04Bounce via linear()

<main class="stage is-demo">
  <div class="track track--v">
    <span class="dot dot--start"></span>
    <span class="dot dot--end"></span>
    <div class="puck"></div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;   // 2s
// linear() — Penner easeOutBounce 공식을 25개 점으로 표본화한 값. cubic-bezier 로는 못 만드는,
// 방향이 꺾이는(제자리로 되튀는) 곡선이라 CSS Easing Functions Level 2 의 linear() 가 필요하다
$easing: linear(0, .013, .053, .118, .21, .328, .473, .643, .84, .97, .875, .807, .766, .75, .761, .798, .861, .951, .973, .943, .939, .962, .995, .984, 1);
$color: $subject-blue;

@include stage($stage-paper);

$track-h: 132px;   // 320×200 모바일 체크에서 세로 여백을 남겨야 한다 (BD9 실측: 180px 는 넘쳤다)
$puck-w: $sp-6;
$drop: $track-h - $puck-w;

.track--v {
  position: relative;
  width: $sp-2;
  height: $track-h;
}

.dot {
  position: absolute;
  left: 50%;
  width: $sp-2;
  height: $sp-2;
  border-radius: $r-pill;
  background: rgba(23, 20, 26, .18);
  transform: translate(-50%, -50%);
}
.dot--start { top: 0; }
.dot--end { top: $track-h; }

.puck {
  position: absolute;
  top: 0;
  left: 50%;
  width: $puck-w;
  height: $puck-w;
  border-radius: $r-pill;
  background: $color;
  box-shadow: $shadow-raised;
  transform: translate(-50%, 0);
}

// 공이 떨어져 튀어오르는 방향이라 alternate 를 안 쓴다 — 한 번 떨어지고 나면 다음 루프에서 제자리로 순간 복귀한다
.stage.is-demo .puck {
  animation: drop $duration $easing infinite;
}

@keyframes drop {
  from { transform: translate(-50%, 0); }
  to   { transform: translate(-50%, $drop); }
}

@media (prefers-reduced-motion: reduce) {
  .stage.is-demo .puck { animation: none; transform: translate(-50%, $drop); }
}

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);
}

.track--v {
  position: relative;
  width: 8px;
  height: 132px;
}

.dot {
  position: absolute;
  left: 50%;
  width: 8px;
  height: 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.18);
  transform: translate(-50%, -50%);
}

.dot--start {
  top: 0;
}

.dot--end {
  top: 132px;
}

.puck {
  position: absolute;
  top: 0;
  left: 50%;
  width: 24px;
  height: 24px;
  border-radius: 999px;
  background: #2f6df6;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  transform: translate(-50%, 0);
}

.stage.is-demo .puck {
  animation: drop 2s linear(0, 0.013, 0.053, 0.118, 0.21, 0.328, 0.473, 0.643, 0.84, 0.97, 0.875, 0.807, 0.766, 0.75, 0.761, 0.798, 0.861, 0.951, 0.973, 0.943, 0.939, 0.962, 0.995, 0.984, 1) infinite;
}

@keyframes drop {
  from {
    transform: translate(-50%, 0);
  }
  to {
    transform: translate(-50%, 108px);
  }
}
@media (prefers-reduced-motion: reduce) {
  .stage.is-demo .puck {
    animation: none;
    transform: translate(-50%, 108px);
  }
}

import type { CSSProperties } from "react";
import styles from "./BounceLinear.module.scss";

export type BounceLinearProps = {
  duration?: number;
  color?: string;
};

export default function BounceLinear({ duration = 2000, color = "#2f6df6" }: BounceLinearProps) {
  const vars: CSSProperties = { "--duration": `${duration}ms`, "--color": color } as CSSProperties;
  return (
    <div className={styles.stage} style={vars}>
      <div className={styles.track}>
        <span className={`${styles.dot} ${styles.dotStart}`} />
        <span className={`${styles.dot} ${styles.dotEnd}`} />
        <div className={styles.puck} />
      </div>
    </div>
  );
}