01Button a Cursor-coordinate Value Rotates

<main class="stage">
<div class="cr is-demo" id="cr">
  <p class="cr__hint"><span>Move the cursor over the button and its coordinates compute rotateX/rotateY</span></p>
  <div class="cr__scene" id="scene">
    <button class="cr__btn" id="btn" type="button" aria-label="Get started — button that tilts to follow the cursor">
      <span class="cr__sheen" id="sheen" aria-hidden="true"></span>
      <span class="cr__label">Get started</span>
    </button>
  </div>
</div>
</main>

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

.cr {
  display: grid;
  gap: $sp-4;
  justify-items: center;
}

.cr__hint {
  margin: 0;
  font-size: 13px;
  line-height: 1.5;
  text-align: center;
  max-width: 260px;
  color: rgba($subject-cream, .78);
}

.cr__scene {
  perspective: 640px;
  padding: $sp-6;
}

.cr__btn {
  position: relative;
  border: 0;
  cursor: pointer;
  padding: $sp-4 $sp-8;
  border-radius: $r-control;
  background: $subject-blue-deep;
  color: $subject-cream;
  font-size: 16px;
  font-weight: 700;
  letter-spacing: .01em;
  transform-style: preserve-3d;
  box-shadow: $shadow-raised;
  overflow: hidden;
  will-change: transform;
}

.cr__btn:focus-visible {
  outline: 2px solid $subject-cream;
  outline-offset: 3px;
}

.cr__sheen {
  position: absolute;
  inset: 0;
  pointer-events: none;
  mix-blend-mode: soft-light;
}

.cr__label {
  position: relative;
  z-index: 1;
}

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

.cr {
  display: grid;
  gap: 16px;
  justify-items: center;
}

.cr__hint {
  margin: 0;
  font-size: 13px;
  line-height: 1.5;
  text-align: center;
  max-width: 260px;
  color: rgba(255, 247, 230, 0.78);
}

.cr__scene {
  perspective: 640px;
  padding: 24px;
}

.cr__btn {
  position: relative;
  border: 0;
  cursor: pointer;
  padding: 16px 32px;
  border-radius: 12px;
  background: #0a4fe8;
  color: #fff7e6;
  font-size: 16px;
  font-weight: 700;
  letter-spacing: 0.01em;
  transform-style: preserve-3d;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  overflow: hidden;
  will-change: transform;
}

.cr__btn:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 3px;
}

.cr__sheen {
  position: absolute;
  inset: 0;
  pointer-events: none;
  mix-blend-mode: soft-light;
}

.cr__label {
  position: relative;
  z-index: 1;
}

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

(function () {
  var root = document.getElementById('cr');
  var scene = document.getElementById('scene');
  var btn = document.getElementById('btn');
  var sheen = document.getElementById('sheen');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var MAX_DEG = 14;          // 커서가 버튼 가장자리에 닿았을 때의 최대 회전각
  var LERP = 0.16;           // 관성 lerp 계수 — 값이 목표를 따라잡는 속도
  var targetNX = 0, targetNY = 0;
  var curNX = 0, curNY = 0;
  var hasPointer = false;
  var clickTimer = null;

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

  // 포인터가 없을 때(자동 시연·헤드리스 측정) 같은 계산을 돌리기 위한 합성 좌표 — 두 축의 위상을 어긋낸 리사주 궤적
  function syntheticTarget(t) {
    return [Math.sin(t * 1.15), Math.sin(t * 0.82 + 1.05)];
  }

  function render(nx, ny) {
    var rotY = nx * MAX_DEG;
    var rotX = -ny * MAX_DEG;
    btn.style.transform = 'rotateX(' + rotX.toFixed(2) + 'deg) rotateY(' + rotY.toFixed(2) + 'deg)';
    var sx = 50 + nx * 34, sy = 50 + ny * 34;
    sheen.style.background = 'radial-gradient(circle at ' + sx.toFixed(1) + '% ' + sy.toFixed(1) + '%, rgba(255,247,230,.5), rgba(255,247,230,0) 58%)';
    var depth = Math.min(1, Math.sqrt(nx * nx + ny * ny));
    btn.style.boxShadow = '0 ' + (10 + depth * 16).toFixed(1) + 'px ' + (18 + depth * 22).toFixed(1) + 'px rgba(8,6,12,' + (0.30 + depth * 0.16).toFixed(2) + ')';
  }

