03Divider fade — the opacity value of the line between buttons fades as the pointer nears an adjacent button

<main class="stage">
<div class="dfh is-demo" id="dfh" role="group" aria-label="View switch">
  <button class="dfh__btn">Grid</button>
  <span class="dfh__seam" id="seam0" aria-hidden="true"></span>
  <button class="dfh__btn">List</button>
  <span class="dfh__seam" id="seam1" aria-hidden="true"></span>
  <button class="dfh__btn">Cards</button>
</div>
</main>

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

// 계산 겹: index.html 이 포인터(x) 와 각 구분선의 실측 중점 사이 거리 값을 매 프레임 재고,
// smoothstep falloff 로 목표 투명도를 구한 뒤 지수 완화(lerp)로 순간이동 없이 따라가게 한다.
@include stage($stage-yellow);

.dfh {
  position: relative;
  display: flex;
  align-items: center;
  border-radius: $r-control;
  background: rgba($subject-ink, .06);
  padding: $sp-1;
}

.dfh__btn {
  appearance: none;
  border: none;
  background: transparent;
  color: $subject-ink;
  font-size: 13px;
  font-weight: 700;
  padding: $sp-2 $sp-4;
  cursor: pointer;
  border-radius: $r-sm;
  transition: background-color $dur-quick $ease-out;

  &:hover, &:focus-visible { background: rgba($subject-ink, .08); }
  &:focus-visible { outline: 2px solid $subject-blue; outline-offset: 2px; }
}

.dfh__seam {
  position: relative;
  width: 4px;
  height: 28px;
  align-self: center;
  opacity: 1;
  will-change: opacity;

  &::before {
    content: "";
    position: absolute;
    inset: 0;
    border-radius: $r-xs;
    background: rgba($subject-ink, .42);
  }

  &::after {
    content: "";
    position: absolute;
    top: 50%;
    left: 50%;
    width: 26px;
    height: 26px;
    transform: translate(-50%, -50%);
    border-radius: 50%;
    background: rgba($subject-ink, .14);
    filter: blur(5px);
  }
}

@media (max-width: 340px) {
  .dfh__btn { font-size: 12px; padding: $sp-2 $sp-3; }
}

@media (prefers-reduced-motion: reduce) {
  .dfh__btn { transition: none; }
  .dfh__seam { opacity: 1 !important; }
}

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);
}

.dfh {
  position: relative;
  display: flex;
  align-items: center;
  border-radius: 12px;
  background: rgba(23, 20, 26, 0.06);
  padding: 4px;
}

.dfh__btn {
  appearance: none;
  border: none;
  background: transparent;
  color: #17141a;
  font-size: 13px;
  font-weight: 700;
  padding: 8px 16px;
  cursor: pointer;
  border-radius: 8px;
  transition: background-color 220ms ease-out;
}
.dfh__btn:hover, .dfh__btn:focus-visible {
  background: rgba(23, 20, 26, 0.08);
}
.dfh__btn:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

.dfh__seam {
  position: relative;
  width: 4px;
  height: 28px;
  align-self: center;
  opacity: 1;
  will-change: opacity;
}
.dfh__seam::before {
  content: "";
  position: absolute;
  inset: 0;
  border-radius: 4px;
  background: rgba(23, 20, 26, 0.42);
}
.dfh__seam::after {
  content: "";
  position: absolute;
  top: 50%;
  left: 50%;
  width: 26px;
  height: 26px;
  transform: translate(-50%, -50%);
  border-radius: 50%;
  background: rgba(23, 20, 26, 0.14);
  filter: blur(5px);
}

@media (max-width: 340px) {
  .dfh__btn {
    font-size: 12px;
    padding: 8px 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .dfh__btn {
    transition: none;
  }
  .dfh__seam {
    opacity: 1 !important;
  }
}

(function () {
  var FALLOFF_PX = 46; // 계산 겹 상수: 포인터 거리 값이 이 px 안이면 구분선 투명도가 낮아지기 시작한다
  var SMOOTH = 0.22; // 계산 겹 상수: 지수 완화(lerp) 비율 — 값이 순간이동하지 않고 따라간다
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var root = document.getElementById('dfh');
  var seams = [document.getElementById('seam0'), document.getElementById('seam1')];
  var isDemo = true;
  var pointerX = null;
  var opac = [1, 1]; // 계산 겹: 각 구분선의 현재 렌더 투명도(완화된 값)

  function seamCenters() {
    var rootRect = root.getBoundingClientRect();
    return seams.map(function (s) {
      var r = s.getBoundingClientRect();
      return r.left + r.width / 2 - rootRect.left;
    });
  }

