01Shared-border merge — the inner border value between two adjoined buttons overlaps into one line so it never renders doubled

<main class="stage">
<div class="bg is-demo" id="bg" role="group" aria-label="Text align">
  <button class="bg__btn" data-align="left" aria-pressed="true">Left</button>
  <button class="bg__btn" data-align="center" aria-pressed="false">Center</button>
  <button class="bg__btn" data-align="right" aria-pressed="false">Right</button>
  <button class="bg__btn" data-align="justify" aria-pressed="false">Justify</button>
  <span class="bg__seam" id="seam0" aria-hidden="true"></span>
  <span class="bg__seam" id="seam1" aria-hidden="true"></span>
  <span class="bg__seam" id="seam2" aria-hidden="true"></span>
</div>
</main>

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

// 계산 겹: index.html 이 getBoundingClientRect 로 버튼 사이 공유 경계선의 실측 좌표를 재고,
// 클릭 파동(ripple)이 그 좌표 + BLEED_PX 만큼만 이웃으로 번지도록 clip-path 를 매 프레임 계산한다.
// 경계선 자체은 이 파일에서 음수 margin 으로 겹쳐 항상 한 겹으로만 보인다.
@include stage($stage-ink);

.bg {
  position: relative;
  display: flex;
  border-radius: $r-control;
  overflow: visible;
}

.bg__btn {
  position: relative;
  appearance: none;
  border: 1.5px solid rgba($subject-cream, .38);
  background: rgba($subject-cream, .06);
  color: $subject-cream;
  font-size: 13px;
  font-weight: 600;
  padding: $sp-2 $sp-4;
  cursor: pointer;
  overflow: hidden;
  transition: background-color $dur-quick $ease-out, color $dur-quick $ease-out, border-color $dur-quick $ease-out;

  // 공유 경계선 병합 — 이웃 버튼과 1.5px 만큼 겹쳐 안쪽 경계선이 항상 한 겹으로만 보인다
  & + & { margin-left: -1.5px; }
  &:first-child { border-radius: $r-control 0 0 $r-control; }
  &:last-child { border-radius: 0 $r-control $r-control 0; }
  &:hover, &:focus-visible { z-index: 1; }
  &:focus-visible { outline: 2px solid $subject-blue-light; outline-offset: 2px; }
}

.bg__btn--on {
  background: $subject-blue-deep;
  border-color: $subject-blue-deep;
  color: $subject-cream;
  z-index: 2;
}

.bg__ripple {
  position: absolute;
  border-radius: 50%;
  background: rgba($subject-cream, .5);
  pointer-events: none;
  will-change: width, height, opacity, clip-path;
}

.bg__seam {
  position: absolute;
  top: 4px;
  bottom: 4px;
  width: 1px;
  left: var(--seam-x, 0);
  transform: translateX(-50%);
  background: rgba($subject-cream, .16);
  pointer-events: none;
  z-index: 3;
}

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

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

.bg {
  position: relative;
  display: flex;
  border-radius: 12px;
  overflow: visible;
}

.bg__btn {
  position: relative;
  appearance: none;
  border: 1.5px solid rgba(255, 247, 230, 0.38);
  background: rgba(255, 247, 230, 0.06);
  color: #fff7e6;
  font-size: 13px;
  font-weight: 600;
  padding: 8px 16px;
  cursor: pointer;
  overflow: hidden;
  transition: background-color 220ms ease-out, color 220ms ease-out, border-color 220ms ease-out;
}
.bg__btn + .bg__btn {
  margin-left: -1.5px;
}
.bg__btn:first-child {
  border-radius: 12px 0 0 12px;
}
.bg__btn:last-child {
  border-radius: 0 12px 12px 0;
}
.bg__btn:hover, .bg__btn:focus-visible {
  z-index: 1;
}
.bg__btn:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

.bg__btn--on {
  background: #0a4fe8;
  border-color: #0a4fe8;
  color: #fff7e6;
  z-index: 2;
}

.bg__ripple {
  position: absolute;
  border-radius: 50%;
  background: rgba(255, 247, 230, 0.5);
  pointer-events: none;
  will-change: width, height, opacity, clip-path;
}

.bg__seam {
  position: absolute;
  top: 4px;
  bottom: 4px;
  width: 1px;
  left: var(--seam-x, 0);
  transform: translateX(-50%);
  background: rgba(255, 247, 230, 0.16);
  pointer-events: none;
  z-index: 3;
}

@media (max-width: 340px) {
  .bg__btn {
    font-size: 12px;
    padding: 8px 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bg__btn {
    transition: none;
  }
  .bg__ripple {
    display: none;
  }
}

