05角度がポインターの方へゆっくり向くグラデーション

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">向きを追う</button>
  <p class="bc__meta"><span class="bc__tag">ANGLE</span> <b id="bcAng">135</b><span>deg</span></p>
</div>
</main>

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

// 핵심: 그라데이션 각도(--ang)는 포인터 방향을 향해 일정 비율씩 도는 숫자다. 0/360 경계는 차이를 접어서 가까운 쪽으로 돈다.

$angle-a: $subject-sky;
$angle-b: $subject-lilac;
$angle-rest: 135;

@include stage($stage-ink);

.bc {
  width: min(100%, 320px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.bc__btn {
  --ang: #{$angle-rest};
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: linear-gradient(calc(var(--ang) * 1deg), $angle-a, $angle-b);
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.bc__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-1;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.bc__tag { margin-right: $sp-1; font-weight: 700; letter-spacing: .08em; }
.bc__meta b { min-width: 3ch; text-align: right; font-variant-numeric: tabular-nums; color: $stage-paper; }

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

.bc {
  width: min(100%, 320px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.bc__btn {
  --ang: 135;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: linear-gradient(calc(var(--ang) * 1deg), #6ec8ff, #b9a0ff);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

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

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 4px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

.bc__tag {
  margin-right: 4px;
  font-weight: 700;
  letter-spacing: 0.08em;
}

.bc__meta b {
  min-width: 3ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #ffffff;
}

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

(function () {
  // 기법: 포인터가 버튼 중심에서 향한 방향을 각도로 바꿔 목표로 삼고, 현재 각도가 그쪽으로 일정 비율씩 돈다.
  // 각도는 0/360 경계가 있어 차이를 -180~180 으로 접어 가까운 쪽으로 돌린다. 포인터가 나가면 REST_ANGLE 로 돌아온다.
  var FOLLOW_RATE = 6;
  var REST_ANGLE = 135;
  var START_MS = 200;
  var ORBIT_MS = 1300;
  var CYCLE_MS = 2000;
  var ORBIT_SPAN = 300;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var angEl = document.getElementById('bcAng');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var angle = REST_ANGLE;
  var goalAngle = REST_ANGLE;
  var pointerIn = false;
  var t0 = null;
  var lastTs = null;

  function wrap(a) { return ((a % 360) + 360) % 360; }
  function fromPoint(x, y) {
    var r = btn.getBoundingClientRect();
    var dx = x - (r.left + r.width / 2);
    var dy = y - (r.top + r.height / 2);
    return wrap(Math.atan2(dx, -dy) * 180 / Math.PI);
  }
  function paint() {
    btn.style.setProperty('--ang', angle.toFixed(2));
    angEl.textContent = String(Math.round(wrap(angle)));
  }
  function stopDemo() { root.classList.remove('is-demo'); }

  btn.addEventListener('pointermove', function (e) {
    stopDemo();
    pointerIn = true;
    goalAngle = fromPoint(e.clientX, e.clientY);
    if (reduce) { angle = goalAngle; paint(); }
  });
  btn.addEventListener('pointerleave', function () {
    pointerIn = false;
    goalAngle = REST_ANGLE;
    if (reduce) { angle = goalAngle; paint(); }
  });

  function demoGoal(now) {
    if (now < START_MS) return REST_ANGLE;
    var c = (now - START_MS) % CYCLE_MS;
    if (c >= ORBIT_MS) return REST_ANGLE;
    return wrap(60 + (c / ORBIT_MS) * ORBIT_SPAN);
  }

  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 = root.classList.contains('is-demo') ? demoGoal(now) : goalAngle;
    var diff = ((g - angle + 540) % 360) - 180;
    angle = wrap(angle + diff * (1 - Math.exp(-dt * FOLLOW_RATE)));
    paint();
    requestAnimationFrame(tick);
  }

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

// AngleFollowButton — vanilla 판(05-btn-gradient-angle-pointer-lerp) 의 React 포트.
// 포인터가 버튼 중심에서 향한 방향을 각도로 바꿔 목표로 삼고, 현재 각도가 일정 비율씩 따라간다.
import { useEffect, useRef } from "react";
import styles from "./AngleFollowButton.module.scss";

type Box<T> = { current: T };
type PointLike = { clientX: number; clientY: number };

const FOLLOW_RATE = 6;
const REST_ANGLE = 135;
const START_MS = 200;
const ORBIT_MS = 1300;
const CYCLE_MS = 2000;
const ORBIT_SPAN = 300;

function wrap(a: number): number {
  return ((a % 360) + 360) % 360;
}

export default function AngleFollowButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const angRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const angEl = angRef.current;
    if (!btn || !angEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let angle = REST_ANGLE;
    let goalAngle = REST_ANGLE;
    let demo = !reduce;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const fromPoint = (x: number, y: number): number => {
      const r = btn.getBoundingClientRect();
      const dx = x - (r.left + r.width / 2);
      const dy = y - (r.top + r.height / 2);
      return wrap((Math.atan2(dx, -dy) * 180) / Math.PI);
    };
    const paint = (): void => {
      btn.style.setProperty("--ang", angle.toFixed(2));
      angEl.textContent = String(Math.round(wrap(angle)));
    };
    const stopDemo = (): void => { demo = false; };
    const onMove = (e: Event): void => {
      const p = e as unknown as PointLike;
      stopDemo();
      goalAngle = fromPoint(p.clientX, p.clientY);
      if (reduce) { angle = goalAngle; paint(); }
    };
    const onLeave = (): void => {
      goalAngle = REST_ANGLE;
      if (reduce) { angle = goalAngle; paint(); }
    };
    btn.addEventListener("pointermove", onMove);
    btn.addEventListener("pointerleave", onLeave);

    const demoGoal = (now: number): number => {
      if (now < START_MS) return REST_ANGLE;
      const c = (now - START_MS) % CYCLE_MS;
      if (c >= ORBIT_MS) return REST_ANGLE;
      return wrap(60 + (c / ORBIT_MS) * ORBIT_SPAN);
    };

    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 = demo ? demoGoal(now) : goalAngle;
      const diff = ((g - angle + 540) % 360) - 180;
      angle = wrap(angle + diff * (1 - Math.exp(-dt * FOLLOW_RATE)));
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("pointermove", onMove);
      btn.removeEventListener("pointerleave", onLeave);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bc}>
        <button className={styles.bc__btn} type="button" ref={btnRef}>방향 따라가기</button>
        <p className={styles.bc__meta}><span className={styles.bc__tag}>ANGLE</span> <b ref={angRef}>135</b><span>deg</span></p>
      </div>
    </main>
  );
}