02배경 흐림 폴백 — 흐림이 안 먹는 환경에서 반투명 배경으로 대신 채우는 카드

<main class="stage">
<div class="wrap is-demo" id="wrap">
  <div class="backdrop"></div>
  <button class="openBtn" id="openBtn" type="button" aria-pressed="false">카드 열기</button>
  <div class="card" id="card">
    <p class="cardText">알림 3건</p>
  </div>
  <div class="foot">
    <span class="modeLabel" id="modeLabel">흐림 지원판</span>
    <button class="simToggle" id="simToggle" type="button" aria-pressed="false">폴백 흉내내기</button>
  </div>
</div>
</main>

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

// 계산 겹은 카드 열림 애니메이션의 스프링 오버슛(scale 0.86→1.05→1) 에 있다 — index.html 은
// FALLBACK_BG_ALPHA 로 폴백 배경의 진하기만 넘긴다. 여기는 두 배경 표기법(흐림 · 반투명 단색)과
// 오버슛 곡선만 맡는다.
@include stage($stage-yellow);

.wrap {
  position: relative;
  width: 240px;
  height: 150px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: $sp-3;
}

.backdrop {
  position: absolute;
  inset: 0;
  border-radius: $r-card;
  background:
    radial-gradient(circle at 20% 30%, rgba($subject-pink, .9), transparent 55%),
    radial-gradient(circle at 80% 70%, rgba($subject-sky, .9), transparent 55%),
    radial-gradient(circle at 50% 50%, rgba($subject-lilac, .85), transparent 60%);
}

.openBtn {
  position: relative;
  z-index: 2;
  appearance: none;
  border: none;
  border-radius: $r-pill;
  background: $subject-ink;
  color: $subject-cream;
  padding: $sp-1 $sp-4;
  font-size: 11px;
  font-weight: 600;
  cursor: pointer;
  transition: transform $dur-instant $ease-out;

  &:hover { transform: translateY(-1px); }
  &:active { transform: scale(.95); }
  &:focus-visible { outline: 2px solid $subject-blue-light; outline-offset: 2px; }
}

.card {
  position: relative;
  z-index: 2;
  width: 160px;
  padding: $sp-3 $sp-4;
  border-radius: $r-card;
  opacity: 0;
  transform: scale(.86);
  transition: transform $dur-base $ease-elastic, opacity $dur-quick $ease-out;

  // 폴백: 흐림이 없을 때 반투명 단색으로 채운다 — 항상 선언, @supports 가 지원판에서 덮어쓴다
  background: rgba($subject-ink, var(--fallback-alpha, .6));
}

@supports (backdrop-filter: blur(1px)) or (-webkit-backdrop-filter: blur(1px)) {
  .wrap:not(.force-fallback) .card {
    background: rgba($subject-ink, .28);
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
  }
}

.card.is-open {
  opacity: 1;
  transform: scale(1);
}

.cardText {
  margin: 0;
  color: $subject-cream;
  font-size: 13px;
  font-weight: 600;
  text-align: center;
}

.foot {
  position: relative;
  z-index: 2;
  display: flex;
  align-items: center;
  gap: $sp-2;
}

.modeLabel {
  color: $subject-ink;
  font-size: 10px;
  font-weight: 700;
  background: rgba($stage-paper, .82);
  padding: $sp-1 $sp-2;
  border-radius: $r-xs;
}

.simToggle {
  appearance: none;
  border: none;
  border-radius: $r-pill;
  background: rgba($subject-ink, .82);
  color: $subject-cream;
  padding: $sp-1 $sp-3;
  font-size: 10px;
  font-weight: 600;
  cursor: pointer;
  transition: transform $dur-instant $ease-out;

  &:active { transform: scale(.96); }
  &[aria-pressed="true"] { background: $subject-orange-deep; }
  &:focus-visible { outline: 2px solid $subject-blue-light; outline-offset: 2px; }
}

