS = ITEM_MS + (FORMATS.length - 1) * MENU_STAGGER;
  var RISE_MS = 320;
  var START_MS = 200;
  var CYCLE_MS = 2000;

  var root = document.getElementById('spRoot');
  var toggle = document.getElementById('spToggle');
  var menu = document.getElementById('spMenu');
  var oldEl = document.getElementById('spOld');
  var newEl = document.getElementById('spNew');
  var textEl = document.getElementById('spFmtText');
  var items = Array.prototype.slice.call(menu.querySelectorAll('.sp__item'));
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var wantOpen = false;
  var wantFmt = 0;
  var shownFmt = 0;
  var open = 0;
  var rise = 1;
  var t0 = null;
  var lastTs = null;

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function stopDemo() { root.classList.remove('is-demo'); }
  function outBack(t) { var c = 1.70158; return 1 + (c + 1) * Math.pow(t - 1, 3) + c * Math.pow(t - 1, 2); }
  function outCubic(t) { return 1 - Math.pow(1 - t, 3); }

  function render() {
    var s = open * OPEN_MS;
    var visible = open > 0.001 || wantOpen;
    for (var i = 0; i < items.length; i += 1) {
      var q = outBack(clamp((s - i * MENU_STAGGER) / ITEM_MS, 0, 1));
      var o = clamp(q, 0, 1);
      items[i].style.opacity = o.toFixed(3);
      items[i].style.transform = 'translateY(' + ((1 - q) * -16).toFixed(2) + 'px) scale(' + (0.8 + 0.2 * q).toFixed(3) + ')';
      items[i].style.visibility = visible ? 'visible' : 'hidden';
      items[i].setAttribute('tabindex', wantOpen ? '0' : '-1');
      items[i].classList.toggle('is-picked', i === wantFmt);
    }
    menu.setAttribute('aria-hidden', wantOpen ? 'false' : 'true');
    toggle.setAttribute('aria-expanded', wantOpen ? 'true' : 'false');
    toggle.style.setProperty('--o', open.toFixed(3));
    root.style.setProperty('--k', (1 - rise).toFixed(3));
    oldEl.style.transform = 'translateY(' + (-LABEL_RISE * outCubic(rise)).toFixed(2) + 'px)';
    newEl.style.transform = 'translateY(' + (LABEL_RISE * (1 - outBack(rise))).toFixed(2) + 'px)';
    oldEl.style.opacity = String((1 - rise).toFixed(2));
    textEl.textContent = FORMATS[wantFmt];
  }

  function pick(i) {
    if (i === wantFmt) return;
    oldEl.textContent = FORMATS[shownFmt === wantFmt ? wantFmt : shownFmt];
    wantFmt = i;
    shownFmt = i;
    newEl.textContent = FORMATS[i];
    rise = reduce ? 1 : 0;
  }
  function setOpen(v) {
    wantOpen = v;
    if (reduce) open = v ? 1 : 0;
  }

  toggle.addEventListener('click', function () {
    stopDemo();
    setOpen(!wantOpen);
    render();
    if (wantOpen) items[wantFmt].focus({ preventScroll: true });
  });
  toggle.addEventListener('keydown', function (e) {
    if (e.key === 'ArrowDown' && !wantOpen) { e.preventDefault(); stopDemo(); setOpen(true); render(); items[wantFmt].focus({ preventScroll: true }); }
  });
  items.forEach(function (it, i) {
    it.addEventListener('click', function () { stopDemo(); pick(i); setOpen(false); render(); toggle.focus({ preventScroll: true }); });
    it.addEventListener('keydown', function (e) {
      if (e.key === 'ArrowRight' || e.key === 'ArrowDown') { e.preventDefault(); items[(i + 1) % items.length].focus({ preventScroll: true }); }
      if (e.key === 'ArrowLeft' || e.key === 'ArrowUp') { e.preventDefault(); items[(i + items.length - 1) % items.length].focus({ preventScroll: true }); }
      if (e.key === 'Escape') { e.preventDefault(); setOpen(false); render(); toggle.focus({ preventScroll: true }); }
    });
  });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    if (root.classList.contains('is-demo') && now >= START_MS) {
      var ph = (now - START_MS) % CYCLE_MS;
      wantOpen = ph < 800;
      var f = ph >= 600 && ph < 1700 ? 2 : 0;
      if (f !== wantFmt) pick(f);
    }
    open = clamp(open + (wantOpen ? 1 : -1) * dt * 1000 / OPEN_MS, 0, 1);
    rise = clamp(rise + dt * 1000 / RISE_MS, 0, 1);
    render();
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    render();
  } else {
    render();
    requestAnimationFrame(tick);
  }
})();

