07紐でつながったカードが先頭のカードに引かれて横へ動き、揺れてから静まる紐に吊るしたカードの列

<main class="stage">
<div class="hs is-demo" id="hs">
  <canvas class="hs__canvas" data-role="canvas" width="480" height="300" tabindex="0" aria-label="柱の間のひもに掛かったカードが、横へ押されるとひもが遅れてついてきて揺れる場面"></canvas>
  <p class="hs__status" data-role="status" aria-live="off">落ち着きました</p>
  <div class="hs__tools">
    <button class="hs__btn" data-role="btn" type="button">揺らす</button>
  </div>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-ink);

.hs {
  position: relative;
  width: min(92vw, calc(160vh - 12.8vw));
  aspect-ratio: 480 / 300;
  margin: 0 auto;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
  background: $subject-ink;
}

.hs__canvas {
  display: block;
  width: 100%;
  height: 100%;
  cursor: grab;
}

.hs__canvas:active {
  cursor: grabbing;
}

.hs__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, .85);
  outline-offset: -6px;
}

.hs__status {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  max-width: 46%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-control;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.hs__tools {
  position: absolute;
  right: $sp-3;
  bottom: $sp-3;
  display: flex;
  gap: $sp-2;
}

.hs__btn {
  margin: 0;
  padding: $sp-1 $sp-3;
  border: 0;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
  transition: background-color $dur-quick ease-out;
}

.hs__btn:hover {
  background: $subject-cream;
}

.hs__btn[aria-pressed="true"] {
  background: $subject-mint;
}

.hs__btn:focus-visible {
  outline: 2px solid rgba(255, 255, 255, .95);
  outline-offset: 2px;
}

// 손가락 가로 끌기는 이 캔버스가 받고, 세로 쓸기는 브라우저에 남겨 페이지 스크롤이 막히지 않게 한다
.hs__canvas { touch-action: pan-y; }

@media (max-width: 340px) {
  .hs__btn { font-size: 10px; padding: $sp-1 $sp-2; }
  .hs__status { font-size: 10px; padding: $sp-1 $sp-2; }
}

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

.hs {
  position: relative;
  width: min(92vw, 160vh - 12.8vw);
  aspect-ratio: 480/300;
  margin: 0 auto;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  background: #17141a;
}

.hs__canvas {
  display: block;
  width: 100%;
  height: 100%;
  cursor: grab;
}

.hs__canvas:active {
  cursor: grabbing;
}

.hs__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, 0.85);
  outline-offset: -6px;
}

.hs__status {
  position: absolute;
  left: 12px;
  bottom: 12px;
  max-width: 46%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 12px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.hs__tools {
  position: absolute;
  right: 12px;
  bottom: 12px;
  display: flex;
  gap: 8px;
}

.hs__btn {
  margin: 0;
  padding: 4px 12px;
  border: 0;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
  transition: background-color 220ms ease-out;
}

.hs__btn:hover {
  background: #fff7e6;
}

.hs__btn[aria-pressed=true] {
  background: #4cd4a6;
}

.hs__btn:focus-visible {
  outline: 2px solid rgba(255, 255, 255, 0.95);
  outline-offset: 2px;
}

.hs__canvas {
  touch-action: pan-y;
}

@media (max-width: 340px) {
  .hs__btn {
    font-size: 10px;
    padding: 4px 8px;
  }
  .hs__status {
    font-size: 10px;
    padding: 4px 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .hs__btn {
    transition: none;
  }
}