@media (max-width: 340px) {
  .wrap { width: 200px; height: 130px; }
  .card { width: 140px; }
}

@media (prefers-reduced-motion: reduce) {
  .card { transition: none; }
  .openBtn, .simToggle { transition: 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);
}

.wrap {
  position: relative;
  width: 240px;
  height: 150px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 12px;
}

.backdrop {
  position: absolute;
  inset: 0;
  border-radius: 16px;
  background: radial-gradient(circle at 20% 30%, rgba(255, 158, 187, 0.9), transparent 55%), radial-gradient(circle at 80% 70%, rgba(110, 200, 255, 0.9), transparent 55%), radial-gradient(circle at 50% 50%, rgba(185, 160, 255, 0.85), transparent 60%);
}

.openBtn {
  position: relative;
  z-index: 2;
  appearance: none;
  border: none;
  border-radius: 999px;
  background: #17141a;
  color: #fff7e6;
  padding: 4px 16px;
  font-size: 11px;
  font-weight: 600;
  cursor: pointer;
  transition: transform 140ms ease-out;
}
.openBtn:hover {
  transform: translateY(-1px);
}
.openBtn:active {
  transform: scale(0.95);
}
.openBtn:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

.card {
  position: relative;
  z-index: 2;
  width: 160px;
  padding: 12px 16px;
  border-radius: 16px;
  opacity: 0;
  transform: scale(0.86);
  transition: transform 300ms cubic-bezier(0.34, 1.56, 0.64, 1), opacity 220ms ease-out;
  background: rgba(23, 20, 26, var(--fallback-alpha, 0.6));
}

@supports (backdrop-filter: blur(1px)) or (-webkit-backdrop-filter: blur(1px)) {
  .wrap:not(.force-fallback) .card {
    background: rgba(23, 20, 26, 0.28);
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
  }
}
.card.is-open {
  opacity: 1;
  transform: scale(1);
}

.cardText {
  margin: 0;
  color: #fff7e6;
  font-size: 13px;
  font-weight: 600;
  text-align: center;
}

.foot {
  position: relative;
  z-index: 2;
  display: flex;
  align-items: center;
  gap: 8px;
}

.modeLabel {
  color: #17141a;
  font-size: 10px;
  font-weight: 700;
  background: rgba(255, 255, 255, 0.82);
  padding: 4px 8px;
  border-radius: 4px;
}

.simToggle {
  appearance: none;
  border: none;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.82);
  color: #fff7e6;
  padding: 4px 12px;
  font-size: 10px;
  font-weight: 600;
  cursor: pointer;
  transition: transform 140ms ease-out;
}
.simToggle:active {
  transform: scale(0.96);
}
.simToggle[aria-pressed=true] {
  background: #c93000;
}
.simToggle:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .wrap {
    width: 200px;
    height: 130px;
  }
  .card {
    width: 140px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .card {
    transition: none;
  }
  .openBtn, .simToggle {
    transition: none;
  }
}

(function () {
  var FALLBACK_BG_ALPHA = 0.6; // 흐림이 없는 환경에서 카드 배경을 채우는 반투명 진하기 — 변형 목표: 0.6 → 0.85
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var wrap = document.getElementById('wrap');
  var card = document.getElementById('card');
  var openBtn = document.getElementById('openBtn');
  var simToggle = document.getElementById('simToggle');
  var modeLabel = document.getElementById('modeLabel');
  var isDemo = true;

  card.style.setProperty('--fallback-alpha', FALLBACK_BG_ALPHA);

  function human() { if (isDemo) { wrap.classList.remove('is-demo'); isDemo = false; } }

  var open = false;
  function setOpen(on) {
    open = on;
    card.classList.toggle('is-open', on);
    openBtn.setAttribute('aria-pressed', String(on));
  }
  openBtn.addEventListener('click', function () { human(); setOpen(!open); });

