03Tab underline synced to scroll

<main class="stage">
  <div class="tus is-demo">
    <nav class="tus__nav" id="tusNav">
      <a class="tus__tab is-active" href="#overview" data-i="0">Overview</a>
      <a class="tus__tab" href="#report" data-i="1">Report</a>
      <a class="tus__tab" href="#setting" data-i="2">Settings</a>
      <span class="tus__underline" aria-hidden="true"></span>
    </nav>
    <div class="tus__panel">The underline slides to whichever tab matches the section scrolling past</div>
  </div>
</main>

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

// ── 변수 3개. 변형은 이것만 바꾼다 ──────────────────────────
$duration: $dur-base;                              // 밑줄이 미끄러지는 시간
$easing: $ease-spring;
$color: $subject-cream;                             // 밑줄·활성 탭 색

@include stage($stage-orange);

.tus { width: min(320px, 88vw); }

.tus__nav {
  position: relative;
  display: flex;
  // 탭마다 내용 폭만큼만 차지해야 실측 offsetLeft·offsetWidth 가 의미 있다 (BD19) — 균등분배 금지
  flex-wrap: nowrap;
}
.tus__tab {
  flex: 0 0 auto;
  padding: $sp-2 $sp-4;
  text-decoration: none;
  white-space: nowrap;
  font-weight: 600;
  font-size: clamp(13px, 3.6vw, 15px);
  line-height: 1;
  font-family: inherit;
  color: $subject-ink;   // 주황 무대 위 탭 글자 — AA 4.5 (rgba(cream,.65) 는 2.04:1 이었다)
  &.is-active { color: $subject-ink; font-weight: 800; }
}
// 폭은 고정(칩 "translateX 고정폭") — 탭마다 넓어지지 않는다. translateX 만 탭 중심으로 움직인다
.tus__underline {
  position: absolute; left: 0; bottom: 0;
  width: 24px;
  height: 3px;
  background: $color;
  border-radius: $r-pill;
  transform: translateX(18.7265625px);
  transition: transform $duration $easing;
}

// ── 데모(.is-demo): 세 탭 중심을 옮겨 다니는 2초 루프 ──────────────
// translateX 값 = 480×300 렌더에서 Playwright getBoundingClientRect 로 실측한 탭 left+width/2 - 밑줄폭/2
// (탭0 개요 left 0·폭 61.453125 / 탭1 리포트 left 61.453125·폭 76.171875 / 탭2 설정 left 137.625·폭 61.453125 — 짐작 56/76/56 류 금지, BD19)
.tus.is-demo .tus__underline { animation: tus-loop $dur-loop $easing infinite; }
@keyframes tus-loop {
  0%, 15%  { transform: translateX(18.7265625px); }
  33%, 48% { transform: translateX(87.5390625px); }
  66%, 81% { transform: translateX(156.3515625px); }
  100%     { transform: translateX(18.7265625px); }
}

.tus__panel {
  margin-top: $sp-4;
  min-height: 32px;
  font-weight: 400;
  font-size: clamp(12px, 3.2vw, 13px);
  line-height: 1.5;
  font-family: inherit;
  color: $subject-ink;   // 주황 무대 위 패널 글자 — AA 4.5
}

// ── 접근성 ──────────────────────────────────────────
@media (prefers-reduced-motion: reduce) {
  .tus__underline { transition: none; }
  .tus.is-demo .tus__underline { animation: none !important; transform: translateX(18.7265625px); }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.tus {
  width: min(320px, 88vw);
}

.tus__nav {
  position: relative;
  display: flex;
  flex-wrap: nowrap;
}

.tus__tab {
  flex: 0 0 auto;
  padding: 8px 16px;
  text-decoration: none;
  white-space: nowrap;
  font-weight: 600;
  font-size: clamp(13px, 3.6vw, 15px);
  line-height: 1;
  font-family: inherit;
  color: #17141a;
}
.tus__tab.is-active {
  color: #17141a;
  font-weight: 800;
}

.tus__underline {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 24px;
  height: 3px;
  background: #fff7e6;
  border-radius: 999px;
  transform: translateX(18.7265625px);
  transition: transform 300ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.tus.is-demo .tus__underline {
  animation: tus-loop 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@keyframes tus-loop {
  0%, 15% {
    transform: translateX(18.7265625px);
  }
  33%, 48% {
    transform: translateX(87.5390625px);
  }
  66%, 81% {
    transform: translateX(156.3515625px);
  }
  100% {
    transform: translateX(18.7265625px);
  }
}
.tus__panel {
  margin-top: 16px;
  min-height: 32px;
  font-weight: 400;
  font-size: clamp(12px, 3.2vw, 13px);
  line-height: 1.5;
  font-family: inherit;
  color: #17141a;
}

@media (prefers-reduced-motion: reduce) {
  .tus__underline {
    transition: none;
  }
  .tus.is-demo .tus__underline {
    animation: none !important;
    transform: translateX(18.7265625px);
  }
}

// 실제 페이지: IntersectionObserver 로 지금 통과 중인 섹션을 찾아 그 탭에 밑줄을 옮긴다.
// 탭 폭이 서로 달라 offsetLeft·offsetWidth 를 실측해 옮긴다 — 짐작 px 금지 (BD19)
var nav = document.getElementById('tusNav');
var tabs = nav.querySelectorAll('.tus__tab');
var underline = nav.querySelector('.tus__underline');
function moveTo(tab) {
  var w = underline.offsetWidth;
  underline.style.transform = 'translateX(' + (tab.offsetLeft + tab.offsetWidth / 2 - w / 2) + 'px)';
}
tabs.forEach(function (t) { t.addEventListener('click', function (e) { e.preventDefault(); document.querySelector('.tus').classList.remove('is-demo'); moveTo(t); }); });

import { useEffect, useRef, useState } from "react";
import type { CSSProperties, MouseEvent } from "react";
import styles from "./TabUnderlineSync.module.scss";

export type TabUnderlineSyncTab = { key: string; label: string; href: string };

export type TabUnderlineSyncProps = {
  tabs: TabUnderlineSyncTab[];
  /** 밑줄이 미끄러지는 시간 (기본 300ms) */
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };

export default function TabUnderlineSync({
  tabs,
  duration = 300,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#fff7e6",
}: TabUnderlineSyncProps) {
  const navRef = useRef<HTMLElement>(null);
  const [active, setActive] = useState(0);
  const [x, setX] = useState(0);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  // 탭 폭이 서로 달라 offsetLeft·offsetWidth 를 실측해 밑줄 중심을 옮긴다 — 짐작 px 금지 (BD19)
  useEffect(() => {
    const nav = navRef.current;
    if (!nav) return;
    const tab = nav.querySelectorAll("a")[active] as HTMLElement | undefined;
    const underline = nav.querySelector<HTMLElement>("[data-underline]");
    if (tab && underline) {
      const w = underline.offsetWidth;
      setX(tab.offsetLeft + tab.offsetWidth / 2 - w / 2);
    }
  }, [active, tabs]);

  return (
    <nav ref={navRef} className={styles.nav} style={vars}>
      {tabs.map((t, i) => (
        <a
          key={t.key}
          href={t.href}
          className={i === active ? `${styles.tab} ${styles.isActive}` : styles.tab}
          onClick={(e: MouseEvent<HTMLAnchorElement>) => { e.preventDefault(); setActive(i); }}
        >
          {t.label}
        </a>
      ))}
      <span data-underline className={styles.underline} style={{ transform: `translateX(${x}px)` }} />
    </nav>
  );
}