03Gạch chân trượt

<main class="stage"><a class="link is-demo" href="#">Tìm hiểu thêm</a></main>

// ── 변수 3개 ──────────────────────────────────────────
$thickness: 3px;
$duration: 260ms;
$color: #fff7e6;

html, body { margin: 0; overflow: hidden; }          // iframe 안에서 스크롤바가 생기지 않게 (실측 2026-09-09: body 여백 8px 이 100vh 를 넘겼다)
.stage { min-height: 100vh; display: grid; place-items: center; margin: 0; background: #c89a8f; font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif; }

.link {
  position: relative;
  display: inline-block;
  padding: 6px 2px;
  font-weight: 800;
  font-size: 30px;
  line-height: 1.2;
  font-family: inherit;
  color: $color;
  text-decoration: none;

  &::after {
    content: "";
    position: absolute;
    left: 0; right: 0; bottom: 0;
    height: $thickness;
    background: currentColor;
    transform: scaleX(0);
    transform-origin: right;                     // 떠날 때는 오른쪽으로 빠진다
    transition: transform $duration cubic-bezier(.2, .8, .2, 1);
  }
  &:hover::after, &:focus-visible::after {
    transform: scaleX(1);
    transform-origin: left;                      // 들어올 때는 왼쪽에서 그어진다
  }

  &.is-demo::after { animation: underline-loop 2s cubic-bezier(.2, .8, .2, 1) infinite; }
}

@keyframes underline-loop {
  0%        { transform: scaleX(0); transform-origin: left; } // 규칙8: 언더라인 왼쪽 진입
  35%, 55%  { transform: scaleX(1); transform-origin: left; } // 규칙8: 언더라인 유지 좌기준
  56%       { transform: scaleX(1); transform-origin: right; } // 규칙8: 기준 오른쪽으로 전환
  90%, 100% { transform: scaleX(0); transform-origin: right; } // 규칙8: 언더라인 오른쪽 소멸
}

@media (prefers-reduced-motion: reduce) {
  .link::after, .link.is-demo::after { transition: none; animation: none; }
  .link:hover::after { transform: scaleX(1); }
}

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

.stage {
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #c89a8f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.link {
  position: relative;
  display: inline-block;
  padding: 6px 2px;
  font-weight: 800;
  font-size: 30px;
  line-height: 1.2;
  font-family: inherit;
  color: #fff7e6;
  text-decoration: none;
}
.link::after {
  content: "";
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 3px;
  background: currentColor;
  transform: scaleX(0);
  transform-origin: right;
  transition: transform 260ms cubic-bezier(0.2, 0.8, 0.2, 1);
}
.link:hover::after, .link:focus-visible::after {
  transform: scaleX(1);
  transform-origin: left;
}
.link.is-demo::after {
  animation: underline-loop 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@keyframes underline-loop {
  0% {
    transform: scaleX(0);
    transform-origin: left;
  }
  35%, 55% {
    transform: scaleX(1);
    transform-origin: left;
  }
  56% {
    transform: scaleX(1);
    transform-origin: right;
  }
  90%, 100% {
    transform: scaleX(0);
    transform-origin: right;
  }
}
@media (prefers-reduced-motion: reduce) {
  .link::after, .link.is-demo::after {
    transition: none;
    animation: none;
  }
  .link:hover::after {
    transform: scaleX(1);
  }
}

document.querySelectorAll('.link').forEach(function (b) {
  b.addEventListener('pointerenter', function () { b.classList.remove('is-demo'); });
  b.addEventListener('click', function (e) { e.preventDefault(); });
});

import type { CSSProperties, ReactNode } from "react";
import styles from "./UnderlineLink.module.scss";

export type UnderlineLinkProps = { children: ReactNode; href: string; thickness?: number; duration?: number; color?: string };

export default function UnderlineLink({ children, href, thickness = 3, duration = 260, color = "currentColor" }: UnderlineLinkProps) {
  const vars = { "--thickness": `${thickness}px`, "--duration": `${duration}ms`, "--color": color } as CSSProperties;
  return (
    <a className={styles.link} href={href} style={vars}>
      {children}
    </a>
  );
}