  var t0 = null;
  function loop(now) {
    if (t0 === null) { t0 = now; }
    var t = (now - t0) / 1000;
    if (!hasPointer && !reduce) {
      var s = syntheticTarget(t);
      targetNX = s[0]; targetNY = s[1];
    }
    curNX += (targetNX - curNX) * LERP;
    curNY += (targetNY - curNY) * LERP;
    render(curNX, curNY);
    requestAnimationFrame(loop);
  }

  function pointFromEvent(e) {
    var r = scene.getBoundingClientRect();
    var nx = ((e.clientX - r.left) / r.width) * 2 - 1;
    var ny = ((e.clientY - r.top) / r.height) * 2 - 1;
    return [clamp(nx, -1, 1), clamp(ny, -1, 1)];
  }

  scene.addEventListener('pointermove', function (e) {
    hasPointer = true;
    root.classList.remove('is-demo');
    var p = pointFromEvent(e);
    targetNX = p[0]; targetNY = p[1];
  });
  scene.addEventListener('pointerleave', function () {
    hasPointer = false;
    targetNX = 0; targetNY = 0;
  });
  btn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    hasPointer = true;
    targetNX = 0.92; targetNY = -0.62;
    if (reduce) { render(targetNX, targetNY); }
    clearTimeout(clickTimer);
    clickTimer = setTimeout(function () { hasPointer = false; targetNX = 0; targetNY = 0; if (reduce) { render(targetNX, targetNY); } }, 260);
  });
  btn.addEventListener('keydown', function (e) {
    var step = 0.24;
    var moves = { ArrowLeft: [-step, 0], ArrowRight: [step, 0], ArrowUp: [0, -step], ArrowDown: [0, step] };
    var m = moves[e.key];
    if (!m) { return; }
    e.preventDefault();
    root.classList.remove('is-demo');
    hasPointer = true;
    targetNX = clamp(targetNX + m[0], -1, 1);
    targetNY = clamp(targetNY + m[1], -1, 1);
    if (reduce) { render(targetNX, targetNY); }
  });

  if (reduce) {
    render(0.5, -0.34);
  } else {
    requestAnimationFrame(loop);
  }
})();

// CursorRotateButton — vanilla 판(run/393/demo/01-pointer-tilt-3d-cursor-rotate/index.html) 의 React 포트.
// 버튼 안 커서 좌표를 -1~1 로 정규화해 목표값(target)을 만들고, 관성 lerp 로 현재값(cur)이 따라간다.
// 포인터가 없을 때(자동 시연)는 위상이 어긋난 두 사인파(리사주 궤적)를 목표값으로 써 같은 계산을 돌린다.
// 값·수치는 vanilla 와 같다.
import { useEffect, useRef, useState } from "react";
import type { PointerEvent as ReactPointerEvent } from "react";
import styles from "./CursorRotateButton.module.scss";

type Box<T> = { current: T };
type KeyEvt = { key: string; preventDefault(): void };

const MAX_DEG = 14;
const LERP = 0.16;
const LABEL = "지금 시작하기 — 커서를 따라 기울어지는 버튼";

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

function syntheticTarget(t: number): [number, number] {
  return [Math.sin(t * 1.15), Math.sin(t * 0.82 + 1.05)];
}

export default function CursorRotateButton() {
  const sceneRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const sheenRef = useRef<HTMLSpanElement | null>(null);
  const targetXRef = useRef<number>(0) as Box<number>;
  const targetYRef = useRef<number>(0) as Box<number>;
  const curXRef = useRef<number>(0) as Box<number>;
  const curYRef = useRef<number>(0) as Box<number>;
  const hasPointerRef = useRef<boolean>(false) as Box<boolean>;
  const frameRef = useRef<number>(0) as Box<number>;
  const t0Ref = useRef<number>(0) as Box<number>;
  const clickTimerRef = useRef<number>(0) as Box<number>;
  const [demo, setDemo] = useState(true);
  const [reduce, setReduce] = useState(false);

