04二枚を重ねてグラデーションを替えるクロスフェード

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <div class="bc__pair">
    <div class="bc__cell">
      <button class="bc__btn" id="bcSwap" type="button"><span class="bc__label">そのまま切替</span></button>
      <p class="bc__meta"><span class="bc__tag">SWAP</span> <b id="bcSwapVal">0</b></p>
    </div>
    <div class="bc__cell">
      <button class="bc__btn" id="bcFade" type="button"><span class="bc__layer" id="bcLayer"></span><span class="bc__label">二枚重ね</span></button>
      <p class="bc__meta"><span class="bc__tag">OPACITY</span> <b id="bcFadeVal">0.00</b></p>
    </div>
  </div>
</div>
</main>

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

// 핵심: 그라데이션은 transition 으로 보간되지 않는다. 아래 겹(버튼 배경)과 위 겹(.bc__layer)을 포개고 위 겹의 opacity 만 옮긴다.

$layer-a: linear-gradient(120deg, $subject-lilac, $subject-pink);
$layer-b: linear-gradient(120deg, $subject-mint, $subject-sky);

@include stage($stage-paper);

.bc { width: min(100%, 360px); }

.bc__pair {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: $sp-3;
}

.bc__cell { display: grid; gap: $sp-3; justify-items: center; min-width: 0; }

.bc__btn {
  position: relative;
  isolation: isolate;
  overflow: hidden;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: $layer-a;
  color: $subject-ink;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 20px;
  cursor: pointer;
}
.bc__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }
.bc__btn.bc__on { background: $layer-b; }

.bc__layer {
  position: absolute;
  inset: 0;
  z-index: -1;
  background: $layer-b;
  opacity: 0;
}
.bc__label { position: relative; }

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: rgba($subject-ink, .78);
}
.bc__tag { font-weight: 700; letter-spacing: .08em; }
.bc__meta b { min-width: 4ch; font-variant-numeric: tabular-nums; color: $subject-ink; }

@media (prefers-reduced-motion: reduce) {
  .bc__btn { 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);
}

.bc {
  width: min(100%, 360px);
}

.bc__pair {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
}

.bc__cell {
  display: grid;
  gap: 12px;
  justify-items: center;
  min-width: 0;
}

.bc__btn {
  position: relative;
  isolation: isolate;
  overflow: hidden;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: linear-gradient(120deg, #b9a0ff, #ff9ebb);
  color: #17141a;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  line-height: 20px;
  cursor: pointer;
}

.bc__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.bc__btn.bc__on {
  background: linear-gradient(120deg, #4cd4a6, #6ec8ff);
}

.bc__layer {
  position: absolute;
  inset: 0;
  z-index: -1;
  background: linear-gradient(120deg, #4cd4a6, #6ec8ff);
  opacity: 0;
}

.bc__label {
  position: relative;
}

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(23, 20, 26, 0.78);
}

.bc__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.bc__meta b {
  min-width: 4ch;
  font-variant-numeric: tabular-nums;
  color: #17141a;
}

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

(function () {
  // 기법: background 의 그라데이션은 transition 으로 보간되지 않는다 (왼쪽처럼 한 번에 바뀐다).
  // 그래서 아래 겹에 첫 그라데이션, 위 겹에 두 번째 그라데이션을 깔고 위 겹의 opacity 만 움직인다 (오른쪽).
  // opacity 는 JS 가 목표로 일정 비율씩 끌어 옮긴다.
  var FADE_RATE = 6;
  var START_MS = 200;
  var ON_MS = 1000;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var swapBtn = document.getElementById('bcSwap');
  var fadeBtn = document.getElementById('bcFade');
  var layer = document.getElementById('bcLayer');
  var swapVal = document.getElementById('bcSwapVal');
  var fadeVal = document.getElementById('bcFadeVal');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var amount = 0;
  var pointerIn = false;
  var kbFocus = false;
  var t0 = null;
  var lastTs = null;

  function demoOn(now) {
    return root.classList.contains('is-demo') && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
  }
  function goal(now) { return demoOn(now) || pointerIn || kbFocus ? 1 : 0; }
  function paint() {
    layer.style.opacity = amount.toFixed(3);
    fadeVal.textContent = amount.toFixed(2);
    var on = amount >= 0.5;
    swapBtn.classList.toggle('bc__on', on);
    swapVal.textContent = on ? '1' : '0';
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function jump() { amount = pointerIn || kbFocus ? 1 : 0; paint(); }

