04Wavy underline

<main class="stage">
  <p class="line">This is a <span class="hl">required</span> field.</p>
</main>

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

// ── 변수 3개 ─────────────────────────────
$duration: 1s;                 // 물결이 한 번 오가는 시간
$easing: $ease-out;
$color: $error;                // 경고 색 밑줄

// @property 로 <length> 타입을 등록해야 브라우저가 이 값을 보간(애니메이션)한다
@property --underline-offset {
  syntax: "<length>";
  inherits: true;
  initial-value: 1px;
}

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

.line {
  max-width: 92%;
  margin: 0;
  color: $subject-ink;   // 주황 무대 위 글자 — AA 4.5 (blog-design 글자 대비)
  font-weight: 600;
  font-size: clamp(16px, 4.6vw, 22px);
  line-height: 1.6;
  font-family: inherit;
  text-align: center;
}

// 네이티브 물결 밑줄 — text-decoration-style: wavy 가 곡선을 그리고,
// text-underline-offset 만 --underline-offset 을 통해 오간다
.hl {
  text-decoration-line: underline;
  text-decoration-style: wavy;
  text-decoration-color: $color;
  text-decoration-thickness: 2px;
  text-underline-offset: var(--underline-offset);
  animation: wave $duration linear infinite;
}

// 한 겹: 펜 두께 불균일 — thickness 진동을 wave 안에 싣는다(같은 요소라 별도 키프레임
// 대신 기존 0/50/100% 스텝은 유지하고 사이 스텝에 thickness 요동을 추가)
@keyframes wave {
  0%   { --underline-offset: 0px;   text-decoration-thickness: 2px;   text-decoration-color: $color; }
  18%  { --underline-offset: 1.8px; text-decoration-thickness: 3px; }
  34%  { --underline-offset: 3.4px; text-decoration-thickness: 2.2px; }
  50%  { --underline-offset: 5px;   text-decoration-thickness: 2.6px; text-decoration-color: color-mix(in srgb, $color 55%, white); }
  66%  { --underline-offset: 3.6px; text-decoration-thickness: 3.2px; }
  82%  { --underline-offset: 1.6px; text-decoration-thickness: 2px; }
  100% { --underline-offset: 0px;   text-decoration-thickness: 2px;   text-decoration-color: $color; }
}

@media (prefers-reduced-motion: reduce) {
  .hl { animation: none; text-underline-offset: 2px; }
}

@property --underline-offset {
  syntax: "<length>";
  inherits: true;
  initial-value: 1px;
}
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);
}

.line {
  max-width: 92%;
  margin: 0;
  color: #17141a;
  font-weight: 600;
  font-size: clamp(16px, 4.6vw, 22px);
  line-height: 1.6;
  font-family: inherit;
  text-align: center;
}

.hl {
  text-decoration-line: underline;
  text-decoration-style: wavy;
  text-decoration-color: #c2361a;
  text-decoration-thickness: 2px;
  text-underline-offset: var(--underline-offset);
  animation: wave 1s linear infinite;
}

@keyframes wave {
  0% {
    --underline-offset: 0px;
    text-decoration-thickness: 2px;
    text-decoration-color: #c2361a;
  }
  18% {
    --underline-offset: 1.8px;
    text-decoration-thickness: 3px;
  }
  34% {
    --underline-offset: 3.4px;
    text-decoration-thickness: 2.2px;
  }
  50% {
    --underline-offset: 5px;
    text-decoration-thickness: 2.6px;
    text-decoration-color: color-mix(in srgb, #c2361a 55%, white);
  }
  66% {
    --underline-offset: 3.6px;
    text-decoration-thickness: 3.2px;
  }
  82% {
    --underline-offset: 1.6px;
    text-decoration-thickness: 2px;
  }
  100% {
    --underline-offset: 0px;
    text-decoration-thickness: 2px;
    text-decoration-color: #c2361a;
  }
}
@media (prefers-reduced-motion: reduce) {
  .hl {
    animation: none;
    text-underline-offset: 2px;
  }
}

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

export type WavyUnderlineProps = {
  before: ReactNode;
  keyword: ReactNode;
  after?: ReactNode;
  /** 한 번 오가는 시간 (기본 1000ms) */
  duration?: number;
  /** 밑줄 색 */
  color?: string;
};

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

export default function WavyUnderline({
  before,
  keyword,
  after,
  duration = 1000,
  color = "#c2361a",
}: WavyUnderlineProps) {
  const vars: Vars = {
    "--duration": `${duration}ms`,
    "--color": color,
  };
  return (
    <p className={styles.line} style={vars}>
      {before} <span className={styles.hl}>{keyword}</span> {after}
    </p>
  );
}