03Drop-and-settle rebound — the position value falls, bounces off a floor value repeatedly, and comes to rest

<main class="stage">
<div class="bgd is-demo" id="bgd">
  <div class="bgd__field" id="bgdField">
    <button class="bgd__chip" id="bgdChip" type="button">Alerts 3</button>
    <span class="bgd__floor" aria-hidden="true"></span>
  </div>
  <button class="bgd__drop" id="bgdDrop" type="button">Drop again</button>
</div>
</main>

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

// 계산 겹(중력 낙하·반동 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-yellow);

.stage { position: relative; }

.bgd {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-4;
}

.bgd__field {
  position: relative;
  width: 200px;
  height: 176px;
  overflow: hidden;
}

.bgd__chip {
  position: absolute;
  left: 50%;
  top: 0;
  margin-left: -46px;
  width: 92px;
  height: 34px;
  border: 0;
  border-radius: $r-pill;
  background: $stage-orange;
  color: $subject-ink;
  font-size: 13px;
  font-weight: 700;
  cursor: pointer;
  box-shadow: $shadow-press;
}

.bgd__floor {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 2px;
  background: rgba($subject-ink, .28);
}

.bgd__drop {
  border: 0;
  border-radius: $r-control;
  padding: $sp-2 $sp-5;
  background: $stage-paper;
  color: $subject-ink;
  font-size: 13px;
  font-weight: 600;
  cursor: pointer;
}

@media (max-width: 340px) {
  .bgd__field { width: 150px; height: 108px; }
}

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

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

.bgd {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.bgd__field {
  position: relative;
  width: 200px;
  height: 176px;
  overflow: hidden;
}

.bgd__chip {
  position: absolute;
  left: 50%;
  top: 0;
  margin-left: -46px;
  width: 92px;
  height: 34px;
  border: 0;
  border-radius: 999px;
  background: #ff4d1f;
  color: #17141a;
  font-size: 13px;
  font-weight: 700;
  cursor: pointer;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
}

.bgd__floor {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 2px;
  background: rgba(23, 20, 26, 0.28);
}

.bgd__drop {
  border: 0;
  border-radius: 12px;
  padding: 8px 20px;
  background: #ffffff;
  color: #17141a;
  font-size: 13px;
  font-weight: 600;
  cursor: pointer;
}

@media (max-width: 340px) {
  .bgd__field {
    width: 150px;
    height: 108px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bgd__chip {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('bgd');
  var field = document.getElementById('bgdField');
  var chip = document.getElementById('bgdChip');
  var dropBtn = document.getElementById('bgdDrop');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: vel += GRAVITY*dt, y += vel*dt 로 낙하시키다 바닥 값(FLOOR)에 닿으면
  // vel 을 -RESTITUTION 배로 뒤집어 반동시킨다. |반동 속도| 가 STOP_VEL 아래로 줄면 정착.
  var GRAVITY = 2200, RESTITUTION = 0.52, STOP_VEL = 40, CHIP_H = 34;
  var y = 0, vel = 0, last = 0, raf = null, running = false;

  function floorY() { return field.clientHeight - CHIP_H; }
  function paint() { chip.style.transform = 'translateY(' + y.toFixed(1) + 'px)'; }
  function tick(now) {
    var dt = Math.min((now - last) / 1000, 0.032);
    last = now;
    vel += GRAVITY * dt;
    y += vel * dt;
    var floor = floorY();
    if (y >= floor) {
      y = floor;
      vel = -vel * RESTITUTION;
      if (Math.abs(vel) < STOP_VEL) { vel = 0; paint(); running = false; return; }
    }
    paint();
    raf = requestAnimationFrame(tick);
  }
  function drop() {
    if (reduce) { y = floorY(); vel = 0; paint(); return; }
    y = 0; vel = 0; last = performance.now();
    if (!running) { running = true; raf = requestAnimationFrame(tick); }
  }

  function wireReal() {
    dropBtn.addEventListener('click', function () { root.classList.remove('is-demo'); stopAutoplay(); drop(); });
    chip.addEventListener('click', function () { root.classList.remove('is-demo'); stopAutoplay(); drop(); });
  }
  wireReal();
  y = floorY(); paint();

  // 자동 루프: 같은 drop() 을 주기적으로 다시 불러 낙하-반동 경로를 반복 재현한다.
  var autoTimer = null;
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { drop(); autoTimer = setInterval(drop, 1600); }
})();

import { useEffect, useRef } from "react";
import styles from "./ButtonPhysicsGravityDropSettle.module.scss";

export type ButtonPhysicsGravityDropSettleProps = {
  color?: string;
};

export default function ButtonPhysicsGravityDropSettle({
  color = "#ff4d1f",
}: ButtonPhysicsGravityDropSettleProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const field = root.querySelector<HTMLDivElement>(`.${styles.field}`);
    const chip = root.querySelector<HTMLButtonElement>(`.${styles.chip}`);
    const dropBtn = root.querySelector<HTMLButtonElement>(`.${styles.drop}`);
    if (!field || !chip || !dropBtn) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹: vel += GRAVITY*dt, y += vel*dt 로 낙하시키다 바닥 값(FLOOR)에 닿으면
    // vel 을 -RESTITUTION 배로 뒤집어 반동시킨다. |반동 속도| 가 STOP_VEL 아래로 줄면 정착.
    const GRAVITY = 2200, RESTITUTION = 0.52, STOP_VEL = 40, CHIP_H = 34;
    let y = 0, vel = 0, last = 0, raf: number | null = null, running = false;

    function floorY() { return field!.clientHeight - CHIP_H; }
    function paint() { chip!.style.transform = "translateY(" + y.toFixed(1) + "px)"; }
    function tick(now: number) {
      const dt = Math.min((now - last) / 1000, 0.032);
      last = now;
      vel += GRAVITY * dt;
      y += vel * dt;
      const floor = floorY();
      if (y >= floor) {
        y = floor;
        vel = -vel * RESTITUTION;
        if (Math.abs(vel) < STOP_VEL) { vel = 0; paint(); running = false; return; }
      }
      paint();
      raf = requestAnimationFrame(tick);
    }
    function drop() {
      if (reduce) { y = floorY(); vel = 0; paint(); return; }
      y = 0; vel = 0; last = performance.now();
      if (!running) { running = true; raf = requestAnimationFrame(tick); }
    }

    function onDropClick() { stopAutoplay(); drop(); }
    function onChipClick() { stopAutoplay(); drop(); }
    dropBtn.addEventListener("click", onDropClick);
    chip.addEventListener("click", onChipClick);
    y = floorY(); paint();

    let autoTimer: number | null = null;
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { drop(); autoTimer = window.setInterval(drop, 1600); }

    return () => {
      stopAutoplay();
      if (raf !== null) cancelAnimationFrame(raf);
      dropBtn.removeEventListener("click", onDropClick);
      chip.removeEventListener("click", onChipClick);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.bgd}>
        <div className={styles.field}>
          <button className={styles.chip} type="button" style={{ background: color }}>알림 3</button>
          <span className={styles.floor} aria-hidden="true" />
        </div>
        <button className={styles.drop} type="button">다시 떨어뜨리기</button>
      </div>
    </div>
  );
}