// SplitFormatButton — vanilla 판(07-dlbtn-format-split-menu) 의 React 포트.
// 메뉴 칸마다 시작 시각을 MENU_STAGGER 씩 늦춰 easeOutBack 으로 펼치고, 고른 형식은 라벨 창 안에서 LABEL_RISE 만큼 올라온다.
import { useEffect, useRef } from "react";
import styles from "./SplitFormatButton.module.scss";

const MENU_STAGGER = 70;
const FORMATS = ["PDF", "PNG", "SVG"];
const LABEL_RISE = 24;
const ITEM_MS = 280;
const OPEN_MS = ITEM_MS + (FORMATS.length - 1) * MENU_STAGGER;
const RISE_MS = 320;
const START_MS = 200;
const CYCLE_MS = 2000;

function clamp(v: number, lo: number, hi: number): number {
  return Math.max(lo, Math.min(hi, v));
}
function outBack(t: number): number {
  const c = 1.70158;
  return 1 + (c + 1) * Math.pow(t - 1, 3) + c * Math.pow(t - 1, 2);
}
function outCubic(t: number): number {
  return 1 - Math.pow(1 - t, 3);
}

export default function SplitFormatButton() {
  const toggleRef = useRef<HTMLButtonElement | null>(null);
  const menuRef = useRef<HTMLDivElement | null>(null);
  const oldRef = useRef<HTMLSpanElement | null>(null);
  const newRef = useRef<HTMLSpanElement | null>(null);
  const textRef = useRef<HTMLSpanElement | null>(null);

  useEffect(() => {
    const toggle = toggleRef.current;
    const menu = menuRef.current;
    const oldEl = oldRef.current;
    const newEl = newRef.current;
    const textEl = textRef.current;
    if (!toggle || !menu || !oldEl || !newEl || !textEl) return;
    const items = Array.from(menu.querySelectorAll<HTMLButtonElement>("button"));
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let wantOpen = false;
    let wantFmt = 0;
    let open = 0;
    let rise = 1;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const render = (): void => {
      const s = open * OPEN_MS;
      const visible = open > 0.001 || wantOpen;
      items.forEach((it, i) => {
        const q = outBack(clamp((s - i * MENU_STAGGER) / ITEM_MS, 0, 1));
        it.style.opacity = clamp(q, 0, 1).toFixed(3);
        it.style.transform = `translateY(${((1 - q) * -16).toFixed(2)}px) scale(${(0.8 + 0.2 * q).toFixed(3)})`;
        it.style.visibility = visible ? "visible" : "hidden";
        it.setAttribute("tabindex", wantOpen ? "0" : "-1");
        it.classList.toggle(styles["is-picked"], i === wantFmt);
      });
      menu.setAttribute("aria-hidden", wantOpen ? "false" : "true");
      toggle.setAttribute("aria-expanded", wantOpen ? "true" : "false");
      toggle.style.setProperty("--o", open.toFixed(3));
      toggle.parentElement!.style.setProperty("--k", (1 - rise).toFixed(3));
      oldEl.style.transform = `translateY(${(-LABEL_RISE * outCubic(rise)).toFixed(2)}px)`;
      newEl.style.transform = `translateY(${(LABEL_RISE * (1 - outBack(rise))).toFixed(2)}px)`;
      oldEl.style.opacity = (1 - rise).toFixed(2);
      textEl.textContent = FORMATS[wantFmt];
    };
    const pick = (i: number): void => {
      if (i === wantFmt) return;
      oldEl.textContent = FORMATS[wantFmt];
      wantFmt = i;
      newEl.textContent = FORMATS[i];
      rise = reduce ? 1 : 0;
    };
    const setOpen = (v: boolean): void => {
      wantOpen = v;
      if (reduce) open = v ? 1 : 0;
    };

    const onToggle = (): void => {
      demo = false;
      setOpen(!wantOpen);
      render();
      if (wantOpen) items[wantFmt].focus({ preventScroll: true });
    };
    const onToggleKey = (e: KeyboardEvent): void => {
      if (e.key === "ArrowDown" && !wantOpen) {
        e.preventDefault();
        demo = false;
        setOpen(true);
        render();
        items[wantFmt].focus({ preventScroll: true });
      }
    };
    toggle.addEventListener("click", onToggle);
    toggle.addEventListener("keydown", onToggleKey);
    const itemHandlers = items.map((it, i) => {
      const onClick = (): void => { demo = false; pick(i); setOpen(false); render(); toggle.focus({ preventScroll: true }); };
      const onKey = (e: KeyboardEvent): void => {
        if (e.key === "ArrowRight" || e.key === "ArrowDown") { e.preventDefault(); items[(i + 1) % items.length].focus({ preventScroll: true }); }
        if (e.key === "ArrowLeft" || e.key === "ArrowUp") { e.preventDefault(); items[(i + items.length - 1) % items.length].focus({ preventScroll: true }); }
        if (e.key === "Escape") { e.preventDefault(); setOpen(false); render(); toggle.focus({ preventScroll: true }); }
      };
      it.addEventListener("click", onClick);
      it.addEventListener("keydown", onKey);
      return { it, onClick, onKey };
    });