(function () {
    const root = document.getElementById("hs");
    if (!root) return;
    const W = 480;
    const H = 300;
    const FIXED_DT = 16;
    // 파스텔 팔레트 — 어두운 무대 위에 밝은 카드를 얹어 명도 차로 읽힌다
    const BG = [29, 26, 44];
    const PAPER = [255, 248, 234];
    const INK = [58, 48, 76];
    const MINT = [126, 226, 192];
    const SKY = [124, 196, 255];
    const LILAC = [190, 164, 255];
    const PINK = [255, 160, 190];
    const PEACH = [255, 208, 168];
    const TINTS = [SKY, PINK, MINT, LILAC, PEACH];
    function clamp(v, lo, hi) { return v < lo ? lo : (v > hi ? hi : v); }
    function lerp(a, b, k) { return a + (b - a) * k; }
    function smooth(x) { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
    function rgba(c, a) {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
    }
    function mix(a, b, k) { return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]; }
    const canvasEl = root.querySelector('[data-role="canvas"]');
    const statusEl = root.querySelector('[data-role="status"]');
    const ctxRaw = canvasEl ? canvasEl.getContext('2d') : null;
    if (!canvasEl || !ctxRaw || !statusEl) {
        return;
    }
    const cv = canvasEl;
    const ctx = ctxRaw;
    const status = statusEl;
    const reduceQuery = window.matchMedia('(prefers-reduced-motion: reduce)');
    const still = reduceQuery.matches;
    // 자동 시연 중인가 — 사람이 처음 만지는 순간 시연을 멈추고 상태 줄을 읽어 주기 시작한다
    let demo = root.classList.contains('is-demo');
    function wake() {
        if (!demo) {
            return;
        }
        demo = false;
        root.classList.remove('is-demo');
        status.setAttribute('aria-live', 'polite');
    }
    function say(text) {
        if (status.textContent !== text) {
            status.textContent = text;
        }
    }
    // 휠 한 번의 이동량을 px 로 — 줄 단위(deltaMode 1)·쪽 단위(deltaMode 2) 장치도 같은 눈금으로 맞춘다.
    // 가로·세로 중 큰 쪽을 쓴다: 트랙패드의 가로 쓸기와 마우스 휠의 세로 굴림이 같은 방향으로 들어온다
    function wheelPx(ev) {
        const unit = ev.deltaMode === 1 ? 16 : (ev.deltaMode === 2 ? W : 1);
        const d = Math.abs(ev.deltaX) > Math.abs(ev.deltaY) ? ev.deltaX : ev.deltaY;
        return d * unit;
    }
    // 시연 시계가 loop 주기 안의 at 시각을 이번 스텝에 막 지났는가
    function crossed(t, at, loop) {
        const p = t % loop;
        return p >= at && p - FIXED_DT < at;
    }
    function fitCanvas() {
        const dpr = window.devicePixelRatio || 1;
        cv.width = Math.round(W * dpr);
        cv.height = Math.round(H * dpr);
        ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    function onResize() {
        fitCanvas();
        draw();
    }
    fitCanvas();
    window.addEventListener('resize', onResize);
    // 화면 좌표(CSS px) → 장면 좌표(480x300). 리사이즈 뒤에도 늘 현재 크기로 다시 잰다
    function toScene(ev) {
        const r = cv.getBoundingClientRect();
        return [((ev.clientX - r.left) / r.width) * W, ((ev.clientY - r.top) / r.height) * H];
    }
    function roundRectPath(c, x, y, w, h, r) {
        const rr = Math.min(r, w / 2, h / 2);
        c.beginPath();
        c.moveTo(x + rr, y);
        c.lineTo(x + w - rr, y);
        c.quadraticCurveTo(x + w, y, x + w, y + rr);
        c.lineTo(x + w, y + h - rr);
        c.quadraticCurveTo(x + w, y + h, x + w - rr, y + h);
        c.lineTo(x + rr, y + h);
        c.quadraticCurveTo(x, y + h, x, y + h - rr);
        c.lineTo(x, y + rr);
        c.quadraticCurveTo(x, y, x + rr, y);
        c.closePath();
    }
    // 카드 한 장 — 사진 대신 계산한 그림 칸(그라디언트 하늘·해·언덕) + 글줄 막대 두 개 + 번호
    function drawCard(c, x, y, w, h, i, light) {
        const t1 = TINTS[i % TINTS.length];
        const t2 = TINTS[(i + 2) % TINTS.length];
        const k = clamp(light, 0, 1);
        roundRectPath(c, x, y, w, h, 10);
        c.fillStyle = rgba(mix(BG, PAPER, k), 1);
        c.fill();
        const ih = h * 0.58;
        const g = c.createLinearGradient(x, y, x + w, y + ih);
        g.addColorStop(0, rgba(mix(BG, t1, k), 1));
        g.addColorStop(1, rgba(mix(BG, t2, k * 0.92), 1));
        c.save();
        roundRectPath(c, x + 6, y + 6, w - 12, ih - 6, 7);
        c.fillStyle = g;
        c.fill();
        c.clip();
        c.fillStyle = rgba(mix(BG, PAPER, k * 0.9), 0.9);
        c.beginPath();
        c.arc(x + w * 0.7, y + ih * 0.36, Math.min(w, ih) * 0.13, 0, Math.PI * 2);
        c.fill();
        c.fillStyle = rgba(mix(BG, INK, k), 0.55);
        c.beginPath();
        c.moveTo(x, y + ih);
        c.lineTo(x + w * 0.32, y + ih * 0.55);
        c.lineTo(x + w * 0.55, y + ih * 0.8);
        c.lineTo(x + w * 0.78, y + ih * 0.62);
