08Fallback side-by-side — two panes take the same radius value, the native result beside a border-radius approximation

<main class="stage">
<div class="wrap is-demo" id="wrap">
  <div class="row">
    <figure class="cell">
      <div class="box native" id="nativeBox"></div>
      <figcaption class="cap" id="capNative">Supported environment</figcaption>
    </figure>
    <figure class="cell">
      <div class="box shim" id="shimBox"></div>
      <figcaption class="cap">Unsupported fallback</figcaption>
    </figure>
  </div>
  <p class="readout" id="readout" aria-live="polite">Current radius 20.0</p>
</div>
</main>

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

// 계산 겹은 index.html 에 있다 — 코사인 왕복으로 두 박스의 반경을 같은 값으로 움직여 차이를 겨룬다.
@include stage($stage-paper);

.wrap {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-4;
}

.row {
  display: flex;
  gap: $sp-6;
}

.cell {
  margin: 0;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-2;
}

.box {
  width: 120px;
  height: 92px;
  background: $subject-lilac;
}

.native {
  background: $subject-blue-deep;
}

.cap {
  font-size: 12px;
  font-weight: 700;
  color: $subject-ink;
}

.readout {
  margin: 0;
  color: $subject-ink;
  font-size: 13px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .box { width: 96px; height: 76px; }
  .row { gap: $sp-4; }
}

@media (prefers-reduced-motion: reduce) {
  .box { transition: none; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.wrap {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.row {
  display: flex;
  gap: 24px;
}

.cell {
  margin: 0;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
}

.box {
  width: 120px;
  height: 92px;
  background: #b9a0ff;
}

.native {
  background: #0a4fe8;
}

.cap {
  font-size: 12px;
  font-weight: 700;
  color: #17141a;
}

.readout {
  margin: 0;
  color: #17141a;
  font-size: 13px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .box {
    width: 96px;
    height: 76px;
  }
  .row {
    gap: 16px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .box {
    transition: none;
  }
}

(function () {
  var R_MIN = 6; // Radius 하한(px)
  var R_MAX = 34; // Radius 상한(px)
  var SWEEP_MS = 2000; // 반경이 하한↔상한을 왕복하는 주기
  var SUPER_N = 2.6; // 슈퍼타원 지수 고정값 — 원과 사각형 사이
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var SUPPORTS = CSS.supports('corner-shape', 'superellipse(1)');

  var wrap = document.getElementById('wrap');
  var nativeBox = document.getElementById('nativeBox');
  var shimBox = document.getElementById('shimBox');
  var capNative = document.getElementById('capNative');
  var readout = document.getElementById('readout');
  var isDemo = true;
  var paused = false;
  var frozenT = 0;

  // 캡션을 실제 지원 여부로 갱신 — 미지원 환경에서 열면 두 칸 다 폴백으로 표시된다
  if (!SUPPORTS) capNative.textContent = 'Unsupported fallback';

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

  // 계산 겹: 반경을 코사인으로 왕복시킨다 — 사인이 아니라 위상을 맞춘 코사인으로 0초에 하한에서 시작한다
  function radiusAt(t) {
    var c = (1 - Math.cos(t * Math.PI * 2)) / 2;
    return R_MIN + (R_MAX - R_MIN) * c;
  }

  function applyBoth(r) {
    if (SUPPORTS) {
      nativeBox.style.cornerShape = 'superellipse(' + SUPER_N.toFixed(1) + ')';
      nativeBox.style.borderRadius = r.toFixed(2) + 'px';
    } else {
      nativeBox.style.borderRadius = r.toFixed(2) + 'px';
    }
    // 오른쪽 칸은 언제나 border-radius 근사만 쓴다 — 두 렌더링의 차이를 나란히 겨룬다
    shimBox.style.borderRadius = r.toFixed(2) + 'px';
    readout.textContent = 'Current radius ' + r.toFixed(1);
  }

  wrap.addEventListener('click', function () {
    human();
    paused = !paused;
    if (paused) frozenT = performance.now();
  });
  wrap.addEventListener('keydown', function (e) {
    if (e.key === ' ' || e.key === 'Enter') { e.preventDefault(); wrap.click(); }
  });

  var start = 0;
  function loop(ts) {
    if (!start) start = ts;
    var t;
    if (reduce) {
      t = 0.5; // 상태(상한)에 고정 — 코사인 계산은 그대로 쓴다
    } else if (paused) {
      t = ((frozenT - start) % SWEEP_MS) / SWEEP_MS;
    } else {
      t = ((ts - start) % SWEEP_MS) / SWEEP_MS;
    }
    applyBoth(radiusAt(t));
    requestAnimationFrame(loop);
  }
  requestAnimationFrame(loop);
})();

