06Wavy underline

<main class="stage">
  <a class="link is-demo" href="#">Today's pick</a>
</main>

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

// ── 변수 3개 ─────────────────────────────
$duration: $dur-quick;
$easing:   $ease-out;
$color:    $stage-ink;

// ── 데모 무대 ───────────────────────────────
@include stage($stage-yellow);

.link {
  position: relative;
  display: inline-block;
  padding: $sp-2 $sp-1 $sp-3;
  color: $color;
  text-decoration: none;
  font-weight: 700; font-size: clamp(18px, 5vw, 26px); line-height: 1.2;

  // 물결은 네이티브 wavy 가 아니라 두 겹 radial-gradient 로 그린다 (repeat-x, currentColor 상속)
  &::after {
    content: "";
    position: absolute;
    left: 0; right: 0; bottom: 0;
    height: 10px;
    background-image:
      radial-gradient(circle at 10px -5px, transparent 12px, currentColor 13px, currentColor 14px, transparent 15px),
      radial-gradient(circle at 30px 5px, transparent 12px, currentColor 13px, currentColor 14px, transparent 15px);
    background-repeat: repeat-x;
    background-size: 40px 10px;
    background-position: 0 0, 0 0;
    opacity: 0;
    transition: opacity $duration $easing;
  }
  // 실제 브라우저 — 등장은 opacity 트랜지션, 흐름은 호버 중에만 도는 별도 애니메이션
  &:hover::after, &:focus-visible::after {
    opacity: 1;
    animation: wave-flow 1.2s linear infinite;
  }

  &.is-demo::after {
    animation: wave-loop $dur-loop $easing infinite;
  }
}

@keyframes wave-flow {
  0%   { background-position: 0 0, 0 0; }
  100% { background-position: -40px 0, -40px 0; }
}

@keyframes wave-loop {
  0%   { opacity: 0; background-position: 0 0, 0 0; }
  15%  { opacity: 1; background-position: 0 0, 0 0; }
  85%  { opacity: 1; background-position: -40px 0, -40px 0; }
  100% { opacity: 0; background-position: -40px 0, -40px 0; }
}

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

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);
}

.link {
  position: relative;
  display: inline-block;
  padding: 8px 4px 12px;
  color: #17141a;
  text-decoration: none;
  font-weight: 700;
  font-size: clamp(18px, 5vw, 26px);
  line-height: 1.2;
}
.link::after {
  content: "";
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 10px;
  background-image: radial-gradient(circle at 10px -5px, transparent 12px, currentColor 13px, currentColor 14px, transparent 15px), radial-gradient(circle at 30px 5px, transparent 12px, currentColor 13px, currentColor 14px, transparent 15px);
  background-repeat: repeat-x;
  background-size: 40px 10px;
  background-position: 0 0, 0 0;
  opacity: 0;
  transition: opacity 220ms ease-out;
}
.link:hover::after, .link:focus-visible::after {
  opacity: 1;
  animation: wave-flow 1.2s linear infinite;
}
.link.is-demo::after {
  animation: wave-loop 2s ease-out infinite;
}

@keyframes wave-flow {
  0% {
    background-position: 0 0, 0 0;
  }
  100% {
    background-position: -40px 0, -40px 0;
  }
}
@keyframes wave-loop {
  0% {
    opacity: 0;
    background-position: 0 0, 0 0;
  }
  15% {
    opacity: 1;
    background-position: 0 0, 0 0;
  }
  85% {
    opacity: 1;
    background-position: -40px 0, -40px 0;
  }
  100% {
    opacity: 0;
    background-position: -40px 0, -40px 0;
  }
}
@media (prefers-reduced-motion: reduce) {
  .link::after, .link.is-demo::after {
    transition: none;
    animation: none;
  }
  .link:hover::after, .link:focus-visible::after {
    opacity: 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 "./WavyUnderline.module.scss";

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

export default function WavyUnderline({ children, href, duration = 220, color = "currentColor" }: WavyUnderlineProps) {
  const vars = { "--duration": `${duration}ms`, "--color": color } as CSSProperties;
  return (
    <a className={styles.link} href={href} style={vars}>
      {children}
    </a>
  );
}