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      if (demo && now >= START_MS) {
        const ph = (now - START_MS) % CYCLE_MS;
        wantOpen = ph < 800;
        const f = ph >= 600 && ph < 1700 ? 2 : 0;
        if (f !== wantFmt) pick(f);
      }
      open = clamp(open + (wantOpen ? 1 : -1) * dt * 1000 / OPEN_MS, 0, 1);
      rise = clamp(rise + dt * 1000 / RISE_MS, 0, 1);
      render();
      raf = requestAnimationFrame(tick);
    }

    render();
    if (!reduce) raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      toggle.removeEventListener("click", onToggle);
      toggle.removeEventListener("keydown", onToggleKey);
      itemHandlers.forEach((h) => {
        h.it.removeEventListener("click", h.onClick);
        h.it.removeEventListener("keydown", h.onKey);
      });
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.sp}>
        <div className={styles.sp__row}>
          <button className={styles.sp__main} type="button">
            <span className={styles.sp__win} aria-hidden="true">
              <span className={styles.sp__fmt} ref={oldRef} />
              <span className={styles.sp__fmt} ref={newRef}>PDF</span>
            </span>
            <span className={styles.sp__sr} ref={textRef}>PDF</span>
            <span>내려받기</span>
          </button>
          <button className={styles.sp__toggle} type="button" aria-haspopup="menu" aria-expanded="false" aria-controls="spMenu" aria-label="형식 선택" ref={toggleRef}>
            <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M7 10l5 5 5-5" /></svg>
          </button>
        </div>
        <div className={styles.sp__menu} id="spMenu" role="menu" aria-label="형식 선택" ref={menuRef}>
          {FORMATS.map((f) => (
            <button className={styles.sp__item} type="button" role="menuitem" key={f}>{f}</button>
          ))}
        </div>
      </div>
    </main>
  );
}

<main class="stage">
<div class="rt is-demo" id="rtRoot">
  <div class="rt__wrap" id="rtBar" role="progressbar" aria-labelledby="rtLabel" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-valuetext="0%">
    <svg class="rt__svg" viewBox="0 0 232 96" aria-hidden="true">
      <rect class="rt__ring" id="rtRing" x="3" y="3" width="226" height="90" rx="16" pathLength="100"/>
    </svg>
    <button class="rt__btn" id="rtBtn" type="button">
      <svg class="rt__icon" viewBox="0 0 24 24" aria-hidden="true"><path id="rtGlyph" d="M12 4v11M6.5 10.5 12 16l5.5-5.5"/></svg>
      <span id="rtLabel">ダウンロード</span>
    </button>
  </div>
  <p class="rt__meta"><span class="rt__tag">X</span> <b id="rtX">0.0</b> <span class="rt__tag">P</span> <b id="rtP">0</b></p>
  <p class="rt__sr" id="rtStatus" role="status"></p>
</div>
</main>

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

// 핵심: 흔들림 진폭은 지수 감쇠 함수로, 링 되감기는 보간 곡선으로 JS 가 계산한다. 실패색(--e)은 글자와 아이콘으로도 구분된다.

$rt-face: $subject-lilac;
$rt-fail: $error-soft;

@include stage($stage-ink);

.rt {
  position: relative;
  width: min(100%, 280px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.rt__wrap {
  --e: 0;
  position: relative;
  width: 232px;
  max-width: 100%;
  aspect-ratio: 232 / 96;
}
.rt__svg {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
}
.rt__ring {
  fill: none;
  stroke: color-mix(in srgb, #{$subject-mint}, #{$tick-down} calc(var(--e) * 100%));
  stroke-width: 5;
  stroke-dasharray: 100 100;
  stroke-dashoffset: 100;
  stroke-linecap: round;
}

.rt__btn {
  position: absolute;
  left: 7%;
  right: 7%;
  top: 21%;
  bottom: 21%;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: $sp-3;
  border: 0;
  border-radius: $r-control;
  background: color-mix(in srgb, #{$rt-face}, #{$rt-fail} calc(var(--e) * 100%));
  color: $subject-ink;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.rt__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 4px; }
.rt__icon {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.rt__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: $stage-paper;
}
.rt__tag { font-weight: 700; letter-spacing: .08em; }
.rt__meta b { min-width: 4ch; text-align: right; font-variant-numeric: tabular-nums; }

.rt__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

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

.rt {
  position: relative;
  width: min(100%, 280px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.rt__wrap {
  --e: 0;
  position: relative;
  width: 232px;
  max-width: 100%;
  aspect-ratio: 232/96;
}

