02Flick to angular velocity — turning pointer travel into spin

<main class="stage">
<div class="fw" id="fw">
  <p class="fw__hint">Each pointermove measures the center angle with atan2; delta over time yields rad/s</p>
  <div class="fw__row">
    <div class="fw__field" id="field" role="application" aria-label="Flick velocity lab" tabindex="0">
      <canvas class="fw__cv" id="cv"></canvas>
    </div>
    <div class="fw__side">
      <b class="fw__vel" id="vel">0.0</b>
      <span class="fw__unit">rad/s — the value at release becomes the initial spin</span>
      <div class="fw__bar" id="bar" aria-hidden="true"></div>
      <p class="fw__meta">Flick strength <b class="fw__count" id="pow">0</b><span class="fw__metaUnit">levels</span></p>
      <p class="fw__meta">Drag near the rim and let go</p>
    </div>
  </div>
</div>
</main>

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

// 튕김 각속도(02) — 움직임은 전부 캔버스 rAF 루프다. CSS @keyframes 를 두지 않는다.
// 무대는 노랑 무대의 파스텔 변형 — 무대 노랑 토큰(채도 1.0) 대신 같은 색상의 연한
// 버터 톤(채도 .48 · 명도 .80)을 쓴다(BD26). 칸 다섯 색도 채도 .40 이하 파스텔.

$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-blue;

@include stage(#f2e4bd);

.fw {
  --w1: #b3bfd4;   // 파랑빛 파스텔 — 1번 칸
  --w2: #d8a3b4;   // 로즈 파스텔 — 2번 칸
  --w3: #a9c4b0;   // 세이지 파스텔 — 3번 칸
  --w4: #d9c48f;   // 골드 파스텔 — 4번 칸
  --w5: #c2a8d1;   // 플럼 파스텔 — 5번 칸
  --ink: #{$subject-ink};
  --cream: #{$subject-cream};

  display: flex; flex-direction: column; gap: $sp-2;
  width: 100%; max-width: 420px;
}

.fw__hint {
  margin: 0; font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: $subject-ink;
}

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

.fw__field {
  position: relative; overflow: hidden;
  flex: 0 0 auto;
  width: clamp(176px, 46vw, 232px);
  height: clamp(176px, 46vw, 232px);
  border-radius: $r-card;
  background: rgba($subject-ink, .05);
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .16);
  cursor: grab; touch-action: none;
}
.fw__field:active { cursor: grabbing; }
.fw__field:focus-visible { outline: 2px solid $color; outline-offset: 2px; }

.fw__cv { display: block; width: 100%; height: 100%; }

.fw__side {
  flex: 1; min-width: 0;
  display: flex; flex-direction: column; gap: $sp-1;
  padding-top: $sp-1;
}
.fw__vel {
  font-weight: 800; font-size: clamp(20px, 6vw, 30px); line-height: 1;
  color: $subject-blue-deep; font-variant-numeric: tabular-nums;
}
.fw__unit { font-weight: 600; font-size: clamp(9px, 2.4vw, 11px); line-height: 1.4; color: rgba($subject-ink, .82); }

.fw__bar { display: flex; gap: $sp-1; margin-top: $sp-1; }
.fw__seg {
  width: clamp(12px, 3.4vw, 20px); height: 12px;
  border-radius: $r-xs;
  background: rgba($subject-ink, .14);
}
.fw__seg.is-on { background: $subject-blue-deep; }

.fw__meta { margin: $sp-1 0 0; font-size: clamp(9px, 2.4vw, 11px); line-height: 1.5; color: rgba($subject-ink, .82); }
.fw__meta b { font-weight: 700; color: $subject-blue-deep; font-variant-numeric: tabular-nums; margin-right: $sp-1; }
.fw__metaUnit { color: rgba($subject-ink, .72); }

@media (max-width: 340px) {
  .fw__hint { font-size: 8px; line-height: 1.3; }
  .fw { gap: $sp-1; }
  .fw__field { width: 150px; height: 150px; }
  .fw__vel { font-size: 16px; }
  .fw__meta { margin-top: 0; font-size: 8px; }
  .fw__seg { width: 10px; height: 8px; }
}

// reduce — 루프가 안 돈다. 자취 표시가 남은 정지 한 장이 남는다.
@media (prefers-reduced-motion: reduce) {
  .fw.is-still .fw__field { cursor: default !important; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #f2e4bd;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.fw {
  --w1: #b3bfd4;
  --w2: #d8a3b4;
  --w3: #a9c4b0;
  --w4: #d9c48f;
  --w5: #c2a8d1;
  --ink: #17141a;
  --cream: #fff7e6;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 420px;
}

