08斜めの光の帯が色の上をなでる光沢

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">始める</button>
  <p class="bc__meta"><span class="bc__tag">BAND</span> <b id="bcX">-30</b><span>%</span> <span class="bc__tag">GLOW</span> <b id="bcA">0.00</b></p>
</div>
</main>

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

// 핵심: 밝은 띠는 별도 요소가 아니라 같은 background 의 위 겹 그라데이션이다. 위치(--x)와 세기(--glow)는 JS 가 매 프레임 계산한다.

$sheen-a: $subject-lilac;
$sheen-b: $subject-pink;
$sheen-skew: 105deg;
$sheen-half: 16%;

@include stage($stage-ink);

.bc {
  width: min(100%, 320px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.bc__btn {
  --x: -30;
  --glow: 0;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background:
    linear-gradient(
      $sheen-skew,
      rgba(255, 255, 255, 0) calc(var(--x) * 1% - #{$sheen-half}),
      rgba(255, 255, 255, var(--glow)) calc(var(--x) * 1%),
      rgba(255, 255, 255, 0) calc(var(--x) * 1% + #{$sheen-half})
    ),
    linear-gradient(100deg, $sheen-a, $sheen-b);
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.bc__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: $sp-1;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.bc__tag { margin: 0 $sp-1 0 $sp-2; font-weight: 700; letter-spacing: .08em; }
.bc__tag:first-child { margin-left: 0; }
.bc__meta b { min-width: 3ch; text-align: right; font-variant-numeric: tabular-nums; color: $stage-paper; }

@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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.bc {
  width: min(100%, 320px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.bc__btn {
  --x: -30;
  --glow: 0;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: linear-gradient(105deg, rgba(255, 255, 255, 0) calc(var(--x) * 1% - 16%), rgba(255, 255, 255, var(--glow)) calc(var(--x) * 1%), rgba(255, 255, 255, 0) calc(var(--x) * 1% + 16%)), linear-gradient(100deg, #b9a0ff, #ff9ebb);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

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

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: 4px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

.bc__tag {
  margin: 0 4px 0 8px;
  font-weight: 700;
  letter-spacing: 0.08em;
}

.bc__tag:first-child {
  margin-left: 0;
}

.bc__meta b {
  min-width: 3ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #ffffff;
}

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

(function () {
  // 기법: 기울어진 밝은 띠의 위치(--x)와 세기(--glow)를 JS 가 계산한다. hover 가 시작되면 SWEEP_MS 동안 띠가 한 번 지나간다.
  // 위치는 smoothstep 곡선, 세기는 사인 반 주기라서 띠가 들어올 때 서서히 밝아지고 나갈 때 서서히 사라진다.
  var SWEEP_MS = 700;
  var BAND_FROM = -30;
  var BAND_TO = 130;
  var PEAK_GLOW = 0.7;
  var START_MS = 200;
  var ON_MS = 900;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var xEl = document.getElementById('bcX');
  var aEl = document.getElementById('bcA');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var pointerIn = false;
  var kbFocus = false;
  var wasOn = false;
  var sweepAt = -1;
  var t0 = null;

  function demoOn(now) {
    return root.classList.contains('is-demo') && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
  }
  function paint(x, glow) {
    btn.style.setProperty('--x', x.toFixed(1));
    btn.style.setProperty('--glow', glow.toFixed(3));
    xEl.textContent = String(Math.round(x));
    aEl.textContent = glow.toFixed(2);
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function staticState() {
    if (pointerIn || kbFocus) paint(50, 0.5); else paint(BAND_FROM, 0);
  }