        c.lineTo(x + w, y + ih);
        c.closePath();
        c.fill();
        c.restore();
        c.fillStyle = rgba(mix(BG, INK, k), 0.75);
        c.fillRect(x + 10, y + ih + 12, w * 0.62, 6);
        c.fillStyle = rgba(mix(BG, INK, k), 0.4);
        c.fillRect(x + 10, y + ih + 24, w * 0.44, 5);
        c.fillStyle = rgba(mix(BG, INK, k), 0.9);
        c.font = '700 13px sans-serif';
        c.textAlign = 'right';
        c.textBaseline = 'alphabetic';
        c.fillText(String(i + 1).padStart(2, '0'), x + w - 9, y + h - 10);
    }
    const NODE_N = 49;
    const TRACK_L = 1440;
    const SEG = TRACK_L / (NODE_N - 1);
    const SLACK = 1.05;
    const REST = SEG * SLACK;
    const POST_EVERY = 12;
    const CORD_Y = 64;
    const PAD = 60;
    const GRAV = 1400;
    const DAMP = 0.99;
    const ITER = 10;
    const CARD_WEIGHT = 2.2;
    const CARD_AT = [3, 6, 9, 15, 18, 21, 27, 30, 33, 39, 42, 45];
    const CARD_W = 72;
    const CARD_H = 92;
    const CLIP = 10;
    const MAX_SCROLL = TRACK_L + PAD * 2 - W;
    const S_LERP = 0.09;
    const CALM_V = 0.06;
    const DEMO_LOOP = 4200;
    const TXT_SWAY = 'ひもが揺れています';
    const TXT_CALM = '落ち着きました';
    const shakeBtn = root.querySelector('[data-role="btn"]');
    const px = [];
    const py = [];
    const ox = [];
    const oy = [];
    let target = 0;
    let scroll = 0;
    let calm = true;
    function isPost(i) { return i % POST_EVERY === 0; }
    // 기둥 자리는 스크롤만큼 옆으로 밀린 화면 좌표에 못 박혀 있다
    function postX(i) { return PAD + i * SEG - scroll; }
    function weight(i) { return CARD_AT.indexOf(i) >= 0 ? CARD_WEIGHT : 1; }
    for (let i = 0; i < NODE_N; i += 1) {
        px.push(postX(i));
        py.push(CORD_Y);
        ox.push(postX(i));
        oy.push(CORD_Y);
    }
    // Verlet 한 스텝 — 속도를 따로 두지 않고 (현재 − 前へ) 위치 차를 속도로 쓴다
    function integrate(dt) {
        for (let i = 0; i < NODE_N; i += 1) {
            if (isPost(i)) {
                continue;
            }
            const vx = (px[i] - ox[i]) * DAMP;
            const vy = (py[i] - oy[i]) * DAMP;
            ox[i] = px[i];
            oy[i] = py[i];
            px[i] += vx;
            py[i] += vy + GRAV * weight(i) * dt * dt;
        }
    }
    // 거리 제약 — 이웃 두 점 사이를 REST 로 되돌리는 일을 ITER 번 되풀이한다. 기둥 점은 움직이지 않는다
    function relax() {
        for (let it = 0; it < ITER; it += 1) {
            for (let i = 0; i < NODE_N; i += 1) {
                if (isPost(i)) {
                    px[i] = postX(i);
                    py[i] = CORD_Y;
                }
            }
            for (let i = 0; i < NODE_N - 1; i += 1) {
                const dx = px[i + 1] - px[i];
                const dy = py[i + 1] - py[i];
                const d = Math.sqrt(dx * dx + dy * dy) || 1;
                if (d <= REST) {
                    continue;
                }
                const diff = (d - REST) / d;
                const a = isPost(i) ? 0 : (isPost(i + 1) ? 1 : 0.5);
                const b = isPost(i + 1) ? 0 : (isPost(i) ? 1 : 0.5);
                px[i] += dx * diff * a;
                py[i] += dy * diff * a;
                px[i + 1] -= dx * diff * b;
                py[i + 1] -= dy * diff * b;
            }
        }
    }
    // 집게 자리의 줄 기울기(라디안) — 양옆 이웃 점을 잇는 선의 각도
    function slopeAt(i) {
        const a = Math.max(0, i - 1);
        const b = Math.min(NODE_N - 1, i + 1);
        return Math.atan2(py[b] - py[a], px[b] - px[a]);
    }
    function step() {
        const prevScroll = scroll;
        scroll += (target - scroll) * S_LERP;
        if (Math.abs(target - scroll) < 0.05) {
            scroll = target;
        }
        const dt = FIXED_DT / 1000;
        integrate(dt);
        relax();
        let maxV = Math.abs(scroll - prevScroll);
        for (let i = 0; i < NODE_N; i += 1) {
            maxV = Math.max(maxV, Math.abs(px[i] - ox[i]));
        }
        const now = maxV < CALM_V;
        if (now !== calm) {
            calm = now;
            say(calm ? TXT_CALM : TXT_SWAY);
        }
    }
    // 처음 모양 — 가만히 둔 줄이 늘어진 모습이 될 때까지 미리 돌려 둔다
    function rest(n) {
        for (let k = 0; k < n; k += 1) {
            integrate(FIXED_DT / 1000);
            relax();
            for (let i = 0; i < NODE_N; i += 1) {
                ox[i] = px[i];
                oy[i] = py[i];
            }
        }
    }
    rest(240);
    function settle() {
        scroll = target;
        rest(240);
        say(TXT_CALM);
    }
    function demoTick(t) {
        if (crossed(t, 200, DEMO_LOOP)) {
            target = clamp(target + 420, 0, MAX_SCROLL);
