02Trailing-value interpolation — the button's position value follows the pointer's position value, narrowing the gap by a fixed ratio each moment

<main class="stage">
<div class="bil is-demo" id="bil">
  <div class="bil__field" id="bilField" role="application" aria-label="Floating button drag test area">
    <button class="bil__fab" id="bilFab" type="button" aria-label="Floating action button">+</button>
  </div>
  <p class="bil__hint">Press and move it — the value trails behind</p>
</div>
</main>

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

// 계산 겹(지연 보간 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-ink);

.stage { position: relative; }

.bil {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
}

.bil__field {
  position: relative;
  width: 220px;
  height: 160px;
  border-radius: $r-card;
  background: rgba($stage-paper, .08);
  border: 1px solid rgba($stage-paper, .18);
}

.bil__fab {
  position: absolute;
  left: 0;
  top: 0;
  width: 52px;
  height: 52px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-mint;
  color: $subject-ink;
  font-size: 24px;
  font-weight: 700;
  line-height: 1;
  cursor: grab;
  touch-action: none;
  box-shadow: $shadow-raised;
}

.bil__hint {
  color: rgba($stage-paper, .72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .bil__field { width: 160px; height: 96px; }
  .bil__fab { width: 40px; height: 40px; font-size: 18px; }
}

@media (prefers-reduced-motion: reduce) {
  .bil__fab { 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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

.bil {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.bil__field {
  position: relative;
  width: 220px;
  height: 160px;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.08);
  border: 1px solid rgba(255, 255, 255, 0.18);
}

.bil__fab {
  position: absolute;
  left: 0;
  top: 0;
  width: 52px;
  height: 52px;
  border: 0;
  border-radius: 999px;
  background: #4cd4a6;
  color: #17141a;
  font-size: 24px;
  font-weight: 700;
  line-height: 1;
  cursor: grab;
  touch-action: none;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.bil__hint {
  color: rgba(255, 255, 255, 0.72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .bil__field {
    width: 160px;
    height: 96px;
  }
  .bil__fab {
    width: 40px;
    height: 40px;
    font-size: 18px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bil__fab {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('bil');
  var field = document.getElementById('bilField');
  var fab = document.getElementById('bilFab');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 포인터 위치 값(targetX/Y)과 버튼 위치 값(x/y)의 차이를 매 프레임 LERP_RATIO
  // 만큼만 좁힌다 — x += (targetX-x)*LERP_RATIO. 비율이 1에 가까울수록 즉시 따라붙고,
  // 낮을수록 지연(트레일)이 길어진다.
  var LERP_RATIO = 0.14;
  var FAB_R = 26;
  var x = 0, y = 0, targetX = 0, targetY = 0, raf = null;

  function bounds() { return { w: field.clientWidth, h: field.clientHeight }; }
  function clampPos(px, py) {
    var b = bounds();
    return [Math.max(FAB_R, Math.min(b.w - FAB_R, px)), Math.max(FAB_R, Math.min(b.h - FAB_R, py))];
  }
  function paint() { fab.style.transform = 'translate(' + (x - FAB_R).toFixed(1) + 'px,' + (y - FAB_R).toFixed(1) + 'px)'; }
  function tick() {
    x += (targetX - x) * LERP_RATIO;
    y += (targetY - y) * LERP_RATIO;
    paint();
    raf = requestAnimationFrame(tick);
  }
  function start() { if (raf === null) { raf = requestAnimationFrame(tick); } }
  function setTarget(px, py) {
    var c = clampPos(px, py);
    targetX = c[0]; targetY = c[1];
    if (reduce) { x = targetX; y = targetY; paint(); return; }
    start();
  }

  function wireReal() {
    var dragging = false;
    fab.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay(); dragging = true;
      fab.setPointerCapture(e.pointerId);
    });
    fab.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      var r = field.getBoundingClientRect();
      setTarget(e.clientX - r.left, e.clientY - r.top);
    });
    fab.addEventListener('pointerup', function () { dragging = false; });
  }
  wireReal();
  var b0 = bounds();
  x = targetX = b0.w / 2; y = targetY = b0.h / 2;
  paint();