.fw__hint {
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: #17141a;
}

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

.fw__field {
  position: relative;
  overflow: hidden;
  flex: 0 0 auto;
  width: clamp(176px, 46vw, 232px);
  height: clamp(176px, 46vw, 232px);
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.05);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: grab;
  touch-action: none;
}

.fw__field:active {
  cursor: grabbing;
}

.fw__field:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

.fw__cv {
  display: block;
  width: 100%;
  height: 100%;
}

.fw__side {
  flex: 1;
  min-width: 0;
  display: flex;
  flex-direction: column;
  gap: 4px;
  padding-top: 4px;
}

.fw__vel {
  font-weight: 800;
  font-size: clamp(20px, 6vw, 30px);
  line-height: 1;
  color: #0a4fe8;
  font-variant-numeric: tabular-nums;
}

.fw__unit {
  font-weight: 600;
  font-size: clamp(9px, 2.4vw, 11px);
  line-height: 1.4;
  color: rgba(23, 20, 26, 0.82);
}

.fw__bar {
  display: flex;
  gap: 4px;
  margin-top: 4px;
}

.fw__seg {
  width: clamp(12px, 3.4vw, 20px);
  height: 12px;
  border-radius: 4px;
  background: rgba(23, 20, 26, 0.14);
}

.fw__seg.is-on {
  background: #0a4fe8;
}

.fw__meta {
  margin: 4px 0 0;
  font-size: clamp(9px, 2.4vw, 11px);
  line-height: 1.5;
  color: rgba(23, 20, 26, 0.82);
}

.fw__meta b {
  font-weight: 700;
  color: #0a4fe8;
  font-variant-numeric: tabular-nums;
  margin-right: 4px;
}

.fw__metaUnit {
  color: rgba(23, 20, 26, 0.72);
}

@media (max-width: 340px) {
  .fw__hint {
    font-size: 8px;
    line-height: 1.3;
  }
  .fw {
    gap: 4px;
  }
  .fw__field {
    width: 150px;
    height: 150px;
  }
  .fw__vel {
    font-size: 16px;
  }
  .fw__meta {
    margin-top: 0;
    font-size: 8px;
  }
  .fw__seg {
    width: 10px;
    height: 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fw.is-still .fw__field {
    cursor: default !important;
  }
}

// 튕김 각속도(02) — 포인터 이동을 회전 속도로 바꾼다. pointermove 마다 중심에서의 각도를
// Math.atan2 로 재고 이전 각도와의 차에 경과 시간을 나누면 rad/s 각속도가 나온다.
// 손을 뗀 순간의 값이 초기 회전 속도가 되어 튕긴 세기 그대로 판이 돈다.
(function () {
  var root = document.getElementById('fw');
  var field = document.getElementById('field');
  var cv = document.getElementById('cv');
  var velEl = document.getElementById('vel');
  var barEl = document.getElementById('bar');
  var powEl = document.getElementById('pow');
  if (!root || !field || !cv || !velEl || !barEl || !powEl) return;
  var ctx = cv.getContext('2d');
  var css = getComputedStyle(root);
  var PAL = [];
  for (var i = 1; i <= 5; i++) PAL.push(css.getPropertyValue('--w' + i).trim());
  var INK = css.getPropertyValue('--ink').trim();
  var CREAM = css.getPropertyValue('--cream').trim();
  var TAU = Math.PI * 2;
  var N = 5, SEG = TAU / N;

  var ang = -1.1, vel = 0;
  var FRICTION = 0.26, STOP = 0.5;
  var FLICKS = [10.2, -15.8, 6.8];   // 자동 시연 튕김 — 고정값이라 항상 같은 시연이 반복된다
  var fi = 1, markAng = -1.1, markOp = 0, idle = 0, dpr = 1;

  function fit() {
    var w = field.clientWidth, h = field.clientHeight;
    dpr = Math.min(2, window.devicePixelRatio || 1);
    cv.width = Math.round(w * dpr); cv.height = Math.round(h * dpr);
  }