        }
        if (crossed(t, 1150, DEMO_LOOP)) {
            target = clamp(target - 420, 0, MAX_SCROLL);
        }
    }
    function draw() {
        ctx.fillStyle = rgba(BG, 1);
        ctx.fillRect(0, 0, W, H);
        // 기둥
        for (let i = 0; i < NODE_N; i += POST_EVERY) {
            const x = postX(i);
            if (x < -10 || x > W + 10) {
                continue;
            }
            ctx.fillStyle = rgba(mix(BG, LILAC, 0.45), 1);
            ctx.fillRect(x - 3, CORD_Y - 10, 6, H);
            ctx.fillStyle = rgba(PEACH, 1);
            ctx.beginPath();
            ctx.arc(x, CORD_Y, 5, 0, Math.PI * 2);
            ctx.fill();
        }
        // 줄
        ctx.strokeStyle = rgba(mix(BG, PAPER, 0.7), 1);
        ctx.lineWidth = 2;
        ctx.beginPath();
        ctx.moveTo(px[0], py[0]);
        for (let i = 1; i < NODE_N; i += 1) {
            ctx.lineTo(px[i], py[i]);
        }
        ctx.stroke();
        // 카드 — 집게 점에 매달려 그 자리의 줄 기울기만큼 기운다
        for (let k = 0; k < CARD_AT.length; k += 1) {
            const i = CARD_AT[k];
            if (px[i] < -CARD_W || px[i] > W + CARD_W) {
                continue;
            }
            ctx.save();
            ctx.translate(px[i], py[i]);
            ctx.rotate(slopeAt(i));
            ctx.fillStyle = rgba([10, 8, 22], 0.35);
            ctx.fillRect(-CARD_W / 2 + 4, CLIP + 6, CARD_W, CARD_H);
            drawCard(ctx, -CARD_W / 2, CLIP, CARD_W, CARD_H, k, 1);
            ctx.fillStyle = rgba(MINT, 1);
            ctx.fillRect(-4, -4, 8, CLIP + 8);
            ctx.restore();
        }
    }
    function canMove(d) { return d > 0 ? target < MAX_SCROLL : target > 0; }
    function moveTo(p) {
        target = clamp(p, 0, MAX_SCROLL);
        if (still) {
            settle();
            draw();
        }
    }
    function onWheel(ev) {
        const d = wheelPx(ev);
        if (d === 0 || !canMove(d)) {
            return;
        }
        ev.preventDefault();
        wake();
        moveTo(target + d);
    }
    cv.addEventListener('wheel', onWheel, { passive: false });
    let dragId = -1;
    let dragX = 0;
    cv.addEventListener('pointerdown', function (ev) {
        wake();
        dragId = ev.pointerId;
        cv.setPointerCapture(ev.pointerId);
        dragX = toScene(ev)[0];
    });
    cv.addEventListener('pointermove', function (ev) {
        if (ev.pointerId !== dragId) {
            return;
        }
        const sx = toScene(ev)[0];
        moveTo(target - (sx - dragX));
        dragX = sx;
    });
    function endDrag(ev) { if (ev.pointerId === dragId) {
        dragId = -1;
    } }
    cv.addEventListener('pointerup', endDrag);
    cv.addEventListener('pointercancel', endDrag);
    cv.addEventListener('keydown', function (ev) {
        let goal = NaN;
        if (ev.key === 'ArrowRight') {
            goal = target + 180;
        }
        else if (ev.key === 'ArrowLeft') {
            goal = target - 180;
        }
        else if (ev.key === 'Home') {
            goal = 0;
        }
        else if (ev.key === 'End') {
            goal = MAX_SCROLL;
        }
        if (Number.isNaN(goal) || !canMove(goal - target)) {
            return;
        }
        ev.preventDefault();
        wake();
        moveTo(goal);
    });
    if (shakeBtn) {
        shakeBtn.addEventListener('click', function () {
            wake();
            if (still) {
                return;
            }
            // 前へ 위치를 옆으로 밀어 두면 次へ 스텝에 그만큼 속도가 생긴다
            for (let i = 0; i < NODE_N; i += 1) {
                if (!isPost(i)) {
                    ox[i] -= 9;
                }
            }
        });
    }
    say(TXT_CALM);
    let rafId = 0;
    let lastTs = -1;
    let acc = 0;
    let clock = 0;
    function frame(ts) {
        if (lastTs < 0) {
            lastTs = ts;
        }
        // 탭이 숨었다 돌아와도 한 번에 100ms 까지만 따라잡는다 — 순간이동 대신 이어서 움직인다
        acc += Math.min(ts - lastTs, 100);
        lastTs = ts;
        let n = 0;
        while (acc >= FIXED_DT && n < 6) {
            clock += FIXED_DT;
            if (demo) {
                demoTick(clock);
            }
            step();
            acc -= FIXED_DT;
            n += 1;
        }
        draw();
        rafId = window.requestAnimationFrame(frame);
    }
    if (still) {
        settle();
        draw();
    }
    else {
        rafId = window.requestAnimationFrame(frame);
    }
    return function () {
        window.cancelAnimationFrame(rafId);
        window.removeEventListener('resize', onResize);
    };
})();