  // 자동 루프: 목표 값을 좌우로 오가게 옮겨 같은 lerp 경로를 반복 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    var b = bounds();
    phase += 1;
    var px = b.w / 2 + Math.sin(phase * 0.9) * (b.w / 2 - FAB_R - 6);
    var py = b.h / 2 + Math.cos(phase * 0.6) * (b.h / 2 - FAB_R - 6) * 0.5;
    setTarget(px, py);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 260); autoTick(); }
})();

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

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

export default function ButtonPhysicsInertiaLerpDrag({
  color = "#4cd4a6",
}: ButtonPhysicsInertiaLerpDragProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

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

    // 계산 겹: 포인터 위치 값(targetX/Y)과 버튼 위치 값(x/y)의 차이를 매 프레임 LERP_RATIO
    // 만큼만 좁힌다 — x += (targetX-x)*LERP_RATIO. 비율이 1에 가까울수록 즉시 따라붙고,
    // 낮을수록 지연(트레일)이 길어진다.
    const LERP_RATIO = 0.14;
    const FAB_R = 26;
    let x = 0, y = 0, targetX = 0, targetY = 0, raf: number | null = null;

    function bounds() { return { w: field!.clientWidth, h: field!.clientHeight }; }
    function clampPos(px: number, py: number): [number, number] {
      const b = bounds();
      return [Math.max(FAB_R, Math.min(b.w - FAB_R, px)), Math.max(FAB_R, Math.min(b.h - FAB_R, py))];
    }
    function paint() { fab!.style.transform = "translate(" + (x - FAB_R).toFixed(1) + "px," + (y - FAB_R).toFixed(1) + "px)"; }
    function tick() {
      x += (targetX - x) * LERP_RATIO;
      y += (targetY - y) * LERP_RATIO;
      paint();
      raf = requestAnimationFrame(tick);
    }
    function start() { if (raf === null) { raf = requestAnimationFrame(tick); } }
    function setTarget(px: number, py: number) {
      const c = clampPos(px, py);
      targetX = c[0]; targetY = c[1];
      if (reduce) { x = targetX; y = targetY; paint(); return; }
      start();
    }

    let dragging = false;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay(); dragging = true;
      fab!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const r = field!.getBoundingClientRect();
      setTarget(e.clientX - r.left, e.clientY - r.top);
    }
    function onPointerUp() { dragging = false; }
    fab.addEventListener("pointerdown", onPointerDown);
    fab.addEventListener("pointermove", onPointerMove);
    fab.addEventListener("pointerup", onPointerUp);
    const b0 = bounds();
    x = targetX = b0.w / 2; y = targetY = b0.h / 2;
    paint();

    let autoTimer: number | null = null, phase = 0;
    function autoTick() {
      const b = bounds();
      phase += 1;
      const px = b.w / 2 + Math.sin(phase * 0.9) * (b.w / 2 - FAB_R - 6);
      const py = b.h / 2 + Math.cos(phase * 0.6) * (b.h / 2 - FAB_R - 6) * 0.5;
      setTarget(px, py);
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 260); autoTick(); }

    return () => {
      stopAutoplay();
      if (raf !== null) cancelAnimationFrame(raf);
      fab.removeEventListener("pointerdown", onPointerDown);
      fab.removeEventListener("pointermove", onPointerMove);
      fab.removeEventListener("pointerup", onPointerUp);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.bil}>
        <div className={styles.field} role="application" aria-label="플로팅 버튼 드래그 실험판">
          <button className={styles.fab} type="button" aria-label="플로팅 액션 버튼" style={{ background: color }}>+</button>
        </div>
        <p className={styles.hint}>눌러서 옮기면 값이 뒤따라옵니다</p>
      </div>
    </div>
  );
}