  function draw(peak) {
    var w = cv.width / dpr, h = cv.height / dpr;
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    ctx.clearRect(0, 0, w, h);
    var cx = w / 2, cy = h / 2, R = Math.min(w, h) / 2 - 10;
    ctx.save();
    ctx.translate(cx, cy);
    ctx.rotate(ang);
    for (var k = 0; k < N; k++) {
      var a0 = k * SEG;
      ctx.beginPath(); ctx.moveTo(0, 0);
      ctx.arc(0, 0, R, a0, a0 + SEG); ctx.closePath();
      ctx.fillStyle = PAL[k]; ctx.fill();
      ctx.strokeStyle = 'rgba(23,20,26,.20)'; ctx.lineWidth = 1; ctx.stroke();
      ctx.save();
      ctx.rotate(a0 + SEG / 2);
      ctx.fillStyle = INK;
      ctx.font = '700 13px sans-serif';
      ctx.textAlign = 'right'; ctx.textBaseline = 'middle';
      ctx.fillText(String(k + 1), R - 14, 0);
      ctx.restore();
    }
    ctx.beginPath(); ctx.arc(0, 0, 13, 0, TAU);
    ctx.fillStyle = CREAM; ctx.fill();
    ctx.strokeStyle = 'rgba(23,20,26,.35)'; ctx.stroke();
    ctx.restore();
    // 12시 고정 포인터
    ctx.beginPath();
    ctx.moveTo(cx - 7, cy - R - 8); ctx.lineTo(cx + 7, cy - R - 8); ctx.lineTo(cx, cy - R + 8);
    ctx.closePath();
    ctx.fillStyle = INK; ctx.fill();
    // 손 뗀 순간의 각도 자취 — 튕김의 시작점을 허브에서 표시한다
    if (markOp > 0) {
      ctx.beginPath();
      ctx.arc(cx, cy, 20, markAng - 0.5, markAng + 0.5);
      ctx.strokeStyle = 'rgba(23,20,26,' + markOp * 0.7 + ')';
      ctx.lineWidth = 4; ctx.stroke();
    }
  }

  var drag = { on: false, lastA: 0, lastT: 0, v: 0 };
  function angleAt(e) {
    var b = field.getBoundingClientRect();
    return Math.atan2(e.clientY - (b.top + b.height / 2), e.clientX - (b.left + b.width / 2));
  }
  field.addEventListener('pointerdown', function (e) {
    drag.on = true; drag.lastA = angleAt(e); drag.lastT = performance.now(); drag.v = 0;
    if (field.setPointerCapture) { try { field.setPointerCapture(e.pointerId); } catch (err) { /* 캡처 실패는 회전에 영향 없다 */ } }
  });
  field.addEventListener('pointermove', function (e) {
    if (!drag.on) return;
    var a = angleAt(e), now = performance.now();
    var dA = a - drag.lastA;
    if (dA > Math.PI) dA -= TAU;
    if (dA < -Math.PI) dA += TAU;
    ang += dA;
    var dt = Math.max(1, now - drag.lastT) / 1000;
    drag.v = 0.65 * drag.v + 0.35 * (dA / dt);
    drag.lastA = a; drag.lastT = now;
    velEl.textContent = (drag.v).toFixed(1);
    idle = 1.2;
  });
  function endDrag() {
    if (!drag.on) return;
    drag.on = false;
    vel = Math.max(-24, Math.min(24, drag.v));
    markAng = ang; markOp = 1;
    powEl.textContent = String(Math.min(5, Math.max(1, Math.ceil(Math.abs(vel) / 4.8))));
  }
  field.addEventListener('pointerup', endDrag);
  field.addEventListener('pointercancel', endDrag);
  field.addEventListener('keydown', function (e) {
    if (e.key === ' ' || e.key === 'Enter') {
      e.preventDefault();
      vel = FLICKS[fi++ % FLICKS.length];
      markAng = ang; markOp = 1; idle = 0;
      powEl.textContent = String(Math.min(5, Math.max(1, Math.ceil(Math.abs(vel) / 4.8))));
    }
  });