.rt__svg {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
}

.rt__ring {
  fill: none;
  stroke: color-mix(in srgb, #4cd4a6, #ff7a5e calc(var(--e) * 100%));
  stroke-width: 5;
  stroke-dasharray: 100 100;
  stroke-dashoffset: 100;
  stroke-linecap: round;
}

.rt__btn {
  position: absolute;
  left: 7%;
  right: 7%;
  top: 21%;
  bottom: 21%;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
  border: 0;
  border-radius: 12px;
  background: color-mix(in srgb, #b9a0ff, #ffe0d8 calc(var(--e) * 100%));
  color: #17141a;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.rt__btn:focus-visible {
  outline: 3px solid #ffffff;
  outline-offset: 4px;
}

.rt__icon {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.rt__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: #ffffff;
}

.rt__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.rt__meta b {
  min-width: 4ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

.rt__sr {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

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

(function () {
  // 기법: 失敗 순간부터 흔들림 x = SHAKE_AMP x e^(-SHAKE_DECAY x t) x sin(2 pi f t) 로 진폭이 지수로 줄고,
  // 링은 실패한 길이에서 처음으로 REWIND_MS 동안 되감긴다. 두 번째 시도는 끝까지 간다.
  var SHAKE_AMP = 14;
  var SHAKE_DECAY = 5;
  var REWIND_MS = 500;
  var SHAKE_HZ = 4.5;
  var FAIL_AT = 0.62;
  var RUN_MS = 700;
  var REWIND_DELAY = 150;
  var START_MS = 200;
  var CYCLE_MS = 2000;

  var root = document.getElementById('rtRoot');
  var bar = document.getElementById('rtBar');
  var btn = document.getElementById('rtBtn');
  var ring = document.getElementById('rtRing');
  var labelEl = document.getElementById('rtLabel');
  var glyph = document.getElementById('rtGlyph');
  var xEl = document.getElementById('rtX');
  var pEl = document.getElementById('rtP');
  var statusEl = document.getElementById('rtStatus');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var ARROW = 'M12 4v11M6.5 10.5 12 16l5.5-5.5';
  var BANG = 'M12 5v9M12 19v.5';
  var clock = 0;
  var runStart = null;
  var outcome = 'fail';
  var attempts = 0;
  var settled = null;
  var lastNote = '';
  var t0 = null;

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function stopDemo() { root.classList.remove('is-demo'); }
  function outQuad(t) { return 1 - (1 - t) * (1 - t); }
  function inOut(t) { return t < 0.5 ? 2 * t * t : 1 - Math.pow(-2 * t + 2, 2) / 2; }
  function busy(now) { return runStart !== null && now - runStart < RUN_MS + REWIND_DELAY + REWIND_MS; }

  function frame(now) {
    var s;
    var res = outcome;
    if (root.classList.contains('is-demo')) {
      s = now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS;
      res = 'fail';
    } else {
      s = runStart;
    }
    if (settled) return settled;
    if (s === null) return { p: 0, x: 0, e: 0, label: 'ダウンロード', glyph: ARROW, note: '' };
    var t = now - s;
    if (t < RUN_MS) {
      var u = outQuad(t / RUN_MS);
      return { p: res === 'fail' ? FAIL_AT * u : u, x: 0, e: 0, label: '受信中', glyph: ARROW, note: '' };
    }
    if (res === 'ok') return { p: 1, x: 0, e: 0, label: '完了', glyph: ARROW, note: 'ダウンロード完了' };
    var tau = (t - RUN_MS) / 1000;
    var rw = clamp((t - RUN_MS - REWIND_DELAY) / REWIND_MS, 0, 1);
    return {
      p: FAIL_AT * (1 - inOut(rw)),
      x: SHAKE_AMP * Math.exp(-SHAKE_DECAY * tau) * Math.sin(2 * Math.PI * SHAKE_HZ * tau),
      e: 1,
      label: rw >= 1 ? '再試行' : '失敗',
      glyph: BANG,
      note: 'ダウンロードが途切れました'
    };
  }

  function paint(f) {
    var pct = Math.round(f.p * 100);
    ring.style.strokeDashoffset = String((100 - f.p * 100).toFixed(2));
    ring.style.opacity = f.p < 0.004 ? '0' : '1';
    bar.style.setProperty('--e', String(f.e));
    bar.style.transform = 'translateX(' + f.x.toFixed(2) + 'px)';
    labelEl.textContent = f.label;
    glyph.setAttribute('d', f.glyph);
    xEl.textContent = f.x.toFixed(1);
    pEl.textContent = String(pct);
    bar.setAttribute('aria-valuenow', String(pct));
    bar.setAttribute('aria-valuetext', pct + '%');
    if (f.note !== lastNote) { lastNote = f.note; statusEl.textContent = f.note; }
  }

