06下線が伸びる

<main class="stage">
  <div class="field is-demo">
    <label class="field__label" for="email">メールアドレス</label>
    <input class="field__input" id="email" type="email" placeholder="メール">
    <span class="field__bar" aria-hidden="true"></span>
  </div>
</main>

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

$duration: $dur-quick;                                  // 실제 포커스 시 밑줄이 벌어지는 시간
$easing: $ease-out;
$color: $subject-blue;                                  // 밑줄 색

@include stage($stage-yellow);
.stage { padding: clamp(6px, 2.4vw, 16px); }

.field {
  position: relative;
  width: min(220px, 70vw);
}
.field__label {
  display: block;
  margin-bottom: 4px;
  font-weight: 600; font-size: clamp(11px, 3vw, 12px); line-height: 1.3; font-family: inherit;
  color: rgba($stage-ink, .7);
}
.field__input {
  box-sizing: border-box;
  width: 100%;
  border: 0;
  border-bottom: 2px solid rgba($stage-ink, .25);
  background: transparent;
  padding: 10px 2px;
  font-weight: 500; font-size: clamp(13px, 3.6vw, 15px); line-height: 1.3; font-family: inherit;
  color: $stage-ink;
}
.field__bar {
  position: absolute;
  left: 50%; right: 50%; bottom: -2px;
  height: 2px;
  background: $color;
  transition: left $duration $easing, right $duration $easing;
}
.field__input:focus + .field__bar { left: 0; right: 0; }

// 데모 루프: 가운데서 시작 → 양쪽으로 채워짐 → 다시 가운데로
.field.is-demo .field__bar { animation: grow $dur-loop $easing infinite; }
@keyframes grow {
  0%, 10%  { left: 50%; right: 50%; }
  40%, 60% { left: 0; right: 0; }
  100%     { left: 50%; right: 50%; }
}

@media (prefers-reduced-motion: reduce) {
  .field__bar { transition: none; }
  .field.is-demo .field__bar { animation: none; left: 0; right: 0; }
}

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);
}

.stage {
  padding: clamp(6px, 2.4vw, 16px);
}

.field {
  position: relative;
  width: min(220px, 70vw);
}

.field__label {
  display: block;
  margin-bottom: 4px;
  font-weight: 600;
  font-size: clamp(11px, 3vw, 12px);
  line-height: 1.3;
  font-family: inherit;
  color: rgba(23, 20, 26, 0.7);
}

.field__input {
  box-sizing: border-box;
  width: 100%;
  border: 0;
  border-bottom: 2px solid rgba(23, 20, 26, 0.25);
  background: transparent;
  padding: 10px 2px;
  font-weight: 500;
  font-size: clamp(13px, 3.6vw, 15px);
  line-height: 1.3;
  font-family: inherit;
  color: #17141a;
}

.field__bar {
  position: absolute;
  left: 50%;
  right: 50%;
  bottom: -2px;
  height: 2px;
  background: #2f6df6;
  transition: left 220ms ease-out, right 220ms ease-out;
}

.field__input:focus + .field__bar {
  left: 0;
  right: 0;
}

.field.is-demo .field__bar {
  animation: grow 2s ease-out infinite;
}

@keyframes grow {
  0%, 10% {
    left: 50%;
    right: 50%;
  }
  40%, 60% {
    left: 0;
    right: 0;
  }
  100% {
    left: 50%;
    right: 50%;
  }
}
@media (prefers-reduced-motion: reduce) {
  .field__bar {
    transition: none;
  }
  .field.is-demo .field__bar {
    animation: none;
    left: 0;
    right: 0;
  }
}

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

export type UnderlineGrowFieldProps = {
  label: string;
  placeholder: string;
  /** 실제 포커스 시 밑줄이 벌어지는 시간 (기본 220ms) */
  duration?: number;
  easing?: string;
  /** 밑줄 색 */
  color?: string;
};

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

export default function UnderlineGrowField({
  label,
  placeholder,
  duration = 220,
  easing = "ease-out",
  color = "#2f6df6",
}: UnderlineGrowFieldProps) {
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };
  const id = "field-" + label.replace(/\s+/g, "-").toLowerCase();

  return (
    <div className={styles.field} style={vars}>
      <label className={styles.label} htmlFor={id}>{label}</label>
      <input className={styles.input} id={id} type="text" placeholder={placeholder} />
      <span className={styles.bar} aria-hidden="true" />
    </div>
  );
}