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var last = performance.now();
  function step(now) {
    var dt = Math.min(64, now - last) / 1000; last = now;
    if (!drag.on) {
      if (vel !== 0) {
        ang += vel * dt;
        vel *= Math.pow(FRICTION, dt);
        velEl.textContent = vel.toFixed(1);
        if (Math.abs(vel) < STOP) { vel = 0; velEl.textContent = '0.0'; }
      } else if (idle > 0) { idle -= dt; }
      else {
        vel = FLICKS[fi++ % FLICKS.length];
        markAng = ang; markOp = 1;
        powEl.textContent = String(Math.min(5, Math.max(1, Math.ceil(Math.abs(vel) / 4.8))));
      }
    }
    if (markOp > 0) markOp = Math.max(0, markOp - dt * 0.9);
    draw(markOp);
    // 속도 바 — 5단계로 채운다 (레이아웃이 아니라 transform 이다)
    var lv = Math.min(5, Math.ceil(Math.abs(drag.on ? drag.v : vel) / 4.8));
    for (var q = 0; q < barEl.children.length; q++) {
      barEl.children[q].className = q < lv ? 'fw__seg is-on' : 'fw__seg';
    }
    requestAnimationFrame(step);
  }

  fit();
  for (var s = 0; s < 5; s++) {
    var seg = document.createElement('span');
    seg.className = 'fw__seg';
    barEl.appendChild(seg);
  }
  window.addEventListener('resize', fit);
  if (reduce) {
    // 정지 한 장 — 자취 표시가 진행 중인 상태로 고정하고 루프는 돌지 않는다
    markOp = 0.5;
    draw(0.5);
    for (var z = 0; z < barEl.children.length; z++) barEl.children[z].className = z < 2 ? 'fw__seg is-on' : 'fw__seg';
    root.classList.add('is-still');
  } else {
    vel = FLICKS[0]; markAng = ang; markOp = 1;
    powEl.textContent = '2';
    requestAnimationFrame(step);
  }
})();

// SpinWheelFlickVelocity — vanilla 판(run/332/demo/02-spin-wheel-flick-velocity/index.html) 의 React 포트.
// pointermove 마다 중심에서의 각도를 Math.atan2 로 재고 이전 각도와의 차에 경과 시간을 나누면
// rad/s 각속도가 나온다. 손을 뗀 순간의 값이 초기 회전 속도가 되어 튕긴 세기 그대로 판이 돈다.
// 자동 시연 튕김(FLICKS)은 시드 없는 고정값 — 항상 같은 계열이 반복된다. 값·수치는 vanilla 와 같다.
// 움직임은 전부 캔버스 rAF 루프가 그리므로 CSS 애니메이션이 없다.
// reduce 에서는 루프를 돌리지 않고 자취 표시가 남은 정지 한 장만 남긴다(.isStill).
import { useEffect, useRef, useState } from "react";
import type { CSSProperties, PointerEvent } from "react";
import styles from "./SpinWheelFlickVelocity.module.scss";

// demo.py 의 ambient react 스텁은 KeyboardEvent 를 export 하지 않는다 — 최소 자체 타입으로 대체
type KeyEvt = { key: string; preventDefault(): void };

export type SpinWheelFlickVelocityProps = {
  duration?: string;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };

const TAU = Math.PI * 2;
const N = 5, SEG = TAU / N;
const FRICTION = 0.26, STOP = 0.5;
const FLICKS = [10.2, -15.8, 6.8];   // 자동 시연 튕김 — 고정값이라 항상 같은 시연이 반복된다
const PAL = ["#b3bfd4", "#d8a3b4", "#a9c4b0", "#d9c48f", "#c2a8d1"];
const INK = "#17141a";
const CREAM = "#fff7e6";
const HINT = "pointermove 마다 atan2 로 중심각을 재고 이전 각도·시간과의 차로 rad/s 를 만듭니다";
const ARIA_FIELD = "튕김 각속도 실험판";
const UNIT = "rad/s 손을 뗀 순간의 값이 초기 속도";
const POW_LABEL = "튕김 세기 ";
const POW_UNIT = "단계";
const META_DRAG = "가장자리를 드래그해 돌리고 놓아 보세요";

export default function SpinWheelFlickVelocity({
  duration = "2s",
  easing = "ease-out",
  color = "#2f6df6"
}: SpinWheelFlickVelocityProps) {
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const cvRef = useRef<HTMLCanvasElement | null>(null);
  const velRef = useRef<HTMLSpanElement | null>(null);
  const powRef = useRef<HTMLSpanElement | null>(null);
  const barRef = useRef<HTMLDivElement | null>(null);
  const ctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const stRef = useRef({
    ang: -1.1, vel: 0, fi: 1, markAng: -1.1, markOp: 0, idle: 0, dpr: 1,
    drag: { on: false, lastA: 0, lastT: 0, v: 0 }
  }) as Box<{
    ang: number; vel: number; fi: number; markAng: number; markOp: number; idle: number; dpr: number;
    drag: { on: boolean; lastA: number; lastT: number; v: number };
  }>;
  const lastRef = useRef(0) as Box<number>;
  const rafRef = useRef(0) as Box<number>;
  const [still, setStill] = useState(false);