  btn.addEventListener('pointerenter', function () { stopDemo(); pointerIn = true; if (reduce) staticState(); });
  btn.addEventListener('pointerleave', function () { pointerIn = false; if (reduce) staticState(); });
  btn.addEventListener('focus', function () { kbFocus = btn.matches(':focus-visible'); if (kbFocus) stopDemo(); if (reduce) staticState(); });
  btn.addEventListener('blur', function () { kbFocus = false; if (reduce) staticState(); });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var on = demoOn(now) || pointerIn || kbFocus;
    if (on && !wasOn) sweepAt = now;
    wasOn = on;
    var p = sweepAt < 0 ? 1 : Math.min(1, (now - sweepAt) / SWEEP_MS);
    var eased = p * p * (3 - 2 * p);
    if (p >= 1) paint(BAND_FROM, 0);
    else paint(BAND_FROM + (BAND_TO - BAND_FROM) * eased, PEAK_GLOW * Math.sin(Math.PI * p));
    requestAnimationFrame(tick);
  }

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

// SheenSweepButton — vanilla 판(08-btn-gradient-sheen-sweep-skew) 의 React 포트.
// hover 가 시작되면 기울어진 밝은 띠가 SWEEP_MS 동안 한 번 지나간다. 위치는 smoothstep, 세기는 사인 반 주기.
import { useEffect, useRef } from "react";
import styles from "./SheenSweepButton.module.scss";

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

const SWEEP_MS = 700;
const BAND_FROM = -30;
const BAND_TO = 130;
const PEAK_GLOW = 0.7;
const START_MS = 200;
const ON_MS = 900;
const CYCLE_MS = 2000;

export default function SheenSweepButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const xRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const aRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const xEl = xRef.current;
    const aEl = aRef.current;
    if (!btn || !xEl || !aEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let pointerIn = false;
    let kbFocus = false;
    let wasOn = false;
    let sweepAt = -1;
    let t0: number | null = null;
    let raf = 0;

    const demoOn = (now: number): boolean => demo && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
    const paint = (x: number, glow: number): void => {
      btn.style.setProperty("--x", x.toFixed(1));
      btn.style.setProperty("--glow", glow.toFixed(3));
      xEl.textContent = String(Math.round(x));
      aEl.textContent = glow.toFixed(2);
    };
    const stopDemo = (): void => { demo = false; };
    const staticState = (): void => {
      if (pointerIn || kbFocus) paint(50, 0.5);
      else paint(BAND_FROM, 0);
    };
    const onEnter = (): void => { stopDemo(); pointerIn = true; if (reduce) staticState(); };
    const onLeave = (): void => { pointerIn = false; if (reduce) staticState(); };
    const onFocus = (): void => { kbFocus = btn.matches(":focus-visible"); if (kbFocus) stopDemo(); if (reduce) staticState(); };
    const onBlur = (): void => { kbFocus = false; if (reduce) staticState(); };
    btn.addEventListener("pointerenter", onEnter);
    btn.addEventListener("pointerleave", onLeave);
    btn.addEventListener("focus", onFocus);
    btn.addEventListener("blur", onBlur);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const on = demoOn(now) || pointerIn || kbFocus;
      if (on && !wasOn) sweepAt = now;
      wasOn = on;
      const p = sweepAt < 0 ? 1 : Math.min(1, (now - sweepAt) / SWEEP_MS);
      const eased = p * p * (3 - 2 * p);
      if (p >= 1) paint(BAND_FROM, 0);
      else paint(BAND_FROM + (BAND_TO - BAND_FROM) * eased, PEAK_GLOW * Math.sin(Math.PI * p));
      raf = requestAnimationFrame(tick);
    }

    if (reduce) staticState();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("pointerenter", onEnter);
      btn.removeEventListener("pointerleave", onLeave);
      btn.removeEventListener("focus", onFocus);
      btn.removeEventListener("blur", onBlur);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bc}>
        <button className={styles.bc__btn} type="button" ref={btnRef}>시작하기</button>
        <p className={styles.bc__meta}>
          <span className={styles.bc__tag}>BAND</span> <b ref={xRef}>-30</b><span>%</span> <span className={styles.bc__tag}>GLOW</span> <b ref={aRef}>0.00</b>
        </p>
      </div>
    </main>
  );
}