// HscVerletCordHangingCards - vanilla 판(07-hsc-verlet-cord-hanging-cards) 의 React 포트. 같은 계산을 useEffect 안에서 돌린다.
import { useEffect, useRef } from "react";
import styles from "./HscVerletCordHangingCards.module.scss";

const cx = (s: string): string => s.split(" ").map((n) => styles[n] ?? n).join(" ");

export default function HscVerletCordHangingCards() {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const rootEl = rootRef.current;
    if (!rootEl) return;
    const root: HTMLElement = rootEl;
    type RGB = [number, number, number];
    const W = 480;
    const H = 300;
    const FIXED_DT = 16;
    // 파스텔 팔레트 — 어두운 무대 위에 밝은 카드를 얹어 명도 차로 읽힌다
    const BG: RGB = [29, 26, 44];
    const PAPER: RGB = [255, 248, 234];
    const INK: RGB = [58, 48, 76];
    const MINT: RGB = [126, 226, 192];
    const SKY: RGB = [124, 196, 255];
    const LILAC: RGB = [190, 164, 255];
    const PINK: RGB = [255, 160, 190];
    const PEACH: RGB = [255, 208, 168];
    const TINTS: RGB[] = [SKY, PINK, MINT, LILAC, PEACH];
    function clamp(v: number, lo: number, hi: number): number { return v < lo ? lo : (v > hi ? hi : v); }
    function lerp(a: number, b: number, k: number): number { return a + (b - a) * k; }
    function smooth(x: number): number { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
    function rgba(c: RGB, a: number): string {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
    }
    function mix(a: RGB, b: RGB, k: number): RGB { return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]; }
    const canvasEl = root.querySelector('[data-role="canvas"]') as HTMLCanvasElement | null;
    const statusEl = root.querySelector('[data-role="status"]') as HTMLElement | null;
    const ctxRaw = canvasEl ? canvasEl.getContext('2d') : null;
    if (!canvasEl || !ctxRaw || !statusEl) { return; }
    const cv: HTMLCanvasElement = canvasEl;
    const ctx: CanvasRenderingContext2D = ctxRaw;
    const status: HTMLElement = statusEl;
    const reduceQuery = window.matchMedia('(prefers-reduced-motion: reduce)');
    const still = reduceQuery.matches;
    // 자동 시연 중인가 — 사람이 처음 만지는 순간 시연을 멈추고 상태 줄을 읽어 주기 시작한다
    let demo = root.classList.contains('is-demo');
    function wake(): void {
        if (!demo) { return; }
        demo = false;
        root.classList.remove('is-demo');
        status.setAttribute('aria-live', 'polite');
    }
    function say(text: string): void {
        if (status.textContent !== text) { status.textContent = text; }
    }
    // 휠 한 번의 이동량을 px 로 — 줄 단위(deltaMode 1)·쪽 단위(deltaMode 2) 장치도 같은 눈금으로 맞춘다.
    // 가로·세로 중 큰 쪽을 쓴다: 트랙패드의 가로 쓸기와 마우스 휠의 세로 굴림이 같은 방향으로 들어온다
    function wheelPx(ev: WheelEvent): number {
        const unit = ev.deltaMode === 1 ? 16 : (ev.deltaMode === 2 ? W : 1);
        const d = Math.abs(ev.deltaX) > Math.abs(ev.deltaY) ? ev.deltaX : ev.deltaY;
        return d * unit;
    }
    // 시연 시계가 loop 주기 안의 at 시각을 이번 스텝에 막 지났는가
    function crossed(t: number, at: number, loop: number): boolean {
        const p = t % loop;
        return p >= at && p - FIXED_DT < at;
    }
    function fitCanvas(): void {
        const dpr = window.devicePixelRatio || 1;
        cv.width = Math.round(W * dpr);
        cv.height = Math.round(H * dpr);
        ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    function onResize(): void {
        fitCanvas();
        draw();
    }
    fitCanvas();
    window.addEventListener('resize', onResize);
    // 화면 좌표(CSS px) → 장면 좌표(480x300). 리사이즈 뒤에도 늘 현재 크기로 다시 잰다
    function toScene(ev: PointerEvent): [number, number] {
        const r = cv.getBoundingClientRect();
        return [((ev.clientX - r.left) / r.width) * W, ((ev.clientY - r.top) / r.height) * H];
    }
    function roundRectPath(c: CanvasRenderingContext2D, x: number, y: number, w: number, h: number, r: number): void {
        const rr = Math.min(r, w / 2, h / 2);
        c.beginPath();
        c.moveTo(x + rr, y);
        c.lineTo(x + w - rr, y);
        c.quadraticCurveTo(x + w, y, x + w, y + rr);
        c.lineTo(x + w, y + h - rr);
        c.quadraticCurveTo(x + w, y + h, x + w - rr, y + h);
        c.lineTo(x + rr, y + h);
        c.quadraticCurveTo(x, y + h, x, y + h - rr);
        c.lineTo(x, y + rr);
        c.quadraticCurveTo(x, y, x + rr, y);
        c.closePath();
    }
    // 카드 한 장 — 사진 대신 계산한 그림 칸(그라디언트 하늘·해·언덕) + 글줄 막대 두 개 + 번호
    function drawCard(c: CanvasRenderingContext2D, x: number, y: number, w: number, h: number, i: number, light: number): void {