(function () {
  var CYCLE_MS = 900; // 자동 루프가 다음 버튼으로 넘어가는 간격
  var BLEED_PX = 6; // 계산 겹 상수: 파동이 이웃 경계선을 넘어 번질 수 있는 최대 여유(px) — 이 값만큼만 넘고 그 이상은 실측 경계선에서 잘린다
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var RIPPLE_MS = reduce ? 1 : 420; // 계산 자체는 유지하되 reduce 에서는 목표 상태로 즉시 놓는다

  var root = document.getElementById('bg');
  var btns = Array.prototype.slice.call(root.querySelectorAll('.bg__btn'));
  var seams = [document.getElementById('seam0'), document.getElementById('seam1'), document.getElementById('seam2')];
  var isDemo = true;

  // 계산 겹: 버튼 사이 공유 경계선의 실제 화면 좌표를 getBoundingClientRect 로 잰다 — 짐작 px 를 쓰지 않는다.
  function measureSeams() {
    var rootRect = root.getBoundingClientRect();
    for (var i = 0; i < seams.length; i++) {
      var a = btns[i].getBoundingClientRect();
      var b = btns[i + 1].getBoundingClientRect();
      var seamX = (a.right + b.left) / 2 - rootRect.left; // 두 버튼이 겹치는 경계선의 실측 중점
      seams[i].style.setProperty('--seam-x', seamX.toFixed(2) + 'px');
    }
  }

  function setActive(btn) {
    btns.forEach(function (b) { b.setAttribute('aria-pressed', String(b === btn)); b.classList.toggle('bg__btn--on', b === btn); });
  }

  var ripples = [];
  function spawnRipple(btn, clientX) {
    var idx = btns.indexOf(btn);
    var rect = btn.getBoundingClientRect();
    var rootRect = root.getBoundingClientRect();
    var originX = (clientX == null ? rect.left + rect.width / 2 : clientX) - rect.left;
    var leftSeam = idx > 0 ? btns[idx - 1].getBoundingClientRect().right - rect.left : null;
    var rightSeam = idx < btns.length - 1 ? btns[idx + 1].getBoundingClientRect().left - rect.left : null;
    var span = document.createElement('span');
    span.className = 'bg__ripple';
    span.style.left = originX + 'px';
    span.style.top = (rect.height / 2) + 'px';
    btn.appendChild(span);
    ripples.push({
      el: span, start: performance.now(), from: 0, to: Math.max(rect.width, rect.height) * 1.3,
      // 계산 겹: 파동이 번질 수 있는 한계를 이웃 경계선 실측값 + BLEED_PX 로 고정한다
      minClip: leftSeam == null ? -9999 : leftSeam - BLEED_PX,
      maxClip: rightSeam == null ? 9999 : rightSeam + BLEED_PX,
      originX: originX
    });
  }

  function easeOutQuad(t) { return 1 - (1 - t) * (1 - t); }

  function tickRipples(ts) {
    for (var i = ripples.length - 1; i >= 0; i--) {
      var r = ripples[i];
      var t = Math.min(1, (ts - r.start) / RIPPLE_MS);
      var radius = r.from + (r.to - r.from) * easeOutQuad(t);
      var left = Math.max(r.minClip, r.originX - radius);
      var right = Math.min(r.maxClip, r.originX + radius);
      r.el.style.width = radius * 2 + 'px';
      r.el.style.height = radius * 2 + 'px';
      r.el.style.marginLeft = -radius + 'px';
      r.el.style.marginTop = -radius + 'px';
      r.el.style.opacity = String(1 - t);
      // clip-path 로 파동이 실측 경계선 밖으로 못 나가게 자른다
      r.el.style.clipPath = 'inset(-9999px ' + Math.max(0, r.originX + radius - right) + 'px -9999px ' + Math.max(0, left - (r.originX - radius)) + 'px)';
      if (t >= 1) { r.el.remove(); ripples.splice(i, 1); }
    }
  }

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

  btns.forEach(function (b) {
    b.addEventListener('click', function (e) {
      human();
      setActive(b);
      spawnRipple(b, reduce ? null : e.clientX);
    });
  });

  window.addEventListener('resize', measureSeams);
  measureSeams();

  var cycleStart = 0, lastIdx = -1;
  function loop(ts) {
    if (!cycleStart) cycleStart = ts;
    if (isDemo && !reduce) {
      var idx = Math.floor((ts - cycleStart) / CYCLE_MS) % btns.length;
      if (idx !== lastIdx) { lastIdx = idx; setActive(btns[idx]); spawnRipple(btns[idx], null); }
    }
    tickRipples(ts);
    requestAnimationFrame(loop);
  }
  requestAnimationFrame(loop);
})();

// SharedBorderMerge — vanilla 판(run/369/demo/01-btn-group-shared-border-merge/index.html) 의 React 포트.
// getBoundingClientRect 로 버튼 사이 공유 경계선의 실측 좌표를 재고, 클릭 파동을 그 좌표 + BLEED_PX
// 만큼만 이웃으로 번지도록 clip-path 로 매 프레임 계산해 자른다.
import { useEffect, useRef } from "react";
import styles from "./SharedBorderMerge.module.scss";

type Box<T> = { current: T };
type RippleState = { el: HTMLSpanElement; start: number; to: number; minClip: number; maxClip: number; originX: number };

const ALIGNS = ["left", "center", "right", "justify"] as const;
const LABELS: Record<string, string> = { left: "왼쪽", center: "중앙", right: "오른쪽", justify: "양쪽" };