  btn.addEventListener('click', function () {
    stopDemo();
    if (busy(clock)) return;
    outcome = attempts % 2 === 0 ? 'fail' : 'ok';
    attempts += 1;
    if (reduce) {
      settled = outcome === 'ok'
        ? { p: 1, x: 0, e: 0, label: '完了', glyph: ARROW, note: 'ダウンロード完了' }
        : { p: 0, x: 0, e: 1, label: '再試行', glyph: BANG, note: 'ダウンロードが途切れました' };
      paint(settled);
      settled = null;
      runStart = null;
      return;
    }
    runStart = clock;
  });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    clock = ts - t0;
    paint(frame(clock));
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    paint(frame(0));
  } else {
    requestAnimationFrame(tick);
  }
})();

// RetryDecayButton — vanilla 판(08-dlbtn-error-retry-decay) 의 React 포트.
// 실패 순간부터 흔들림 진폭이 지수로 줄고, 링은 실패한 길이에서 처음으로 REWIND_MS 동안 되감긴다. 두 번째 시도는 끝까지 간다.
import { useEffect, useRef } from "react";
import styles from "./RetryDecayButton.module.scss";

const SHAKE_AMP = 14;
const SHAKE_DECAY = 5;
const REWIND_MS = 500;
const SHAKE_HZ = 4.5;
const FAIL_AT = 0.62;
const RUN_MS = 700;
const REWIND_DELAY = 150;
const START_MS = 200;
const CYCLE_MS = 2000;
const ARROW = "M12 4v11M6.5 10.5 12 16l5.5-5.5";
const BANG = "M12 5v9M12 19v.5";

type Frame = { p: number; x: number; e: number; label: string; glyph: string; note: string };
type Outcome = "fail" | "ok";

function clamp(v: number, lo: number, hi: number): number {
  return Math.max(lo, Math.min(hi, v));
}
function outQuad(t: number): number {
  return 1 - (1 - t) * (1 - t);
}
function inOut(t: number): number {
  return t < 0.5 ? 2 * t * t : 1 - Math.pow(-2 * t + 2, 2) / 2;
}

export default function RetryDecayButton() {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const barRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const ringRef = useRef<SVGRectElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const glyphRef = useRef<SVGPathElement | null>(null);
  const xRef = useRef<HTMLElement | null>(null);
  const pRef = useRef<HTMLElement | null>(null);
  const statusRef = useRef<HTMLParagraphElement | null>(null);

  useEffect(() => {
    const bar = barRef.current;
    const btn = btnRef.current;
    const ring = ringRef.current;
    const labelEl = labelRef.current;
    const glyph = glyphRef.current;
    const xEl = xRef.current;
    const pEl = pRef.current;
    const statusEl = statusRef.current;
    if (!bar || !btn || !ring || !labelEl || !glyph || !xEl || !pEl || !statusEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let clock = 0;
    let runStart: number | null = null;
    let outcome: Outcome = "fail";
    let attempts = 0;
    let lastNote = "";
    let t0: number | null = null;
    let raf = 0;

    const busy = (now: number): boolean =>
      runStart !== null && now - runStart < RUN_MS + REWIND_DELAY + REWIND_MS;