  function render(nx: number, ny: number): void {
    const btn = btnRef.current;
    const sheen = sheenRef.current;
    if (!btn || !sheen) return;
    const rotY = nx * MAX_DEG;
    const rotX = -ny * MAX_DEG;
    btn.style.transform = "rotateX(" + rotX.toFixed(2) + "deg) rotateY(" + rotY.toFixed(2) + "deg)";
    const sx = 50 + nx * 34;
    const sy = 50 + ny * 34;
    sheen.style.background =
      "radial-gradient(circle at " + sx.toFixed(1) + "% " + sy.toFixed(1) + "%, rgba(255,247,230,.5), rgba(255,247,230,0) 58%)";
    const depth = Math.min(1, Math.sqrt(nx * nx + ny * ny));
    btn.style.boxShadow =
      "0 " + (10 + depth * 16).toFixed(1) + "px " + (18 + depth * 22).toFixed(1) + "px rgba(8,6,12," + (0.3 + depth * 0.16).toFixed(2) + ")";
  }

  useEffect(() => {
    setReduce(window.matchMedia("(prefers-reduced-motion: reduce)").matches);
  }, []);

  useEffect(() => {
    if (reduce) {
      render(0.5, -0.34);
      return;
    }
    function loop(now: number): void {
      if (t0Ref.current === 0) t0Ref.current = now;
      const t = (now - t0Ref.current) / 1000;
      if (!hasPointerRef.current) {
        const s = syntheticTarget(t);
        targetXRef.current = s[0];
        targetYRef.current = s[1];
      }
      curXRef.current += (targetXRef.current - curXRef.current) * LERP;
      curYRef.current += (targetYRef.current - curYRef.current) * LERP;
      render(curXRef.current, curYRef.current);
      frameRef.current = requestAnimationFrame(loop);
    }
    frameRef.current = requestAnimationFrame(loop);
    return () => cancelAnimationFrame(frameRef.current);
  }, [reduce]);

  function pointFromEvent(event: ReactPointerEvent<HTMLDivElement>): [number, number] {
    const scene = sceneRef.current;
    if (!scene) return [0, 0];
    const r = scene.getBoundingClientRect();
    const nx = ((event.clientX - r.left) / r.width) * 2 - 1;
    const ny = ((event.clientY - r.top) / r.height) * 2 - 1;
    return [clamp(nx, -1, 1), clamp(ny, -1, 1)];
  }

  function onPointerMove(event: ReactPointerEvent<HTMLDivElement>): void {
    hasPointerRef.current = true;
    setDemo(false);
    const p = pointFromEvent(event);
    targetXRef.current = p[0];
    targetYRef.current = p[1];
  }

  function onPointerLeave(): void {
    hasPointerRef.current = false;
    targetXRef.current = 0;
    targetYRef.current = 0;
  }

  function onClick(): void {
    setDemo(false);
    hasPointerRef.current = true;
    targetXRef.current = 0.92;
    targetYRef.current = -0.62;
    if (reduce) render(targetXRef.current, targetYRef.current);
    window.clearTimeout(clickTimerRef.current);
    clickTimerRef.current = window.setTimeout(() => {
      hasPointerRef.current = false;
      targetXRef.current = 0;
      targetYRef.current = 0;
      if (reduce) render(targetXRef.current, targetYRef.current);
    }, 260);
  }

  function onKeyDown(event: KeyEvt): void {
    const step = 0.24;
    const moves: { [key: string]: [number, number] } = {
      ArrowLeft: [-step, 0],
      ArrowRight: [step, 0],
      ArrowUp: [0, -step],
      ArrowDown: [0, step],
    };
    const m = moves[event.key];
    if (!m) return;
    event.preventDefault();
    setDemo(false);
    hasPointerRef.current = true;
    targetXRef.current = clamp(targetXRef.current + m[0], -1, 1);
    targetYRef.current = clamp(targetYRef.current + m[1], -1, 1);
    if (reduce) render(targetXRef.current, targetYRef.current);
  }

  const cls = [styles.cr, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls}>
        <p className={styles.hint}>
          <span>커서를 버튼 위에서 움직이면 좌표 값이 rotateX·rotateY 를 계산합니다</span>
        </p>
        <div className={styles.scene} ref={sceneRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <button className={styles.btn} ref={btnRef} type="button" aria-label={LABEL} onClick={onClick} onKeyDown={onKeyDown}>
            <span className={styles.sheen} ref={sheenRef} aria-hidden="true" />
            <span className={styles.label}>지금 시작하기</span>
          </button>
        </div>
      </div>
    </main>
  );
}