06下線グロー

<main class="stage is-demo">
  <nav class="nav" aria-hidden="true">
    <span class="tab"></span>
    <span class="tab"></span>
    <span class="indicator"></span>
  </nav>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;   // 2s — 왕복 한 번
$easing: $ease-spring;
$color: #2df5c0;        // 네온 틸

@include stage($stage-ink);

// 탭 두 개가 같은 고정 폭이라 인디케이터는 그 폭+간격만큼만 옮기면 된다 (짐작 없는 값 — BD19)
$tab-w: 96px;
$tab-gap: $sp-4;

.nav { position: relative; display: flex; gap: $tab-gap; }

.tab {
  flex: 0 0 auto;
  width: $tab-w; height: $sp-8;
  border-radius: $r-sm;
  background: rgba($stage-paper, .1);
}

.indicator {
  position: absolute; left: 0; bottom: -$sp-2;
  width: $tab-w; height: 3px;
  border-radius: $r-xs;
  background: linear-gradient(90deg, transparent, $color, transparent);
  box-shadow: 0 0 $sp-3 $color, 0 0 $sp-6 rgba($color, .6);
}

// translateX 만 움직인다 — 폭이 고정값이라 도착 지점도 고정값이다
.stage.is-demo .indicator {
  animation: slide $duration $easing infinite;
}

@keyframes slide {
  0%, 40%  { transform: translateX(0); }
  60%, 90% { transform: translateX(#{$tab-w + $tab-gap}); }
  100%     { transform: translateX(0); }
}

@media (prefers-reduced-motion: reduce) {
  .stage.is-demo .indicator { animation: 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);
}

.nav {
  position: relative;
  display: flex;
  gap: 16px;
}

.tab {
  flex: 0 0 auto;
  width: 96px;
  height: 32px;
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.1);
}

.indicator {
  position: absolute;
  left: 0;
  bottom: -8px;
  width: 96px;
  height: 3px;
  border-radius: 4px;
  background: linear-gradient(90deg, transparent, #2df5c0, transparent);
  box-shadow: 0 0 12px #2df5c0, 0 0 24px rgba(45, 245, 192, 0.6);
}

.stage.is-demo .indicator {
  animation: slide 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@keyframes slide {
  0%, 40% {
    transform: translateX(0);
  }
  60%, 90% {
    transform: translateX(112px);
  }
  100% {
    transform: translateX(0);
  }
}
@media (prefers-reduced-motion: reduce) {
  .stage.is-demo .indicator {
    animation: none;
  }
}

import { useState } from "react";
import type { CSSProperties } from "react";
import styles from "./UnderlineGlow.module.scss";

export type UnderlineGlowProps = {
  /** 슬라이드 시간 (기본 300ms) */
  duration?: number;
  easing?: string;
  color?: string;
  labels?: [string, string];
};

export default function UnderlineGlow({
  duration = 300,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#2df5c0",
  labels = ["탭 1", "탭 2"],
}: UnderlineGlowProps) {
  const [active, setActive] = useState(0);
  const vars: CSSProperties = { "--duration": `${duration}ms`, "--easing": easing, "--color": color } as CSSProperties;

  return (
    <nav className={styles.nav} style={vars}>
      {labels.map((label, i) => (
        <button
          key={label}
          type="button"
          className={styles.tab}
          aria-current={active === i}
          onClick={() => setActive(i)}
        >
          {label}
        </button>
      ))}
      <span className={styles.indicator} data-active={active} />
    </nav>
  );
}