01색을 목표로 끌어당기는 기본 hover

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">저장하기</button>
  <p class="bc__meta"><span class="bc__tag">GAP</span> <b id="bcGap">0</b> <span class="bc__hex" id="bcHex">#b9a0ff</span></p>
</div>
</main>

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

// 핵심: 색은 CSS transition 이 아니라 JS 가 매 프레임 계산한다.
// 현재 색이 목표 색까지 남은 거리의 일정 비율(PULL_RATE)만큼 다가가므로 끝에서 저절로 느려진다.

$pull-from: $subject-lilac;
$pull-to: $subject-pink;

@include stage($stage-ink);

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

.bc__btn {
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: $pull-from;
  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;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.bc__tag { font-weight: 700; letter-spacing: .08em; }
.bc__meta b { min-width: 3ch; text-align: right; font-variant-numeric: tabular-nums; color: $stage-paper; }
.bc__hex { font-variant-numeric: tabular-nums; }

@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 {
  --c-from: #b9a0ff;
  --c-to: #ff9ebb;
  width: min(100%, 320px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.bc__btn {
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: #b9a0ff;
  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;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

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

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

.bc__hex {
  font-variant-numeric: tabular-nums;
}

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

(function () {
  // 기법: 매 프레임 현재 색이 목표 색까지 남은 거리의 일정 비율만큼 다가간다 (시간 곡선 없음).
  // 비율은 프레임 간격(dt)으로 보정해 60Hz 든 120Hz 든 같은 속도로 도착한다.
  var PULL_RATE = 7;
  var SETTLE_GAP = 0.5;
  var START_MS = 200;
  var ON_MS = 900;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var gapEl = document.getElementById('bcGap');
  var hexEl = document.getElementById('bcHex');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function readRgb(name) {
    var v = getComputedStyle(btn).getPropertyValue(name).trim().replace('#', '');
    if (v.length === 3) v = v.split('').map(function (c) { return c + c; }).join('');
    return [parseInt(v.slice(0, 2), 16), parseInt(v.slice(2, 4), 16), parseInt(v.slice(4, 6), 16)];
  }
  function toHex(rgb) {
    return '#' + rgb.map(function (c) { return ('0' + Math.round(c).toString(16)).slice(-2); }).join('');
  }

  var from = readRgb('--c-from');
  var to = readRgb('--c-to');
  var cur = from.slice();
  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 target(now) { return demoOn(now) || pointerIn || kbFocus ? to : from; }

  function gapOf(goal) {
    return Math.abs(goal[0] - cur[0]) + Math.abs(goal[1] - cur[1]) + Math.abs(goal[2] - cur[2]);
  }
  function paint(goal) {
    btn.style.background = toHex(cur);
    gapEl.textContent = String(Math.round(gapOf(goal)));
    hexEl.textContent = toHex(cur);
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function jump() { cur = (pointerIn || kbFocus ? to : from).slice(); paint(pointerIn || kbFocus ? to : from); }

  btn.addEventListener('pointerenter', function () { stopDemo(); pointerIn = true; if (reduce) jump(); });
  btn.addEventListener('pointerleave', function () { pointerIn = false; if (reduce) jump(); });
  btn.addEventListener('focus', function () { kbFocus = btn.matches(':focus-visible'); if (kbFocus) stopDemo(); if (reduce) jump(); });
  btn.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 goal = target(now);
    var k = 1 - Math.exp(-dt * PULL_RATE);
    for (var i = 0; i < 3; i += 1) cur[i] += (goal[i] - cur[i]) * k;
    if (gapOf(goal) < SETTLE_GAP) cur = goal.slice();
    paint(goal);
    requestAnimationFrame(tick);
  }

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

// PullColorButton — vanilla 판(01-btn-color-exp-ease-fill) 의 React 포트.
// 현재 색이 목표 색까지 남은 거리의 일정 비율(PULL_RATE)만큼 매 프레임 다가간다. CSS transition 은 쓰지 않는다.
import { useEffect, useRef } from "react";
import styles from "./PullColorButton.module.scss";

type Box<T> = { current: T };
type Rgb = [number, number, number];

const PULL_RATE = 7;
const SETTLE_GAP = 0.5;
const START_MS = 200;
const ON_MS = 900;
const CYCLE_MS = 2000;

function toHex(rgb: Rgb): string {
  return "#" + rgb.map((c) => ("0" + Math.round(c).toString(16)).slice(-2)).join("");
}

export default function PullColorButton() {
  const rootRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const gapRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const hexRef = useRef<HTMLSpanElement | null>(null) as Box<HTMLSpanElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const gapEl = gapRef.current;
    const hexEl = hexRef.current;
    if (!btn || !gapEl || !hexEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const readRgb = (name: string): Rgb => {
      let v = getComputedStyle(btn).getPropertyValue(name).trim().replace("#", "");
      if (v.length === 3) v = v.split("").map((c) => c + c).join("");
      return [parseInt(v.slice(0, 2), 16), parseInt(v.slice(2, 4), 16), parseInt(v.slice(4, 6), 16)];
    };
    const from = readRgb("--c-from");
    const to = readRgb("--c-to");
    let cur: Rgb = [from[0], from[1], from[2]];
    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 gapOf = (goal: Rgb): number =>
      Math.abs(goal[0] - cur[0]) + Math.abs(goal[1] - cur[1]) + Math.abs(goal[2] - cur[2]);
    const paint = (goal: Rgb): void => {
      btn.style.background = toHex(cur);
      gapEl.textContent = String(Math.round(gapOf(goal)));
      hexEl.textContent = toHex(cur);
    };
    const stopDemo = (): void => { demo = false; };
    const jump = (): void => {
      const goal = pointerIn || kbFocus ? to : from;
      cur = [goal[0], goal[1], goal[2]];
      paint(goal);
    };

    const onEnter = (): void => { stopDemo(); pointerIn = true; if (reduce) jump(); };
    const onLeave = (): void => { pointerIn = false; if (reduce) jump(); };
    const onFocus = (): void => { kbFocus = btn.matches(":focus-visible"); if (kbFocus) stopDemo(); if (reduce) jump(); };
    const onBlur = (): void => { kbFocus = false; if (reduce) jump(); };
    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 dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      const goal = demoOn(now) || pointerIn || kbFocus ? to : from;
      const k = 1 - Math.exp(-dt * PULL_RATE);
      cur = [cur[0] + (goal[0] - cur[0]) * k, cur[1] + (goal[1] - cur[1]) * k, cur[2] + (goal[2] - cur[2]) * k];
      if (gapOf(goal) < SETTLE_GAP) cur = [goal[0], goal[1], goal[2]];
      paint(goal);
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint(from);
    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} ref={rootRef}>
        <button className={styles.bc__btn} type="button" ref={btnRef}>저장하기</button>
        <p className={styles.bc__meta}>
          <span className={styles.bc__tag}>GAP</span> <b ref={gapRef}>0</b> <span className={styles.bc__hex} ref={hexRef}>#b9a0ff</span>
        </p>
      </div>
    </main>
  );
}