        const t1 = TINTS[i % TINTS.length];
        const t2 = TINTS[(i + 2) % TINTS.length];
        const k = clamp(light, 0, 1);
        roundRectPath(c, x, y, w, h, 10);
        c.fillStyle = rgba(mix(BG, PAPER, k), 1);
        c.fill();
        const ih = h * 0.58;
        const g = c.createLinearGradient(x, y, x + w, y + ih);
        g.addColorStop(0, rgba(mix(BG, t1, k), 1));
        g.addColorStop(1, rgba(mix(BG, t2, k * 0.92), 1));
        c.save();
        roundRectPath(c, x + 6, y + 6, w - 12, ih - 6, 7);
        c.fillStyle = g;
        c.fill();
        c.clip();
        c.fillStyle = rgba(mix(BG, PAPER, k * 0.9), 0.9);
        c.beginPath();
        c.arc(x + w * 0.7, y + ih * 0.36, Math.min(w, ih) * 0.13, 0, Math.PI * 2);
        c.fill();
        c.fillStyle = rgba(mix(BG, INK, k), 0.55);
        c.beginPath();
        c.moveTo(x, y + ih);
        c.lineTo(x + w * 0.32, y + ih * 0.55);
        c.lineTo(x + w * 0.55, y + ih * 0.8);
        c.lineTo(x + w * 0.78, y + ih * 0.62);
        c.lineTo(x + w, y + ih);
        c.closePath();
        c.fill();
        c.restore();
        c.fillStyle = rgba(mix(BG, INK, k), 0.75);
        c.fillRect(x + 10, y + ih + 12, w * 0.62, 6);
        c.fillStyle = rgba(mix(BG, INK, k), 0.4);
        c.fillRect(x + 10, y + ih + 24, w * 0.44, 5);
        c.fillStyle = rgba(mix(BG, INK, k), 0.9);
        c.font = '700 13px sans-serif';
        c.textAlign = 'right';
        c.textBaseline = 'alphabetic';
        c.fillText(String(i + 1).padStart(2, '0'), x + w - 9, y + h - 10);
    }
    const NODE_N = 49;
    const TRACK_L = 1440;
    const SEG = TRACK_L / (NODE_N - 1);
    const SLACK = 1.05;
    const REST = SEG * SLACK;
    const POST_EVERY = 12;
    const CORD_Y = 64;
    const PAD = 60;
    const GRAV = 1400;
    const DAMP = 0.99;
    const ITER = 10;
    const CARD_WEIGHT = 2.2;
    const CARD_AT = [3, 6, 9, 15, 18, 21, 27, 30, 33, 39, 42, 45];
    const CARD_W = 72;
    const CARD_H = 92;
    const CLIP = 10;
    const MAX_SCROLL = TRACK_L + PAD * 2 - W;
    const S_LERP = 0.09;
    const CALM_V = 0.06;
    const DEMO_LOOP = 4200;
    const TXT_SWAY = '줄이 흔들리는 중';
    const TXT_CALM = '가라앉음';
    const shakeBtn = root.querySelector('[data-role="btn"]') as HTMLButtonElement | null;
    const px: number[] = [];
    const py: number[] = [];
    const ox: number[] = [];
    const oy: number[] = [];
    let target = 0;
    let scroll = 0;
    let calm = true;
    function isPost(i: number): boolean { return i % POST_EVERY === 0; }
    // 기둥 자리는 스크롤만큼 옆으로 밀린 화면 좌표에 못 박혀 있다
    function postX(i: number): number { return PAD + i * SEG - scroll; }
    function weight(i: number): number { return CARD_AT.indexOf(i) >= 0 ? CARD_WEIGHT : 1; }
    for (let i = 0; i < NODE_N; i += 1) {
        px.push(postX(i));
        py.push(CORD_Y);
        ox.push(postX(i));
        oy.push(CORD_Y);
    }
    // Verlet 한 스텝 — 속도를 따로 두지 않고 (현재 − 이전) 위치 차를 속도로 쓴다
    function integrate(dt: number): void {
        for (let i = 0; i < NODE_N; i += 1) {
            if (isPost(i)) { continue; }
            const vx = (px[i] - ox[i]) * DAMP;
            const vy = (py[i] - oy[i]) * DAMP;
            ox[i] = px[i];
            oy[i] = py[i];
            px[i] += vx;
            py[i] += vy + GRAV * weight(i) * dt * dt;
        }
    }
    // 거리 제약 — 이웃 두 점 사이를 REST 로 되돌리는 일을 ITER 번 되풀이한다. 기둥 점은 움직이지 않는다
    function relax(): void {
        for (let it = 0; it < ITER; it += 1) {
            for (let i = 0; i < NODE_N; i += 1) {
                if (isPost(i)) { px[i] = postX(i); py[i] = CORD_Y; }
            }
            for (let i = 0; i < NODE_N - 1; i += 1) {
                const dx = px[i + 1] - px[i];
                const dy = py[i + 1] - py[i];
                const d = Math.sqrt(dx * dx + dy * dy) || 1;
                if (d <= REST) { continue; }
                const diff = (d - REST) / d;
                const a = isPost(i) ? 0 : (isPost(i + 1) ? 1 : 0.5);
                const b = isPost(i + 1) ? 0 : (isPost(i) ? 1 : 0.5);
                px[i] += dx * diff * a;
                py[i] += dy * diff * a;
                px[i + 1] -= dx * diff * b;
                py[i + 1] -= dy * diff * b;
            }
        }
    }
    // 집게 자리의 줄 기울기(라디안) — 양옆 이웃 점을 잇는 선의 각도
    function slopeAt(i: number): number {
        const a = Math.max(0, i - 1);
        const b = Math.min(NODE_N - 1, i + 1);
        return Math.atan2(py[b] - py[a], px[b] - px[a]);
    }