  function reduced(): boolean {
    return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
  }

  function fit(): void {
    const st = stRef.current;
    const field = fieldRef.current, cv = cvRef.current;
    if (!field || !cv) return;
    const ctx = cv.getContext("2d");
    if (!ctx) return;
    st.dpr = Math.min(2, window.devicePixelRatio || 1);
    cv.width = Math.round(field.clientWidth * st.dpr);
    cv.height = Math.round(field.clientHeight * st.dpr);
    ctxRef.current = ctx;
  }

  function powLevel(v: number): number {
    return Math.min(5, Math.max(1, Math.ceil(Math.abs(v) / 4.8)));
  }

  function draw(peak: number): void {
    const st = stRef.current;
    const ctx = ctxRef.current;
    if (!ctx) return;
    const w = cvRef.current!.width / st.dpr, h = cvRef.current!.height / st.dpr;
    ctx.setTransform(st.dpr, 0, 0, st.dpr, 0, 0);
    ctx.clearRect(0, 0, w, h);
    const cx = w / 2, cy = h / 2, R = Math.min(w, h) / 2 - 10;
    ctx.save();
    ctx.translate(cx, cy);
    ctx.rotate(st.ang);
    for (let k = 0; k < N; k++) {
      const a0 = k * SEG;
      ctx.beginPath(); ctx.moveTo(0, 0);
      ctx.arc(0, 0, R, a0, a0 + SEG); ctx.closePath();
      ctx.fillStyle = PAL[k]; ctx.fill();
      ctx.strokeStyle = "rgba(23,20,26,.20)"; ctx.lineWidth = 1; ctx.stroke();
      ctx.save();
      ctx.rotate(a0 + SEG / 2);
      ctx.fillStyle = INK;
      ctx.font = "700 13px sans-serif";
      ctx.textAlign = "right"; ctx.textBaseline = "middle";
      ctx.fillText(String(k + 1), R - 14, 0);
      ctx.restore();
    }
    ctx.beginPath(); ctx.arc(0, 0, 13, 0, TAU);
    ctx.fillStyle = CREAM; ctx.fill();
    ctx.strokeStyle = "rgba(23,20,26,.35)"; ctx.stroke();
    ctx.restore();
    // 12시 고정 포인터
    ctx.beginPath();
    ctx.moveTo(cx - 7, cy - R - 8); ctx.lineTo(cx + 7, cy - R - 8); ctx.lineTo(cx, cy - R + 8);
    ctx.closePath();
    ctx.fillStyle = INK; ctx.fill();
    // 손 뗀 순간의 각도 자취 — 튕김의 시작점을 허브에서 표시한다
    if (peak > 0) {
      ctx.beginPath();
      ctx.arc(cx, cy, 20, st.markAng - 0.5, st.markAng + 0.5);
      ctx.strokeStyle = "rgba(23,20,26," + peak * 0.7 + ")";
      ctx.lineWidth = 4; ctx.stroke();
    }
  }

  function angleAt(e: PointerEvent<HTMLDivElement>): number {
    const b = e.currentTarget.getBoundingClientRect();
    return Math.atan2(e.clientY - (b.top + b.height / 2), e.clientX - (b.left + b.width / 2));
  }
  function onPointerDown(e: PointerEvent<HTMLDivElement>): void {
    const st = stRef.current;
    st.drag.on = true; st.drag.lastA = angleAt(e); st.drag.lastT = performance.now(); st.drag.v = 0;
    try { e.currentTarget.setPointerCapture(e.pointerId); } catch { /* 캡처 실패는 회전에 영향 없다 */ }
  }
  function onPointerMove(e: PointerEvent<HTMLDivElement>): void {
    const st = stRef.current;
    if (!st.drag.on) return;
    const a = angleAt(e), now = performance.now();
    let dA = a - st.drag.lastA;
    if (dA > Math.PI) dA -= TAU;
    if (dA < -Math.PI) dA += TAU;
    st.ang += dA;
    const dt = Math.max(1, now - st.drag.lastT) / 1000;
    st.drag.v = 0.65 * st.drag.v + 0.35 * (dA / dt);
    st.drag.lastA = a; st.drag.lastT = now;
    const vel = velRef.current;
    if (vel) vel.textContent = st.drag.v.toFixed(1);
    st.idle = 1.2;
  }
  function endDrag(): void {
    const st = stRef.current;
    if (!st.drag.on) return;
    st.drag.on = false;
    st.vel = Math.max(-24, Math.min(24, st.drag.v));
    st.markAng = st.ang; st.markOp = 1;
    const pow = powRef.current;
    if (pow) pow.textContent = String(powLevel(st.vel));
  }
  function onKeyDown(e: KeyEvt): void {
    const st = stRef.current;
    if (e.key === " " || e.key === "Enter") {
      e.preventDefault();
      st.vel = FLICKS[st.fi++ % FLICKS.length];
      st.markAng = st.ang; st.markOp = 1; st.idle = 0;
      const pow = powRef.current;
      if (pow) pow.textContent = String(powLevel(st.vel));
    }
  }

