03色の境目がバネで押されるグラデーション

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">次のステップ</button>
  <p class="bc__meta"><span class="bc__tag">STOP</span> <b id="bcPos">20</b><span>%</span></p>
</div>
</main>

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

// 핵심: 그라데이션의 색 경계(--stop)는 스프링으로 움직이는 숫자다. CSS transition 으로는 목표를 지나쳤다 돌아오는 값을 못 만든다.

$stop-a: $subject-lilac;
$stop-b: $subject-pink;
$stop-soft: 10%;

@include stage($stage-ink);

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

.bc__btn {
  --stop: 20;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: linear-gradient(100deg, $stop-a calc(var(--stop) * 1% - #{$stop-soft}), $stop-b calc(var(--stop) * 1% + #{$stop-soft}));
  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: $sp-1;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.bc__tag { margin-right: $sp-1; font-weight: 700; letter-spacing: .08em; }
.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 {
  --stop: 20;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: linear-gradient(100deg, #b9a0ff calc(var(--stop) * 1% - 10%), #ff9ebb calc(var(--stop) * 1% + 10%));
  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: 4px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

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

.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: 그라데이션 색 경계의 위치(--stop, 퍼센트)를 스프링 방정식으로 푼다.
  // 가속도 = 강성 x (목표 - 위치) - 감쇠 x 속도. 고정 간격으로 쪼개 적분하므로 프레임 속도와 상관없이 같은 흔들림이 나온다.
  var STIFFNESS = 170;
  var DAMPING = 11;
  var STOP_REST = 20;
  var STOP_HOVER = 70;
  var STEP = 1 / 120;
  var START_MS = 200;
  var ON_MS = 800;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var posEl = document.getElementById('bcPos');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var pos = STOP_REST;
  var vel = 0;
  var acc = 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 ? STOP_HOVER : STOP_REST; }
  function paint() {
    btn.style.setProperty('--stop', pos.toFixed(2));
    posEl.textContent = String(Math.round(pos));
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function jump() { pos = pointerIn || kbFocus ? STOP_HOVER : STOP_REST; vel = 0; paint(); }

  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 g = goal(now);
    acc += dt;
    while (acc >= STEP) {
      vel += (STIFFNESS * (g - pos) - DAMPING * vel) * STEP;
      pos += vel * STEP;
      acc -= STEP;
    }
    paint();
    requestAnimationFrame(tick);
  }

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

// SpringStopButton — vanilla 판(03-btn-gradient-stop-spring-overshoot) 의 React 포트.
// 그라데이션 색 경계 위치(--stop)를 스프링 방정식으로 푼다. 목표를 지나쳤다가 돌아와 안착한다.
import { useEffect, useRef } from "react";
import styles from "./SpringStopButton.module.scss";

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

const STIFFNESS = 170;
const DAMPING = 11;
const STOP_REST = 20;
const STOP_HOVER = 70;
const STEP = 1 / 120;
const START_MS = 200;
const ON_MS = 800;
const CYCLE_MS = 2000;

export default function SpringStopButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const posRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const posEl = posRef.current;
    if (!btn || !posEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let pos = STOP_REST;
    let vel = 0;
    let acc = 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 => {
      btn.style.setProperty("--stop", pos.toFixed(2));
      posEl.textContent = String(Math.round(pos));
    };
    const stopDemo = (): void => { demo = false; };
    const jump = (): void => { pos = pointerIn || kbFocus ? STOP_HOVER : STOP_REST; vel = 0; paint(); };
    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 g = demoOn(now) || pointerIn || kbFocus ? STOP_HOVER : STOP_REST;
      acc += dt;
      while (acc >= STEP) {
        vel += (STIFFNESS * (g - pos) - DAMPING * vel) * STEP;
        pos += vel * STEP;
        acc -= STEP;
      }
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint();
    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}>다음 단계</button>
        <p className={styles.bc__meta}><span className={styles.bc__tag}>STOP</span> <b ref={posRef}>20</b><span>%</span></p>
      </div>
    </main>
  );
}