    function step(): void {
        const prevScroll = scroll;
        scroll += (target - scroll) * S_LERP;
        if (Math.abs(target - scroll) < 0.05) { scroll = target; }
        const dt = FIXED_DT / 1000;
        integrate(dt);
        relax();
        let maxV = Math.abs(scroll - prevScroll);
        for (let i = 0; i < NODE_N; i += 1) { maxV = Math.max(maxV, Math.abs(px[i] - ox[i])); }
        const now = maxV < CALM_V;
        if (now !== calm) { calm = now; say(calm ? TXT_CALM : TXT_SWAY); }
    }
    // 처음 모양 — 가만히 둔 줄이 늘어진 모습이 될 때까지 미리 돌려 둔다
    function rest(n: number): void {
        for (let k = 0; k < n; k += 1) {
            integrate(FIXED_DT / 1000);
            relax();
            for (let i = 0; i < NODE_N; i += 1) { ox[i] = px[i]; oy[i] = py[i]; }
        }
    }
    rest(240);
    function settle(): void {
        scroll = target;
        rest(240);
        say(TXT_CALM);
    }
    function demoTick(t: number): void {
        if (crossed(t, 200, DEMO_LOOP)) { target = clamp(target + 420, 0, MAX_SCROLL); }
        if (crossed(t, 1150, DEMO_LOOP)) { target = clamp(target - 420, 0, MAX_SCROLL); }
    }
    function draw(): void {
        ctx.fillStyle = rgba(BG, 1);
        ctx.fillRect(0, 0, W, H);
        // 기둥
        for (let i = 0; i < NODE_N; i += POST_EVERY) {
            const x = postX(i);
            if (x < -10 || x > W + 10) { continue; }
            ctx.fillStyle = rgba(mix(BG, LILAC, 0.45), 1);
            ctx.fillRect(x - 3, CORD_Y - 10, 6, H);
            ctx.fillStyle = rgba(PEACH, 1);
            ctx.beginPath();
            ctx.arc(x, CORD_Y, 5, 0, Math.PI * 2);
            ctx.fill();
        }
        // 줄
        ctx.strokeStyle = rgba(mix(BG, PAPER, 0.7), 1);
        ctx.lineWidth = 2;
        ctx.beginPath();
        ctx.moveTo(px[0], py[0]);
        for (let i = 1; i < NODE_N; i += 1) { ctx.lineTo(px[i], py[i]); }
        ctx.stroke();
        // 카드 — 집게 점에 매달려 그 자리의 줄 기울기만큼 기운다
        for (let k = 0; k < CARD_AT.length; k += 1) {
            const i = CARD_AT[k];
            if (px[i] < -CARD_W || px[i] > W + CARD_W) { continue; }
            ctx.save();
            ctx.translate(px[i], py[i]);
            ctx.rotate(slopeAt(i));
            ctx.fillStyle = rgba([10, 8, 22], 0.35);
            ctx.fillRect(-CARD_W / 2 + 4, CLIP + 6, CARD_W, CARD_H);
            drawCard(ctx, -CARD_W / 2, CLIP, CARD_W, CARD_H, k, 1);
            ctx.fillStyle = rgba(MINT, 1);
            ctx.fillRect(-4, -4, 8, CLIP + 8);
            ctx.restore();
        }
    }
    function canMove(d: number): boolean { return d > 0 ? target < MAX_SCROLL : target > 0; }
    function moveTo(p: number): void {
        target = clamp(p, 0, MAX_SCROLL);
        if (still) { settle(); draw(); }
    }
    function onWheel(ev: WheelEvent): void {
        const d = wheelPx(ev);
        if (d === 0 || !canMove(d)) { return; }
        ev.preventDefault();
        wake();
        moveTo(target + d);
    }
    cv.addEventListener('wheel', onWheel, { passive: false });
    let dragId = -1;
    let dragX = 0;
    cv.addEventListener('pointerdown', function (ev: PointerEvent): void {
        wake();
        dragId = ev.pointerId;
        cv.setPointerCapture(ev.pointerId);
        dragX = toScene(ev)[0];
    });
    cv.addEventListener('pointermove', function (ev: PointerEvent): void {
        if (ev.pointerId !== dragId) { return; }
        const sx = toScene(ev)[0];
        moveTo(target - (sx - dragX));
        dragX = sx;
    });
    function endDrag(ev: PointerEvent): void { if (ev.pointerId === dragId) { dragId = -1; } }
    cv.addEventListener('pointerup', endDrag);
    cv.addEventListener('pointercancel', endDrag);
    cv.addEventListener('keydown', function (ev: KeyboardEvent): void {
        let goal = NaN;
        if (ev.key === 'ArrowRight') { goal = target + 180; }
        else if (ev.key === 'ArrowLeft') { goal = target - 180; }
        else if (ev.key === 'Home') { goal = 0; }
        else if (ev.key === 'End') { goal = MAX_SCROLL; }
        if (Number.isNaN(goal) || !canMove(goal - target)) { return; }
        ev.preventDefault();
        wake();
        moveTo(goal);
    });
    if (shakeBtn) {
        shakeBtn.addEventListener('click', function (): void {
            wake();
            if (still) { return; }
            // 이전 위치를 옆으로 밀어 두면 다음 스텝에 그만큼 속도가 생긴다
            for (let i = 0; i < NODE_N; i += 1) { if (!isPost(i)) { ox[i] -= 9; } }
        });
    }
    say(TXT_CALM);