    const frame = (now: number): Frame => {
      let s: number | null;
      let res: Outcome = outcome;
      if (demo) {
        s = now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS;
        res = "fail";
      } else {
        s = runStart;
      }
      if (s === null) return { p: 0, x: 0, e: 0, label: "내려받기", glyph: ARROW, note: "" };
      const t = now - s;
      if (t < RUN_MS) {
        const u = outQuad(t / RUN_MS);
        return { p: res === "fail" ? FAIL_AT * u : u, x: 0, e: 0, label: "받는 중", glyph: ARROW, note: "" };
      }
      if (res === "ok") return { p: 1, x: 0, e: 0, label: "완료", glyph: ARROW, note: "받기 완료" };
      const tau = (t - RUN_MS) / 1000;
      const rw = clamp((t - RUN_MS - REWIND_DELAY) / REWIND_MS, 0, 1);
      return {
        p: FAIL_AT * (1 - inOut(rw)),
        x: SHAKE_AMP * Math.exp(-SHAKE_DECAY * tau) * Math.sin(2 * Math.PI * SHAKE_HZ * tau),
        e: 1,
        label: rw >= 1 ? "다시 시도" : "실패",
        glyph: BANG,
        note: "받기가 끊겼어요"
      };
    };
    const paint = (f: Frame): void => {
      const pct = Math.round(f.p * 100);
      ring.style.strokeDashoffset = (100 - f.p * 100).toFixed(2);
      ring.style.opacity = f.p < 0.004 ? "0" : "1";
      bar.style.setProperty("--e", String(f.e));
      bar.style.transform = `translateX(${f.x.toFixed(2)}px)`;
      labelEl.textContent = f.label;
      glyph.setAttribute("d", f.glyph);
      xEl.textContent = f.x.toFixed(1);
      pEl.textContent = String(pct);
      bar.setAttribute("aria-valuenow", String(pct));
      bar.setAttribute("aria-valuetext", pct + "%");
      if (f.note !== lastNote) { lastNote = f.note; statusEl.textContent = f.note; }
    };
    const onClick = (): void => {
      demo = false;
      if (busy(clock)) return;
      outcome = attempts % 2 === 0 ? "fail" : "ok";
      attempts += 1;
      if (reduce) {
        paint(outcome === "ok"
          ? { p: 1, x: 0, e: 0, label: "완료", glyph: ARROW, note: "받기 완료" }
          : { p: 0, x: 0, e: 1, label: "다시 시도", glyph: BANG, note: "받기가 끊겼어요" });
        runStart = null;
        return;
      }
      runStart = clock;
    };
    btn.addEventListener("click", onClick);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      clock = ts - t0;
      paint(frame(clock));
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint(frame(0));
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", onClick);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.rt} ref={rootRef}>
        <div className={styles.rt__wrap} ref={barRef} role="progressbar" aria-labelledby="rtLabel" aria-valuemin={0} aria-valuemax={100} aria-valuenow={0} aria-valuetext="0%">
          <svg className={styles.rt__svg} viewBox="0 0 232 96" aria-hidden="true">
            <rect className={styles.rt__ring} ref={ringRef} x="3" y="3" width="226" height="90" rx="16" pathLength="100" />
          </svg>
          <button className={styles.rt__btn} type="button" ref={btnRef}>
            <svg className={styles.rt__icon} viewBox="0 0 24 24" aria-hidden="true"><path d={ARROW} ref={glyphRef} /></svg>
            <span id="rtLabel" ref={labelRef}>내려받기</span>
          </button>
        </div>
        <p className={styles.rt__meta}>
          <span className={styles.rt__tag}>X</span> <b ref={xRef}>0.0</b> <span className={styles.rt__tag}>P</span> <b ref={pRef}>0</b>
        </p>
        <p className={styles.rt__sr} role="status" ref={statusRef} />
      </div>
    </main>
  );
}

<main class="stage">
<div class="mg is-demo" id="mgRoot">
  <button class="mg__btn" id="mgBtn" type="button">
    <span class="mg__sheen" id="mgSheen" aria-hidden="true"></span>
    <svg class="mg__icon" viewBox="0 0 24 24" aria-hidden="true"><path d="M12 4v11M6.5 10.5 12 16l5.5-5.5M5 20h14"/></svg>
    <span class="mg__text">ダウンロード</span>
  </button>
</div>
</main>

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

// 핵심: 버튼 이동량과 광택 위치는 커서와의 거리에서 JS 가 계산한다. 광택 띠와 번쩍임 중심(--gx --gy)이 같은 커서 값을 쓴다.

$mg-face: $subject-lilac;

@include stage($stage-orange);

.mg {
  width: min(100%, 280px);
  height: 144px;
  display: grid;
  place-items: center;
}

.mg__btn {
  --gx: 50%;
  --gy: 50%;
  --ga: 0;
  position: relative;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 72px;
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: $sp-3;
  border: 0;
  border-radius: $r-control;
  background: radial-gradient(circle at var(--gx) var(--gy), rgb(255 255 255 / var(--ga)), rgb(255 255 255 / 0) 60%), $mg-face;
  box-shadow: $shadow-raised;
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.mg__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.mg__sheen {
  position: absolute;
  left: 0;
  top: -10%;
  height: 120%;
  background: linear-gradient(90deg, rgba($stage-paper, 0), rgba($stage-paper, .7), rgba($stage-paper, 0));
  opacity: 0;
  pointer-events: none;
}
.mg__icon {
  position: relative;
  width: 28px;
  height: 28px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}
.mg__text { position: relative; }

@media (prefers-reduced-motion: reduce) {
  .mg__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: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.mg {
  width: min(100%, 280px);
  height: 144px;
  display: grid;
  place-items: center;
}

.mg__btn {
  --gx: 50%;
  --gy: 50%;
  --ga: 0;
  position: relative;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 72px;
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
  border: 0;
  border-radius: 12px;
  background: radial-gradient(circle at var(--gx) var(--gy), rgb(255, 255, 255, var(--ga)), rgba(255, 255, 255, 0) 60%), #b9a0ff;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.mg__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.mg__sheen {
  position: absolute;
  left: 0;
  top: -10%;
  height: 120%;
  background: linear-gradient(90deg, rgba(255, 255, 255, 0), rgba(255, 255, 255, 0.7), rgba(255, 255, 255, 0));
  opacity: 0;
  pointer-events: none;
}