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

  useEffect(() => {
    const CYCLE_MS = 900;
    const BLEED_PX = 6; // 계산 겹 상수: 파동이 이웃 경계선을 넘어 번질 수 있는 최대 여유(px)
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const RIPPLE_MS = reduce ? 1 : 420;

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

    let isDemo = true;
    const ripples: RippleState[] = [];

    function measureSeams() {
      const rootRect = root.getBoundingClientRect();
      for (let i = 0; i < seams.length; i++) {
        const a = btns[i].getBoundingClientRect();
        const b = btns[i + 1].getBoundingClientRect();
        const seamX = (a.right + b.left) / 2 - rootRect.left;
        seams[i].style.setProperty("--seam-x", seamX.toFixed(2) + "px");
      }
    }

    function setActive(btn: HTMLButtonElement) {
      btns.forEach((b) => {
        b.setAttribute("aria-pressed", String(b === btn));
        b.classList.toggle(styles.btnOn, b === btn);
      });
    }

    function spawnRipple(btn: HTMLButtonElement, clientX: number | null) {
      const idx = btns.indexOf(btn);
      const rect = btn.getBoundingClientRect();
      const originX = (clientX == null ? rect.left + rect.width / 2 : clientX) - rect.left;
      const leftSeam = idx > 0 ? btns[idx - 1].getBoundingClientRect().right - rect.left : null;
      const rightSeam = idx < btns.length - 1 ? btns[idx + 1].getBoundingClientRect().left - rect.left : null;
      const span = document.createElement("span");
      span.className = styles.ripple;
      span.style.left = originX + "px";
      span.style.top = rect.height / 2 + "px";
      btn.appendChild(span);
      ripples.push({
        el: span, start: performance.now(), to: Math.max(rect.width, rect.height) * 1.3,
        minClip: leftSeam == null ? -9999 : leftSeam - BLEED_PX,
        maxClip: rightSeam == null ? 9999 : rightSeam + BLEED_PX,
        originX,
      });
    }

    function easeOutQuad(t: number) { return 1 - (1 - t) * (1 - t); }

    function tickRipples(ts: number) {
      for (let i = ripples.length - 1; i >= 0; i--) {
        const r = ripples[i];
        const t = Math.min(1, (ts - r.start) / RIPPLE_MS);
        const radius = r.to * easeOutQuad(t);
        const left = Math.max(r.minClip, r.originX - radius);
        const right = Math.min(r.maxClip, r.originX + radius);
        r.el.style.width = radius * 2 + "px";
        r.el.style.height = radius * 2 + "px";
        r.el.style.marginLeft = -radius + "px";
        r.el.style.marginTop = -radius + "px";
        r.el.style.opacity = String(1 - t);
        r.el.style.clipPath = "inset(-9999px " + Math.max(0, r.originX + radius - right) + "px -9999px " + Math.max(0, left - (r.originX - radius)) + "px)";
        if (t >= 1) { r.el.remove(); ripples.splice(i, 1); }
      }
    }

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

    function onClick(this: HTMLButtonElement, e: MouseEvent) {
      human();
      setActive(this);
      spawnRipple(this, reduce ? null : e.clientX);
    }
    btns.forEach((b) => b.addEventListener("click", onClick as unknown as EventListener));

    window.addEventListener("resize", measureSeams);
    measureSeams();

    let cycleStart = 0, lastIdx = -1;
    function loop(ts: number) {
      if (!cycleStart) cycleStart = ts;
      if (isDemo && !reduce) {
        const idx = Math.floor((ts - cycleStart) / CYCLE_MS) % btns.length;
        if (idx !== lastIdx) { lastIdx = idx; setActive(btns[idx]); spawnRipple(btns[idx], null); }
      }
      tickRipples(ts);
      rafRef.current = requestAnimationFrame(loop);
    }
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      btns.forEach((b) => b.removeEventListener("click", onClick as unknown as EventListener));
      window.removeEventListener("resize", measureSeams);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bg + " " + styles.isDemo} ref={rootRef} role="group" aria-label="텍스트 정렬">
        {ALIGNS.map((align, i) => (
          <button
            key={align}
            className={styles.btn}
            data-align={align}
            aria-pressed={i === 0}
            ref={(el: HTMLButtonElement | null) => { if (el) btnRefs.current[i] = el; }}
          >
            {LABELS[align]}
          </button>
        ))}
        {[0, 1, 2].map((i) => (
          <span key={i} className={styles.seam} ref={(el: HTMLSpanElement | null) => { if (el) seamRefs.current[i] = el; }} aria-hidden="true" />
        ))}
      </div>
    </main>
  );
}