  [swapBtn, fadeBtn].forEach(function (b) {
    b.addEventListener('pointerenter', function () { stopDemo(); pointerIn = true; if (reduce) jump(); });
    b.addEventListener('pointerleave', function () { pointerIn = false; if (reduce) jump(); });
    b.addEventListener('focus', function () { kbFocus = b.matches(':focus-visible'); if (kbFocus) stopDemo(); if (reduce) jump(); });
    b.addEventListener('blur', function () { kbFocus = false; if (reduce) jump(); });
  });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    var g = goal(now);
    amount += (g - amount) * (1 - Math.exp(-dt * FADE_RATE));
    if (Math.abs(g - amount) < 0.004) amount = g;
    paint();
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    paint();
  } else {
    requestAnimationFrame(tick);
  }
})();

// TwoLayerGradientFade — vanilla 판(04-btn-gradient-two-layer-crossfade) 의 React 포트.
// 왼쪽은 그라데이션을 통째로 교체(보간 없음), 오른쪽은 위 겹의 opacity 만 옮겨 두 그라데이션을 섞는다.
import { useEffect, useRef } from "react";
import styles from "./TwoLayerGradientFade.module.scss";

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

const FADE_RATE = 6;
const START_MS = 200;
const ON_MS = 1000;
const CYCLE_MS = 2000;

export default function TwoLayerGradientFade() {
  const swapRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const fadeRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const layerRef = useRef<HTMLSpanElement | null>(null) as Box<HTMLSpanElement | null>;
  const swapValRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const fadeValRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const swapBtn = swapRef.current;
    const fadeBtn = fadeRef.current;
    const layer = layerRef.current;
    const swapVal = swapValRef.current;
    const fadeVal = fadeValRef.current;
    if (!swapBtn || !fadeBtn || !layer || !swapVal || !fadeVal) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let amount = 0;
    let demo = !reduce;
    let pointerIn = false;
    let kbFocus = false;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const demoOn = (now: number): boolean => demo && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
    const paint = (): void => {
      layer.style.opacity = amount.toFixed(3);
      fadeVal.textContent = amount.toFixed(2);
      const on = amount >= 0.5;
      swapBtn.classList.toggle(styles.bc__on, on);
      swapVal.textContent = on ? "1" : "0";
    };
    const stopDemo = (): void => { demo = false; };
    const jump = (): void => { amount = pointerIn || kbFocus ? 1 : 0; paint(); };
    const cleanups: Array<() => void> = [];
    [swapBtn, fadeBtn].forEach((b) => {
      const onEnter = (): void => { stopDemo(); pointerIn = true; if (reduce) jump(); };
      const onLeave = (): void => { pointerIn = false; if (reduce) jump(); };
      const onFocus = (): void => { kbFocus = b.matches(":focus-visible"); if (kbFocus) stopDemo(); if (reduce) jump(); };
      const onBlur = (): void => { kbFocus = false; if (reduce) jump(); };
      b.addEventListener("pointerenter", onEnter);
      b.addEventListener("pointerleave", onLeave);
      b.addEventListener("focus", onFocus);
      b.addEventListener("blur", onBlur);
      cleanups.push(() => {
        b.removeEventListener("pointerenter", onEnter);
        b.removeEventListener("pointerleave", onLeave);
        b.removeEventListener("focus", onFocus);
        b.removeEventListener("blur", onBlur);
      });
    });

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      const g = demoOn(now) || pointerIn || kbFocus ? 1 : 0;
      amount += (g - amount) * (1 - Math.exp(-dt * FADE_RATE));
      if (Math.abs(g - amount) < 0.004) amount = g;
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      cleanups.forEach((fn) => fn());
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bc}>
        <div className={styles.bc__pair}>
          <div className={styles.bc__cell}>
            <button className={styles.bc__btn} type="button" ref={swapRef}><span className={styles.bc__label}>그냥 교체</span></button>
            <p className={styles.bc__meta}><span className={styles.bc__tag}>SWAP</span> <b ref={swapValRef}>0</b></p>
          </div>
          <div className={styles.bc__cell}>
            <button className={styles.bc__btn} type="button" ref={fadeRef}>
              <span className={styles.bc__layer} ref={layerRef}></span>
              <span className={styles.bc__label}>두 겹</span>
            </button>
            <p className={styles.bc__meta}><span className={styles.bc__tag}>OPACITY</span> <b ref={fadeValRef}>0.00</b></p>
          </div>
        </div>
      </div>
    </main>
  );
}