03:has(:focus)でグロー

<main class="stage">
  <div class="fg is-demo">
    <input class="fg__input" type="text" placeholder="メモを残す">
    <span class="fg__glow" aria-hidden="true"></span>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;                  // 글로우가 한 번 숨쉬는 시간
$easing: $ease-out;
$color: $subject-cream;                // 글로우 링 색

@include stage($stage-orange);

.fg { position: relative; width: min(240px, 82vw); }

.fg__input {
  box-sizing: border-box;
  width: 100%;
  border: 2px solid rgba($subject-cream, .4);
  border-radius: $r-control;
  padding: $sp-4 $sp-4;
  font-weight: 500;
  font-size: clamp(14px, 4vw, 16px);
  line-height: 1;
  font-family: inherit;
  color: $subject-cream;
  background: transparent;
  position: relative;
  z-index: 1;

  &:focus { outline: none; }
}

.fg__glow {
  position: absolute;
  inset: -$sp-2;
  border-radius: $r-card;
  border: 2px solid $color;
  opacity: 0;
  transform: scale(.92);
  pointer-events: none;
}

// :has() 로 부모(.fg)가 자식 입력의 실제 포커스 여부를 안다 — 형제가 아니어도 스타일링된다
.fg:has(.fg__input:focus) .fg__glow {
  animation: glow-in $dur-base cubic-bezier(.3, 1.6, .4, 1) both,
             glow-pulse $duration $easing $dur-base infinite;
}

// 스프링 오버슛 등장 — 링이 1.08 까지 뻗었다가 1 로 감아 들어간 뒤 숨쉬기로 넘어간다
@keyframes glow-in {
  0%   { opacity: 0;   transform: scale(.8); }
  60%  { opacity: .95; transform: scale(1.08); }
  100% { opacity: .35; transform: scale(.92); }
}

@keyframes glow-pulse {
  0%, 100% { opacity: .35; transform: scale(.92); }
  50%      { opacity: .9;  transform: scale(1.04); }
}

@media (prefers-reduced-motion: reduce) {
  .fg:has(.fg__input:focus) .fg__glow { animation: none; opacity: .7; transform: scale(1); }
}

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);
}

.fg {
  position: relative;
  width: min(240px, 82vw);
}

.fg__input {
  box-sizing: border-box;
  width: 100%;
  border: 2px solid rgba(255, 247, 230, 0.4);
  border-radius: 12px;
  padding: 16px 16px;
  font-weight: 500;
  font-size: clamp(14px, 4vw, 16px);
  line-height: 1;
  font-family: inherit;
  color: #fff7e6;
  background: transparent;
  position: relative;
  z-index: 1;
}
.fg__input:focus {
  outline: none;
}

.fg__glow {
  position: absolute;
  inset: -8px;
  border-radius: 16px;
  border: 2px solid #fff7e6;
  opacity: 0;
  transform: scale(0.92);
  pointer-events: none;
}

.fg:has(.fg__input:focus) .fg__glow {
  animation: glow-in 300ms cubic-bezier(0.3, 1.6, 0.4, 1) both, glow-pulse 2s ease-out 300ms infinite;
}

@keyframes glow-in {
  0% {
    opacity: 0;
    transform: scale(0.8);
  }
  60% {
    opacity: 0.95;
    transform: scale(1.08);
  }
  100% {
    opacity: 0.35;
    transform: scale(0.92);
  }
}
@keyframes glow-pulse {
  0%, 100% {
    opacity: 0.35;
    transform: scale(0.92);
  }
  50% {
    opacity: 0.9;
    transform: scale(1.04);
  }
}
@media (prefers-reduced-motion: reduce) {
  .fg:has(.fg__input:focus) .fg__glow {
    animation: none;
    opacity: 0.7;
    transform: scale(1);
  }
}

// .is-demo 자동 재생: 진짜 focus 를 오가며 :has(:focus) 를 그대로 탄다 (3줄)
var el = document.querySelector('.is-demo .fg__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, 1600);

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

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

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

export default function FocusGlow({
  placeholder = "Leave a note",
  duration = 2000,
  easing = "ease-out",
  color = "#fff7e6",
}: FocusGlowProps) {
  const [value, setValue] = useState("");
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

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