    let rafId = 0;
    let lastTs = -1;
    let acc = 0;
    let clock = 0;
    function frame(ts: number): void {
        if (lastTs < 0) { lastTs = ts; }
        // 탭이 숨었다 돌아와도 한 번에 100ms 까지만 따라잡는다 — 순간이동 대신 이어서 움직인다
        acc += Math.min(ts - lastTs, 100);
        lastTs = ts;
        let n = 0;
        while (acc >= FIXED_DT && n < 6) {
            clock += FIXED_DT;
            if (demo) { demoTick(clock); }
            step();
            acc -= FIXED_DT;
            n += 1;
        }
        draw();
        rafId = window.requestAnimationFrame(frame);
    }
    if (still) {
        settle();
        draw();
    } else {
        rafId = window.requestAnimationFrame(frame);
    }
    return function (): void {
        window.cancelAnimationFrame(rafId);
        window.removeEventListener('resize', onResize);
    };

  }, []);

  return (
    <div className={cx("hs") + " is-demo"} ref={rootRef}>
        <canvas className={cx("hs__canvas")} data-role="canvas" width={480} height={300} tabIndex={0} aria-label="기둥 사이 줄에 매달린 카드들이 옆으로 밀릴 때 줄이 늦게 따라오며 흔들리는 장면"></canvas>
        <p className={cx("hs__status")} data-role="status" aria-live="off">가라앉음</p>
        <div className={cx("hs__tools")}>
          <button className={cx("hs__btn")} data-role="btn" type="button">흔들기</button>
        </div>
    </div>
  );
}