02フォーカスで下線が伸びる

<main class="stage">
  <div class="ug is-demo">
    <span class="ug__label">ウェブサイト</span>
    <input class="ug__input" type="url" placeholder="https://example.com" autocomplete="url">
    <span class="ug__underline" aria-hidden="true"></span>
    <span class="ug__bloom" aria-hidden="true"></span>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-base;                  // 밑줄이 자라는 시간
$easing: cubic-bezier(.2, 1.5, .35, 1); // 스프링 오버슛 — scaleX 가 1 을 지나 1.08 까지 뻗었다가 돌아온다
$color: $subject-ink;                  // 밑줄 색

@include stage($stage-yellow);

.ug {
  position: relative;
  width: min(240px, 82vw);
  padding-bottom: $sp-3;
}
.ug__label {
  display: block;
  margin-bottom: $sp-2;
  font-weight: 700;
  font-size: 13px;
  line-height: 1;
  font-family: inherit;
  color: rgba($stage-ink, .7);
}
.ug__input {
  box-sizing: border-box;
  width: 100%;
  border: 0;
  border-bottom: 2px solid rgba($stage-ink, .3);
  padding: $sp-2 $sp-1;
  font-weight: 500;
  font-size: clamp(14px, 4vw, 16px);
  line-height: 1;
  font-family: inherit;
  color: $stage-ink;
  background: transparent;

  &:focus { outline: none; }
}
.ug__underline {
  position: absolute;
  left: 0; right: 0; bottom: 0;
  height: 2px;
  background: $color;
  transform: scaleX(0);
  transform-origin: center;
  transition: transform $duration $easing;
}

// 실사용: 입력이나 그 자식 어떤 곳에 포커스가 가도(:focus-within) 밑줄이 자란다
.ug:focus-within .ug__underline { transform: scaleX(1); }

// 포커스 순간 밑줄 중심에서 아래로 번지는 radial 글로우 확산 한 겹
.ug__bloom {
  position: absolute;
  left: 0; right: 0; bottom: -10px;
  height: 30px;
  background: radial-gradient(ellipse 60% 100% at 50% 0%, rgba($color, .25) 0%, rgba($color, 0) 72%);
  opacity: 0;
  pointer-events: none;
}
.ug:focus-within .ug__bloom { animation: ug-bloom $dur-base $ease-out; }

@keyframes ug-bloom {
  0%   { opacity: 0;  transform: scaleX(.4); }
  40%  { opacity: 1;  transform: scaleX(1); }
  100% { opacity: 0;  transform: scaleX(1.15); }
}

@media (prefers-reduced-motion: reduce) {
  .ug__underline { transition: none; }
  .ug:focus-within .ug__bloom { animation: none; }
}

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);
}

.ug {
  position: relative;
  width: min(240px, 82vw);
  padding-bottom: 12px;
}

.ug__label {
  display: block;
  margin-bottom: 8px;
  font-weight: 700;
  font-size: 13px;
  line-height: 1;
  font-family: inherit;
  color: rgba(23, 20, 26, 0.7);
}

.ug__input {
  box-sizing: border-box;
  width: 100%;
  border: 0;
  border-bottom: 2px solid rgba(23, 20, 26, 0.3);
  padding: 8px 4px;
  font-weight: 500;
  font-size: clamp(14px, 4vw, 16px);
  line-height: 1;
  font-family: inherit;
  color: #17141a;
  background: transparent;
}
.ug__input:focus {
  outline: none;
}

.ug__underline {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 2px;
  background: #17141a;
  transform: scaleX(0);
  transform-origin: center;
  transition: transform 300ms cubic-bezier(0.2, 1.5, 0.35, 1);
}

.ug:focus-within .ug__underline {
  transform: scaleX(1);
}

.ug__bloom {
  position: absolute;
  left: 0;
  right: 0;
  bottom: -10px;
  height: 30px;
  background: radial-gradient(ellipse 60% 100% at 50% 0%, rgba(23, 20, 26, 0.25) 0%, rgba(23, 20, 26, 0) 72%);
  opacity: 0;
  pointer-events: none;
}

.ug:focus-within .ug__bloom {
  animation: ug-bloom 300ms ease-out;
}

@keyframes ug-bloom {
  0% {
    opacity: 0;
    transform: scaleX(0.4);
  }
  40% {
    opacity: 1;
    transform: scaleX(1);
  }
  100% {
    opacity: 0;
    transform: scaleX(1.15);
  }
}
@media (prefers-reduced-motion: reduce) {
  .ug__underline {
    transition: none;
  }
  .ug:focus-within .ug__bloom {
    animation: none;
  }
}

// .is-demo 자동 재생: 진짜 focus 를 오가며 :focus-within 을 그대로 탄다 (3줄)
var el = document.querySelector('.is-demo .ug__input');
var t = function () { document.activeElement === el ? el.blur() : el.focus({ preventScroll: true }); };  // preventScroll — iframe 안 포커스가 홈 그리드의 부모 스크롤을 끌어당기지 않게 (사용자 2026-09-10 실측)
t(); setInterval(t, 450);

import { useState } from "react";
import type { CSSProperties, ChangeEvent } from "react";
import styles from "./UnderlineGrow.module.scss";

export type UnderlineGrowProps = {
  label?: string;
  placeholder?: string;
  duration?: number;
  easing?: string;
  color?: string;
};

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

export default function UnderlineGrow({
  label = "Website",
  placeholder = "https://example.com",
  duration = 300,
  easing = "cubic-bezier(.2,1.5,.35,1)",
  color = "#17141a",
}: UnderlineGrowProps) {
  const [value, setValue] = useState("");
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  return (
    <div className={styles.ug} style={vars}>
      <span className={styles.label}>{label}</span>
      <input
        className={styles.input}
        type="url"
        placeholder={placeholder}
        value={value}
        onChange={(e: ChangeEvent<HTMLInputElement>) => setValue(e.target.value)}
      />
      <span className={styles.underline} aria-hidden="true" />
      <span className={styles.bloom} aria-hidden="true" />
    </div>
  );
}