06선 길이로 화살표가 체크로 바뀌는 완료

<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">내려받기</span>
  </button>
  <span class="ck__ruler" id="ckRulerA" aria-hidden="true">내려받기</span>
  <span class="ck__ruler" id="ckRulerB" aria-hidden="true">완료</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 ? '완료' : '내려받기';
    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 = '받기 완료'; }
    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>
  );
}