.mg__icon {
  position: relative;
  width: 28px;
  height: 28px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2.5;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.mg__text {
  position: relative;
}

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

(function () {
  // 기법: 커서와 버튼 중심의 거리 x PULL_RATE 만큼 버튼이 끌려가고 (LEAN_MAX 에서 멈춤), 광택 띠는 커서의 가로 위치를 따라 지수 보간으로 쫓아간다.
  // 포인터가 없는 데모에서는 같은 계산에 궤도 위 가짜 커서를 넣는다.
  var PULL_RATE = 0.12;
  var SHEEN_W = 64;
  var LEAN_MAX = 10;
  var LEAN_LERP = 10;
  var SHEEN_LERP = 14;
  var START_MS = 200;
  var CYCLE_MS = 2000;
  var ORBIT_X = 150;
  var ORBIT_Y = 48;

  var root = document.getElementById('mgRoot');
  var btn = document.getElementById('mgBtn');
  var sheen = document.getElementById('mgSheen');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var lx = 0;
  var ly = 0;
  var sx = -SHEEN_W * 2;
  var alpha = 0;
  var cursor = null;
  var kbFocus = false;
  var t0 = null;
  var lastTs = null;

  sheen.style.width = SHEEN_W + 'px';

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function stopDemo() { root.classList.remove('is-demo'); }

  function goal(now) {
    var w = btn.offsetWidth;
    var h = btn.offsetHeight;
    var d = null;
    if (root.classList.contains('is-demo') && now >= START_MS) {
      var a = 2 * Math.PI * ((now - START_MS) % CYCLE_MS) / CYCLE_MS;
      d = { x: ORBIT_X * Math.cos(a), y: ORBIT_Y * Math.sin(a) };
    } else if (cursor) {
      d = cursor;
    }
    if (kbFocus && !d) return { lx: 0, ly: 0, sx: w / 2, a: 1, gx: 50, gy: 50 };
    if (!d) return { lx: 0, ly: 0, sx: -SHEEN_W * 2, a: 0, gx: 50, gy: 50 };
    return {
      lx: clamp(d.x * PULL_RATE, -LEAN_MAX, LEAN_MAX),
      ly: clamp(d.y * PULL_RATE, -LEAN_MAX, LEAN_MAX),
      sx: clamp(d.x + w / 2, -SHEEN_W, w + SHEEN_W),
      a: 1,
      gx: clamp((d.x + w / 2) / w * 100, -20, 120),
      gy: clamp((d.y + h / 2) / h * 100, -40, 140)
    };
  }

  function render(g) {
    btn.style.transform = 'translate(' + lx.toFixed(2) + 'px,' + ly.toFixed(2) + 'px)';
    sheen.style.transform = 'translateX(' + (sx - SHEEN_W / 2).toFixed(1) + 'px) skewX(-20deg)';
    sheen.style.opacity = alpha.toFixed(3);
    btn.style.setProperty('--gx', g.gx.toFixed(1) + '%');
    btn.style.setProperty('--gy', g.gy.toFixed(1) + '%');
    btn.style.setProperty('--ga', (alpha * 0.55).toFixed(3));
  }

  function snap() {
    var g = goal(0);
    lx = g.lx; ly = g.ly; sx = g.sx; alpha = g.a;
    render(g);
  }

  root.addEventListener('pointermove', function (e) {
    var r = btn.getBoundingClientRect();
    stopDemo();
    cursor = { x: e.clientX - (r.left - lx) - r.width / 2, y: e.clientY - (r.top - ly) - r.height / 2 };
    if (reduce) snap();
  });
  root.addEventListener('pointerleave', function () { cursor = null; if (reduce) snap(); });
  btn.addEventListener('focus', function () { kbFocus = btn.matches(':focus-visible'); if (kbFocus) stopDemo(); if (reduce) snap(); });
  btn.addEventListener('blur', function () { kbFocus = false; if (reduce) snap(); });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    var g = goal(now);
    var kl = 1 - Math.exp(-dt * LEAN_LERP);
    var ks = 1 - Math.exp(-dt * SHEEN_LERP);
    lx += (g.lx - lx) * kl;
    ly += (g.ly - ly) * kl;
    sx += (g.sx - sx) * ks;
    alpha += (g.a - alpha) * ks;
    render(g);
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    snap();
  } else {
    requestAnimationFrame(tick);
  }
})();

