06距離値の分だけ枠線が描かれるボタン

<main class="stage">
<div class="bt is-demo" id="bt">
  <p class="bt__badge" id="btBadge">縁の値 0%</p>
  <div class="bt__scene" id="btScene">
    <div class="bt__wrap" id="btWrap">
      <svg class="bt__ring" id="btRing" viewBox="0 0 200 64" preserveAspectRatio="none" aria-hidden="true">
        <rect id="btRect" x="2" y="2" width="196" height="60" rx="12" ry="12"></rect>
      </svg>
      <button class="bt__btn" id="btBtn" type="button">もっと見る</button>
    </div>
  </div>
</div>
</main>

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

// 핵심: SVG rect 의 실제 둘레 길이(getTotalLength)를 재서 stroke-dasharray 에 쓰고,
// 거리 값이 줄어드는 만큼 stroke-dashoffset 을 그 길이만큼 되돌려 테두리가 한 바퀴 그려지게 한다.

$duration: $dur-loop;
$easing: linear;
$color: $subject-ink;

@include stage($stage-yellow);

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

.bt__badge {
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(23, 20, 26, .12);
  color: $subject-ink;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.bt__scene {
  padding: $sp-10;
}

.bt__wrap {
  position: relative;
  width: 200px;
  height: 64px;
  --p: 0;
}

.bt__wrap::before {
  content: '';
  position: absolute;
  inset: -16px;
  border-radius: calc($r-control + 16px);
  background: radial-gradient(circle, rgba($subject-blue-deep, .5), transparent 70%);
  filter: blur(calc(4px + var(--p) * 18px));
  opacity: var(--p);
  pointer-events: none;
  z-index: 0;
}

.bt__ring {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  overflow: visible;
  z-index: 1;

  rect {
    fill: none;
    stroke: $subject-blue-deep;
    stroke-width: 3;
    will-change: stroke-dashoffset;
  }
}

.bt__btn {
  position: relative;
  z-index: 2;
  width: 100%;
  height: 100%;
  border: 0;
  cursor: pointer;
  border-radius: $r-control;
  background: linear-gradient(calc(var(--p) * 360deg), $color 0%, $subject-blue-deep calc(15% + var(--p) * 55%), $color 100%);
  color: $subject-cream;
  font-size: 15px;
  font-weight: 700;
}

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

@media (max-width: 340px) {
  .bt__scene { padding: $sp-6; }
  .bt__wrap { width: 168px; height: 52px; }
}

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

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

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

.bt__badge {
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.12);
  color: #17141a;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.bt__scene {
  padding: 40px;
}

.bt__wrap {
  position: relative;
  width: 200px;
  height: 64px;
  --p: 0;
}

.bt__wrap::before {
  content: "";
  position: absolute;
  inset: -16px;
  border-radius: 28px;
  background: radial-gradient(circle, rgba(10, 79, 232, 0.5), transparent 70%);
  filter: blur(calc(4px + var(--p) * 18px));
  opacity: var(--p);
  pointer-events: none;
  z-index: 0;
}

.bt__ring {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  overflow: visible;
  z-index: 1;
}
.bt__ring rect {
  fill: none;
  stroke: #0a4fe8;
  stroke-width: 3;
  will-change: stroke-dashoffset;
}

.bt__btn {
  position: relative;
  z-index: 2;
  width: 100%;
  height: 100%;
  border: 0;
  cursor: pointer;
  border-radius: 12px;
  background: linear-gradient(calc(var(--p) * 360deg), #17141a 0%, #0a4fe8 calc(15% + var(--p) * 55%), #17141a 100%);
  color: #fff7e6;
  font-size: 15px;
  font-weight: 700;
}

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

@media (max-width: 340px) {
  .bt__scene {
    padding: 24px;
  }
  .bt__wrap {
    width: 168px;
    height: 52px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bt__ring rect {
    transition: none;
  }
}

(function () {
  var PROX_RADIUS_PX = 170;
  var LERP = 0.18;

  var root = document.getElementById('bt');
  var scene = document.getElementById('btScene');
  var wrap = document.getElementById('btWrap');
  var btn = document.getElementById('btBtn');
  var rect = document.getElementById('btRect');
  var badge = document.getElementById('btBadge');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