  function step(now: number): void {
    const st = stRef.current;
    const dt = Math.min(64, now - lastRef.current) / 1000; lastRef.current = now;
    const vel = velRef.current, pow = powRef.current;
    if (!st.drag.on) {
      if (st.vel !== 0) {
        st.ang += st.vel * dt;
        st.vel *= Math.pow(FRICTION, dt);
        if (vel) vel.textContent = st.vel.toFixed(1);
        if (Math.abs(st.vel) < STOP) { st.vel = 0; if (vel) vel.textContent = "0.0"; }
      } else if (st.idle > 0) { st.idle -= dt; }
      else {
        st.vel = FLICKS[st.fi++ % FLICKS.length];
        st.markAng = st.ang; st.markOp = 1;
        if (pow) pow.textContent = String(powLevel(st.vel));
      }
    }
    if (st.markOp > 0) st.markOp = Math.max(0, st.markOp - dt * 0.9);
    draw(st.markOp);
    // 속도 바 — 5단계로 채운다 (레이아웃이 아니라 transform 이다)
    const lv = Math.min(5, Math.ceil(Math.abs(st.drag.on ? st.drag.v : st.vel) / 4.8));
    const bar = barRef.current;
    if (bar) {
      for (let q = 0; q < bar.children.length; q++) {
        (bar.children[q] as HTMLElement).className = q < lv ? styles.seg + " " + styles.isOn : styles.seg;
      }
    }
    rafRef.current = requestAnimationFrame(step);
  }

  useEffect(() => {
    fit();
    function onResize(): void { fit(); }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      // 정지 한 장 — 자취 표시가 진행 중인 상태로 고정하고 루프는 돌지 않는다
      const st = stRef.current;
      st.markOp = 0.5;
      draw(0.5);
      const bar = barRef.current;
      if (bar) {
        for (let z = 0; z < bar.children.length; z++) {
          (bar.children[z] as HTMLElement).className = z < 2 ? styles.seg + " " + styles.isOn : styles.seg;
        }
      }
      setStill(true);
    } else {
      const st = stRef.current;
      st.vel = FLICKS[0]; st.markAng = st.ang; st.markOp = 1;
      const pow = powRef.current;
      if (pow) pow.textContent = "2";
      lastRef.current = performance.now();
      rafRef.current = requestAnimationFrame(step);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  const rootCls = still ? styles.fw + " " + styles.isStill : styles.fw;
  return (
    <main className={styles.stage}>
      <div className={rootCls} style={{ "--duration": duration, "--easing": easing, "--color": color } as Vars}>
        <p className={styles.hint}>{HINT}</p>
        <div className={styles.row}>
          <div
            className={styles.field}
            ref={fieldRef}
            role="application"
            aria-label={ARIA_FIELD}
            tabIndex={0}
            onKeyDown={onKeyDown}
            onPointerDown={onPointerDown}
            onPointerMove={onPointerMove}
            onPointerUp={endDrag}
            onPointerCancel={endDrag}
          >
            <canvas className={styles.cv} ref={cvRef}></canvas>
          </div>
          <div className={styles.side}>
            <b className={styles.vel} ref={velRef}>0.0</b>
            <span className={styles.unit}>{UNIT}</span>
            <div className={styles.bar} ref={barRef} aria-hidden="true">
              {Array.from({ length: 5 }, (_, i) => <span key={i} className={styles.seg}></span>)}
            </div>
            <p className={styles.meta}>{POW_LABEL}<b className={styles.count} ref={powRef}>0</b><span className={styles.metaUnit}>{POW_UNIT}</span></p>
            <p className={styles.meta}>{META_DRAG}</p>
          </div>
        </div>
      </div>
    </main>
  );
}