01거리 값만큼 후광이 넓어지는 버튼

<main class="stage">
<div class="pg is-demo" id="pg">
  <p class="pg__badge" id="pgBadge">거리 값 100%</p>
  <div class="pg__scene" id="pgScene">
    <button class="pg__btn" id="pgBtn" type="button">지금 시작하기</button>
  </div>
</div>
</main>

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

// 핵심: pointermove 로 커서–버튼 중심 거리(px)를 재고 1 - clamp(d / PROX_RADIUS_PX) 로
// 거리 값(0~1)을 만든다. 그 값이 후광의 blur-radius·spread·밝기를 함께 키운다.
// 계산 한 겹: 매 프레임 lerp 로 목표값을 관성 있게 뒤쫓는다(속성 트윈이 아니라 계산).

$duration: $dur-loop;
$easing: linear;
$color: $subject-blue-deep;

@include stage($stage-ink);

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

.pg__badge {
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 247, 230, .1);
  color: $subject-cream;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.pg__scene {
  padding: $sp-10;
}

.pg__btn {
  border: 0;
  cursor: pointer;
  padding: $sp-4 $sp-8;
  border-radius: $r-control;
  background: $color;
  color: $subject-cream;
  font-size: 16px;
  font-weight: 700;
  will-change: box-shadow, filter;
}

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

@media (max-width: 340px) {
  .pg__scene { padding: $sp-6; }
  .pg__btn { padding: $sp-3 $sp-6; font-size: 14px; }
}

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

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

.pg__badge {
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.1);
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.pg__scene {
  padding: 40px;
}

.pg__btn {
  border: 0;
  cursor: pointer;
  padding: 16px 32px;
  border-radius: 12px;
  background: #0a4fe8;
  color: #fff7e6;
  font-size: 16px;
  font-weight: 700;
  will-change: box-shadow, filter;
}

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

@media (max-width: 340px) {
  .pg__scene {
    padding: 24px;
  }
  .pg__btn {
    padding: 12px 24px;
    font-size: 14px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .pg__btn {
    transition: none;
  }
}

(function () {
  // PROX_GLOW_RADIUS_MAX_PX — 변형 1: 40 → 80 로 늘리면 후광이 버튼 전체를 덮을 만큼 크게 번진다.
  var PROX_GLOW_RADIUS_MAX_PX = 40;
  var PROX_RADIUS_PX = 170;
  var LERP = 0.18;

  var root = document.getElementById('pg');
  var scene = document.getElementById('pgScene');
  var btn = document.getElementById('pgBtn');
  var badge = document.getElementById('pgBadge');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

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

  var targetP = 0;
  var curP = 0;
  var hasPointer = false;

  function distanceToButton(x, y) {
    var r = btn.getBoundingClientRect();
    var cx = r.left + r.width / 2;
    var cy = r.top + r.height / 2;
    return Math.hypot(x - cx, y - cy);
  }

  function render(p) {
    var blurRadius = p * PROX_GLOW_RADIUS_MAX_PX;
    var spread = p * PROX_GLOW_RADIUS_MAX_PX * 0.3;
    var alpha = (0.18 + p * 0.55).toFixed(2);
    btn.style.boxShadow = '0 0 ' + blurRadius.toFixed(1) + 'px ' + spread.toFixed(1) + 'px rgba(255,247,230,' + alpha + ')';
    btn.style.filter = 'brightness(' + (1 + p * 0.22).toFixed(3) + ')';
    badge.textContent = '거리 값 ' + Math.round((1 - p) * 100) + '%';
  }

  // 자동 시연: 가상 커서가 궤도(반지름이 2초 동안 0 → 최대 → 0)를 돌며 접근·이탈을 만든다.
  function syntheticOffset(t) {
    var ang = t * 1.7;
    var rad = 110 * (1 + Math.cos(t * Math.PI));
    return [Math.cos(ang) * rad, Math.sin(ang) * rad];
  }

  var startTime = null;
  function loop(now) {
    if (startTime === null) { startTime = now; }
    var t = (now - startTime) / 1000;
    if (!hasPointer && !reduce) {
      var r = btn.getBoundingClientRect();
      var cx = r.left + r.width / 2;
      var cy = r.top + r.height / 2;
      var off = syntheticOffset(t);
      var d = Math.hypot(off[0], off[1]);
      targetP = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
      void cx; void cy;
    }
    curP += (targetP - curP) * LERP;
    render(curP);
    requestAnimationFrame(loop);
  }

  scene.addEventListener('pointermove', function (e) {
    hasPointer = true;
    root.classList.remove('is-demo');
    var d = distanceToButton(e.clientX, e.clientY);
    targetP = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
    if (reduce) { curP = targetP; render(curP); }
  });
  scene.addEventListener('pointerleave', function () {
    hasPointer = false;
    targetP = 0;
    if (reduce) { curP = 0; render(0); }
  });
  btn.addEventListener('focus', function () {
    hasPointer = true;
    targetP = 1;
    if (reduce) { curP = 1; render(1); }
  });
  btn.addEventListener('blur', function () {
    hasPointer = false;
    targetP = 0;
    if (reduce) { curP = 0; render(0); }
  });

  if (reduce) {
    render(0);
  } else {
    requestAnimationFrame(loop);
  }
})();