  // 계산 겹: 포인터 거리 값 → 투명도 목표값(0~1)을 매끄러운 falloff 곡선으로 계산
  function targetOpacity(dist) {
    var t = Math.min(1, Math.max(0, dist / FALLOFF_PX));
    return t * t * (3 - 2 * t); // smoothstep — 선형보다 자연스러운 감쇠
  }

  function human() {
    if (isDemo) { root.classList.remove('is-demo'); isDemo = false; }
  }

  root.addEventListener('pointermove', function (e) {
    human();
    pointerX = e.clientX - root.getBoundingClientRect().left;
  });
  root.addEventListener('pointerleave', function () { pointerX = null; });

  var demoStart = 0;
  function demoPointerX(ts, width) {
    var t = (ts - demoStart) / 1000;
    return width / 2 + Math.sin(t * 1.1) * (width / 2 - 10); // 좌우로 오가는 시뮬레이션 좌표
  }

  function loop(ts) {
    if (!demoStart) demoStart = ts;
    var rect = root.getBoundingClientRect();
    var px = isDemo && !reduce ? demoPointerX(ts, rect.width) : pointerX;
    var centers = seamCenters();

    for (var i = 0; i < seams.length; i++) {
      var target = px == null ? 1 : targetOpacity(Math.abs(px - centers[i]));
      opac[i] += (target - opac[i]) * (reduce ? 1 : SMOOTH);
      seams[i].style.opacity = opac[i].toFixed(3);
    }

    requestAnimationFrame(loop);
  }
  requestAnimationFrame(loop);
})();

// DividerFadeOnHover — vanilla 판(run/369/demo/03-btn-group-divider-fade-on-hover/index.html) 의 React 포트.
// 포인터(x) 와 각 구분선의 실측 중점 사이 거리 값을 매 프레임 재고, smoothstep falloff 로 목표
// 투명도를 구한 뒤 지수 완화(lerp)로 순간이동 없이 따라가게 한다.
import { useEffect, useRef } from "react";
import styles from "./DividerFadeOnHover.module.scss";

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

export default function DividerFadeOnHover() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const seamRefs = useRef([]) as unknown as Box<HTMLSpanElement[]>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const FALLOFF_PX = 46; // 계산 겹 상수: 포인터 거리 값이 이 px 안이면 구분선 투명도가 낮아지기 시작한다
    const SMOOTH = 0.22; // 계산 겹 상수: 지수 완화(lerp) 비율
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    const root = rootRef.current;
    const seams = seamRefs.current;
    if (!root || seams.length === 0) return;

    let isDemo = true;
    let pointerX: number | null = null;
    const opac = [1, 1];

    function seamCenters() {
      const rootRect = root!.getBoundingClientRect();
      return seams.map((s) => {
        const r = s.getBoundingClientRect();
        return r.left + r.width / 2 - rootRect.left;
      });
    }

    function targetOpacity(dist: number) {
      const t = Math.min(1, Math.max(0, dist / FALLOFF_PX));
      return t * t * (3 - 2 * t);
    }

    function human() { if (isDemo) { root!.classList.remove(styles.isDemo); isDemo = false; } }

    function onMove(e: PointerEvent) { human(); pointerX = e.clientX - root!.getBoundingClientRect().left; }
    function onLeave() { pointerX = null; }
    root.addEventListener("pointermove", onMove);
    root.addEventListener("pointerleave", onLeave);

    let demoStart = 0;
    function demoPointerX(ts: number, width: number) {
      const t = (ts - demoStart) / 1000;
      return width / 2 + Math.sin(t * 1.1) * (width / 2 - 10);
    }

    function loop(ts: number) {
      if (!demoStart) demoStart = ts;
      const rect = root!.getBoundingClientRect();
      const px = isDemo && !reduce ? demoPointerX(ts, rect.width) : pointerX;
      const centers = seamCenters();

      for (let i = 0; i < seams.length; i++) {
        const target = px == null ? 1 : targetOpacity(Math.abs(px - centers[i]));
        opac[i] += (target - opac[i]) * (reduce ? 1 : SMOOTH);
        seams[i].style.opacity = opac[i].toFixed(3);
      }

      rafRef.current = requestAnimationFrame(loop);
    }
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      root.removeEventListener("pointermove", onMove);
      root.removeEventListener("pointerleave", onLeave);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.dfh + " " + styles.isDemo} ref={rootRef} role="group" aria-label="보기 전환">
        <button className={styles.btn}>그리드</button>
        <span className={styles.seam} ref={(el: HTMLSpanElement | null) => { if (el) seamRefs.current[0] = el; }} aria-hidden="true" />
        <button className={styles.btn}>리스트</button>
        <span className={styles.seam} ref={(el: HTMLSpanElement | null) => { if (el) seamRefs.current[1] = el; }} aria-hidden="true" />
        <button className={styles.btn}>카드</button>
      </div>
    </main>
  );
}