  var simOn = false;
  function setSim(on) {
    simOn = on;
    wrap.classList.toggle('force-fallback', on);
    simToggle.setAttribute('aria-pressed', String(on));
    modeLabel.textContent = on ? '흐림 폴백판' : '흐림 지원판';
  }
  simToggle.addEventListener('click', function () { human(); setSim(!simOn); });

  var last = 0;
  var STEP_MS = 620;
  var phase = 0;
  function loop(ts) {
    if (!last) last = ts;
    if (isDemo && !reduce && ts - last >= STEP_MS) {
      last = ts;
      phase = (phase + 1) % 4;
      setOpen(phase === 1 || phase === 2);
      if (phase === 0) setSim(!simOn);
    }
    requestAnimationFrame(loop);
  }
  requestAnimationFrame(loop);
})();

// BackdropFilterPrefix — vanilla 판(run/375/demo/02-compat-backdrop-filter-prefix/index.html) 의 React 포트.
// 폴백 배경(반투명 단색)을 항상 선언하고 @supports(backdrop-filter) 가 지원판에서 흐림으로 덮는다.
import { useEffect, useRef } from "react";
import styles from "./BackdropFilterPrefix.module.scss";

type Box<T> = { current: T };

const FALLBACK_BG_ALPHA = 0.6; // 흐림이 없는 환경에서 카드 배경을 채우는 반투명 진하기 — 변형 목표: 0.6 → 0.85
const STEP_MS = 620;

export default function BackdropFilterPrefix() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const cardRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const openBtnRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const simToggleRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const modeLabelRef = useRef(null) as unknown as Box<HTMLSpanElement>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const wrap = rootRef.current;
    const card = cardRef.current;
    const openBtn = openBtnRef.current;
    const simToggle = simToggleRef.current;
    const modeLabel = modeLabelRef.current;
    if (!wrap || !card || !openBtn || !simToggle || !modeLabel) return;

    let isDemo = true;
    card.style.setProperty("--fallback-alpha", String(FALLBACK_BG_ALPHA));

    function human() { if (isDemo) { wrap!.classList.remove(styles.isDemo); isDemo = false; } }

    let open = false;
    function setOpen(on: boolean) {
      open = on;
      card!.classList.toggle(styles.isOpen, on);
      openBtn!.setAttribute("aria-pressed", String(on));
    }
    function onOpenClick() { human(); setOpen(!open); }
    openBtn.addEventListener("click", onOpenClick);

    let simOn = false;
    function setSim(on: boolean) {
      simOn = on;
      wrap!.classList.toggle(styles.forceFallback, on);
      simToggle!.setAttribute("aria-pressed", String(on));
      modeLabel!.textContent = on ? "흐림 폴백판" : "흐림 지원판";
    }
    function onSimClick() { human(); setSim(!simOn); }
    simToggle.addEventListener("click", onSimClick);

    let last = 0;
    let phase = 0;
    function loop(ts: number) {
      if (!last) last = ts;
      if (isDemo && !reduce && ts - last >= STEP_MS) {
        last = ts;
        phase = (phase + 1) % 4;
        setOpen(phase === 1 || phase === 2);
        if (phase === 0) setSim(!simOn);
      }
      rafRef.current = requestAnimationFrame(loop);
    }
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      openBtn!.removeEventListener("click", onOpenClick);
      simToggle!.removeEventListener("click", onSimClick);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.wrap + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.backdrop}></div>
        <button className={styles.openBtn} ref={openBtnRef} type="button" aria-pressed="false">카드 열기</button>
        <div className={styles.card} ref={cardRef}>
          <p className={styles.cardText}>알림 3건</p>
        </div>
        <div className={styles.foot}>
          <span className={styles.modeLabel} ref={modeLabelRef}>흐림 지원판</span>
          <button className={styles.simToggle} ref={simToggleRef} type="button" aria-pressed="false">폴백 흉내내기</button>
        </div>
      </div>
    </main>
  );
}