// MagnetSheenButton — vanilla 판(09-dlbtn-magnet-sheen-hover) 의 React 포트.
// 커서와 버튼 중심의 거리 x PULL_RATE 만큼 버튼이 끌려가고(LEAN_MAX 에서 멈춤), 광택 띠는 커서의 가로 위치를 지수 보간으로 쫓는다.
import { useEffect, useRef } from "react";
import styles from "./MagnetSheenButton.module.scss";

const PULL_RATE = 0.12;
const SHEEN_W = 64;
const LEAN_MAX = 10;
const LEAN_LERP = 10;
const SHEEN_LERP = 14;
const START_MS = 200;
const CYCLE_MS = 2000;
const ORBIT_X = 150;
const ORBIT_Y = 48;

type Point = { x: number; y: number };
type Goal = { lx: number; ly: number; sx: number; a: number; gx: number; gy: number };

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

export default function MagnetSheenButton() {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const sheenRef = useRef<HTMLSpanElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    const btn = btnRef.current;
    const sheen = sheenRef.current;
    if (!root || !btn || !sheen) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let lx = 0;
    let ly = 0;
    let sx = -SHEEN_W * 2;
    let alpha = 0;
    let cursor: Point | null = null;
    let kbFocus = false;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    sheen.style.width = SHEEN_W + "px";

    const goal = (now: number): Goal => {
      const w = btn.offsetWidth;
      const h = btn.offsetHeight;
      let d: Point | null = null;
      if (demo && now >= START_MS) {
        const a = 2 * Math.PI * ((now - START_MS) % CYCLE_MS) / CYCLE_MS;
        d = { x: ORBIT_X * Math.cos(a), y: ORBIT_Y * Math.sin(a) };
      } else if (cursor) {
        d = cursor;
      }
      if (kbFocus && !d) return { lx: 0, ly: 0, sx: w / 2, a: 1, gx: 50, gy: 50 };
      if (!d) return { lx: 0, ly: 0, sx: -SHEEN_W * 2, a: 0, gx: 50, gy: 50 };
      return {
        lx: clamp(d.x * PULL_RATE, -LEAN_MAX, LEAN_MAX),
        ly: clamp(d.y * PULL_RATE, -LEAN_MAX, LEAN_MAX),
        sx: clamp(d.x + w / 2, -SHEEN_W, w + SHEEN_W),
        a: 1,
        gx: clamp((d.x + w / 2) / w * 100, -20, 120),
        gy: clamp((d.y + h / 2) / h * 100, -40, 140)
      };
    };
    const render = (g: Goal): void => {
      btn.style.transform = `translate(${lx.toFixed(2)}px,${ly.toFixed(2)}px)`;
      sheen.style.transform = `translateX(${(sx - SHEEN_W / 2).toFixed(1)}px) skewX(-20deg)`;
      sheen.style.opacity = alpha.toFixed(3);
      btn.style.setProperty("--gx", g.gx.toFixed(1) + "%");
      btn.style.setProperty("--gy", g.gy.toFixed(1) + "%");
      btn.style.setProperty("--ga", (alpha * 0.55).toFixed(3));
    };
    const snap = (): void => {
      const g = goal(0);
      lx = g.lx; ly = g.ly; sx = g.sx; alpha = g.a;
      render(g);
    };

    const onMove = (e: PointerEvent): void => {
      const r = btn.getBoundingClientRect();
      demo = false;
      cursor = { x: e.clientX - (r.left - lx) - r.width / 2, y: e.clientY - (r.top - ly) - r.height / 2 };
      if (reduce) snap();
    };
    const onLeave = (): void => { cursor = null; if (reduce) snap(); };
    const onFocus = (): void => { kbFocus = btn.matches(":focus-visible"); if (kbFocus) demo = false; if (reduce) snap(); };
    const onBlur = (): void => { kbFocus = false; if (reduce) snap(); };
    root.addEventListener("pointermove", onMove);
    root.addEventListener("pointerleave", onLeave);
    btn.addEventListener("focus", onFocus);
    btn.addEventListener("blur", onBlur);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      const g = goal(now);
      const kl = 1 - Math.exp(-dt * LEAN_LERP);
      const ks = 1 - Math.exp(-dt * SHEEN_LERP);
      lx += (g.lx - lx) * kl;
      ly += (g.ly - ly) * kl;
      sx += (g.sx - sx) * ks;
      alpha += (g.a - alpha) * ks;
      render(g);
      raf = requestAnimationFrame(tick);
    }

    if (reduce) snap();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      root.removeEventListener("pointermove", onMove);
      root.removeEventListener("pointerleave", onLeave);
      btn.removeEventListener("focus", onFocus);
      btn.removeEventListener("blur", onBlur);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.mg} ref={rootRef}>
        <button className={styles.mg__btn} type="button" ref={btnRef}>
          <span className={styles.mg__sheen} aria-hidden="true" ref={sheenRef} />
          <svg className={styles.mg__icon} viewBox="0 0 24 24" aria-hidden="true">
            <path d="M12 4v11M6.5 10.5 12 16l5.5-5.5M5 20h14" />
          </svg>
          <span className={styles.mg__text}>내려받기</span>
        </button>
      </div>
    </main>
  );
}