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

  // 실제 도형 둘레 길이(px) — 짐작값이 아니라 SVG 가 직접 잰 값이다.
  var totalLength = rect.getTotalLength();
  rect.style.strokeDasharray = String(totalLength);

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

  function distanceToButton(x, y) {
    var r = wrap.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 offset = (1 - p) * totalLength;
    rect.style.strokeDashoffset = String(offset);
    rect.style.opacity = String(0.2 + p * 0.8);
    badge.textContent = '縁の値 ' + Math.round(p * 100) + '%';
    // 계산 레이어: 같은 진행률(p)을 링 주변 글로우·버튼 배경 하이라이트 각(그라디언트 각)에도 전파한다.
    wrap.style.setProperty('--p', p.toFixed(3));
  }

  function syntheticOffset(t) {
    var ang = t * 1.6;
    var rad = 120 * (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 off = syntheticOffset(t);
      var d = Math.hypot(off[0], off[1]);
      targetP = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
    }
    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);
  }
})();

// ProximityBorderTraceButton — vanilla 판(run/408/demo/06-btn-proximity-border-trace-draw/index.html) 의 React 포트.
// SVG rect 의 실제 둘레 길이(getTotalLength)를 재서 stroke-dasharray 에 쓰고, 거리 값이 줄어드는
// 만큼 stroke-dashoffset 을 그 길이만큼 되돌려 테두리가 한 바퀴 그려지게 한다 — 짐작값이 아니라 실측값이다.
import { useEffect, useRef, useState } from "react";
import type { PointerEvent as ReactPointerEvent } from "react";
import styles from "./ProximityBorderTraceButton.module.scss";

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

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 * 1.6;
  const rad = 120 * (1 + Math.cos(t * Math.PI));
  return [Math.cos(ang) * rad, Math.sin(ang) * rad];
}

export default function ProximityBorderTraceButton() {
  const sceneRef = useRef<HTMLDivElement | null>(null);
  const wrapRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const rectRef = useRef<SVGRectElement | null>(null);
  const badgeRef = useRef<HTMLParagraphElement | null>(null);
  const totalLengthRef = useRef<number>(0) as Box<number>;
  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 rect = rectRef.current;
    const badge = badgeRef.current;
    const wrap = wrapRef.current;
    if (!rect || !badge || !wrap) return;
    const offset = (1 - p) * totalLengthRef.current;
    rect.style.strokeDashoffset = String(offset);
    rect.style.opacity = String(0.2 + p * 0.8);
    // 계산 레이어: 같은 진행률(p)을 링 글로우·버튼 배경 그라디언트 각도에도 전파한다.
    wrap.style.setProperty("--p", p.toFixed(3));
    badge.textContent = "테두리 값 " + Math.round(p * 100) + "%";
  }

  function distanceToButton(x: number, y: number): number {
    const wrap = wrapRef.current;
    if (!wrap) return PROX_RADIUS_PX;
    const r = wrap.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(() => {
    const rect = rectRef.current;
    if (!rect) return;
    // 실제 도형 둘레 길이(px) — 짐작값이 아니라 SVG 가 직접 잰 값이다.
    totalLengthRef.current = rect.getTotalLength();
    rect.style.strokeDasharray = String(totalLengthRef.current);

    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.bt, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls}>
        <p className={styles.badge} ref={badgeRef}>테두리 값 0%</p>
        <div className={styles.scene} ref={sceneRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <div className={styles.wrap} ref={wrapRef}>
            <svg className={styles.ring} viewBox="0 0 200 64" preserveAspectRatio="none" aria-hidden="true">
              <rect ref={rectRef} x={2} y={2} width={196} height={60} rx={12} ry={12} />
            </svg>
            <button className={styles.btn} ref={btnRef} type="button" onFocus={onFocus} onBlur={onBlur}>더 보기</button>
          </div>
        </div>
      </div>
    </main>
  );
}