// FallbackPolyfill — vanilla 판(run/372/demo/08-corner-shape-fallback-polyfill/index.html) 의 React 포트.
// 반경을 코사인으로 왕복시킨다 — 사인이 아니라 위상을 맞춘 코사인으로 0초에 하한에서 시작한다.
import { useEffect, useRef } from "react";
import styles from "./FallbackPolyfill.module.scss";

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

const R_MIN = 6; // 반경 하한(px)
const R_MAX = 34; // 반경 상한(px)
const SWEEP_MS = 2000; // 반경이 하한↔상한을 왕복하는 주기
const SUPER_N = 2.6; // 슈퍼타원 지수 고정값 — 원과 사각형 사이

export default function FallbackPolyfill() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const nativeRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const shimRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const capRef = useRef(null) as unknown as Box<HTMLElement>;
  const readoutRef = useRef(null) as unknown as Box<HTMLParagraphElement>;
  const rafRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const SUPPORTS = CSS.supports("corner-shape", "superellipse(1)");

    const root = rootRef.current;
    const nativeBox = nativeRef.current;
    const shimBox = shimRef.current;
    const capNative = capRef.current;
    const readout = readoutRef.current;
    if (!root || !nativeBox || !shimBox || !capNative || !readout) return;

    let isDemo = true;
    let paused = false;
    let frozenT = 0;

    // 캡션을 실제 지원 여부로 갱신 — 미지원 환경에서 열면 두 칸 다 폴백으로 표시된다
    if (!SUPPORTS) capNative.textContent = "미지원 폴백";

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

    // 계산 겹: 반경을 코사인으로 왕복시킨다 — 사인이 아니라 위상을 맞춘 코사인으로 0초에 하한에서 시작한다
    function radiusAt(t: number) {
      const c = (1 - Math.cos(t * Math.PI * 2)) / 2;
      return R_MIN + (R_MAX - R_MIN) * c;
    }

    function applyBoth(r: number) {
      if (SUPPORTS) {
        (nativeBox!.style as any).cornerShape = "superellipse(" + SUPER_N.toFixed(1) + ")";
      }
      nativeBox!.style.borderRadius = r.toFixed(2) + "px";
      shimBox!.style.borderRadius = r.toFixed(2) + "px";
      readout!.textContent = "지금 반경 " + r.toFixed(1);
    }

    function onClick() {
      human();
      paused = !paused;
      if (paused) frozenT = performance.now();
    }
    function onKeyDown(e: KeyboardEvent) {
      if (e.key === " " || e.key === "Enter") { e.preventDefault(); onClick(); }
    }
    root.addEventListener("click", onClick);
    root.addEventListener("keydown", onKeyDown);

    let start = 0;
    function loop(ts: number) {
      if (!start) start = ts;
      let t: number;
      if (reduce) {
        t = 0.5;
      } else if (paused) {
        t = ((frozenT - start) % SWEEP_MS) / SWEEP_MS;
      } else {
        t = ((ts - start) % SWEEP_MS) / SWEEP_MS;
      }
      applyBoth(radiusAt(t));
      rafRef.current = requestAnimationFrame(loop);
    }
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      root.removeEventListener("click", onClick);
      root.removeEventListener("keydown", onKeyDown);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.wrap + " " + styles.isDemo} ref={rootRef} tabIndex={0}>
        <div className={styles.row}>
          <figure className={styles.cell}>
            <div className={styles.box + " " + styles.native} ref={nativeRef} />
            <figcaption className={styles.cap} ref={capRef}>지원 환경</figcaption>
          </figure>
          <figure className={styles.cell}>
            <div className={styles.box + " " + styles.shim} ref={shimRef} />
            <figcaption className={styles.cap}>미지원 폴백</figcaption>
          </figure>
        </div>
        <p className={styles.readout} ref={readoutRef} aria-live="polite">지금 반경 20.0</p>
      </div>
    </main>
  );
}