// ProximityGlowButton — vanilla 판(run/408/demo/01-btn-proximity-glow-radius-grow/index.html) 의 React 포트.
// 버튼까지 거리 값(0~1)을 lerp 로 관성 있게 따라가며 후광(box-shadow blur·spread)과 밝기(filter)를 계산한다.
// 포인터가 없을 때(자동 시연)는 궤도 반지름이 2초 동안 0 → 최대 → 0 으로 도는 가상 커서 거리로 같은 계산을 돌린다.
import { useEffect, useRef, useState } from "react";
import type { PointerEvent as ReactPointerEvent } from "react";
import styles from "./ProximityGlowButton.module.scss";

type Box<T> = { current: T };

// PROX_GLOW_RADIUS_MAX_PX — 변형 1: 40 → 80 로 늘리면 후광이 버튼 전체를 덮을 만큼 크게 번진다.
const PROX_GLOW_RADIUS_MAX_PX = 40;
const PROX_RADIUS_PX = 170;
const LERP = 0.18;

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

function syntheticOffset(t: number): [number, number] {
  const ang = t * 1.7;
  const rad = 110 * (1 + Math.cos(t * Math.PI));
  return [Math.cos(ang) * rad, Math.sin(ang) * rad];
}

export default function ProximityGlowButton() {
  const sceneRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const badgeRef = useRef<HTMLParagraphElement | null>(null);
  const targetPRef = useRef<number>(0) as Box<number>;
  const curPRef = 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 [demo, setDemo] = useState(true);
  const [reduce, setReduce] = useState(false);

  function render(p: number): void {
    const btn = btnRef.current;
    const badge = badgeRef.current;
    if (!btn || !badge) return;
    const blurRadius = p * PROX_GLOW_RADIUS_MAX_PX;
    const spread = p * PROX_GLOW_RADIUS_MAX_PX * 0.3;
    const alpha = (0.18 + p * 0.55).toFixed(2);
    btn.style.boxShadow = "0 0 " + blurRadius.toFixed(1) + "px " + spread.toFixed(1) + "px rgba(255,247,230," + alpha + ")";
    btn.style.filter = "brightness(" + (1 + p * 0.22).toFixed(3) + ")";
    badge.textContent = "거리 값 " + Math.round((1 - p) * 100) + "%";
  }

  function distanceToButton(x: number, y: number): number {
    const btn = btnRef.current;
    if (!btn) return PROX_RADIUS_PX;
    const r = btn.getBoundingClientRect();
    const cx = r.left + r.width / 2;
    const cy = r.top + r.height / 2;
    return Math.hypot(x - cx, y - cy);
  }

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

  useEffect(() => {
    if (reduce) {
      render(0);
      return;
    }
    function loop(now: number): void {
      if (t0Ref.current === 0) t0Ref.current = now;
      const t = (now - t0Ref.current) / 1000;
      if (!hasPointerRef.current) {
        const off = syntheticOffset(t);
        const d = Math.hypot(off[0], off[1]);
        targetPRef.current = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
      }
      curPRef.current += (targetPRef.current - curPRef.current) * LERP;
      render(curPRef.current);
      frameRef.current = requestAnimationFrame(loop);
    }
    frameRef.current = requestAnimationFrame(loop);
    return () => cancelAnimationFrame(frameRef.current);
  }, [reduce]);

  function onPointerMove(event: ReactPointerEvent<HTMLDivElement>): void {
    hasPointerRef.current = true;
    setDemo(false);
    const d = distanceToButton(event.clientX, event.clientY);
    targetPRef.current = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
    if (reduce) { curPRef.current = targetPRef.current; render(curPRef.current); }
  }

  function onPointerLeave(): void {
    hasPointerRef.current = false;
    targetPRef.current = 0;
    if (reduce) { curPRef.current = 0; render(0); }
  }

  function onFocus(): void {
    hasPointerRef.current = true;
    targetPRef.current = 1;
    if (reduce) { curPRef.current = 1; render(1); }
  }

  function onBlur(): void {
    hasPointerRef.current = false;
    targetPRef.current = 0;
    if (reduce) { curPRef.current = 0; render(0); }
  }

  const cls = [styles.pg, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls}>
        <p className={styles.badge} ref={badgeRef}>거리 값 100%</p>
        <div className={styles.scene} ref={sceneRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <button className={styles.btn} ref={btnRef} type="button" onFocus={onFocus} onBlur={onBlur}>지금 시작하기</button>
        </div>
      </div>
    </main>
  );
}