07Checkout bar lift — a bottom bar whose shadow and height values grow with scroll position

<main class="stage">
<div class="cscr is-demo" id="cscr">
  <div class="cscr__panel">
    <div class="cscr__scroll" id="cscrScroll">
      <ul class="cscr__list">
        <li class="cscr__row"><span>Tumbler</span><b>$16</b></li>
        <li class="cscr__row"><span>Notebook</span><b>$8</b></li>
        <li class="cscr__row"><span>Pouch</span><b>$14</b></li>
        <li class="cscr__row"><span>Keyring</span><b>$6</b></li>
        <li class="cscr__row"><span>Candle</span><b>$18</b></li>
      </ul>
    </div>
    <footer class="cscr__bar" id="cscrBar">
      <span>Total</span>
      <b id="cscrTotal">$62</b>
    </footer>
  </div>
</div>
</main>

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

// 계산 겹: 스크롤 진행률을 목표값으로 두고 매 프레임 lerp() 로 그림자 세기 쪽에 관성 있게
// 다가간다(tick(), index.html) — 스크롤 위치를 그대로 쓰지 않고 보간된 값만 opacity 로 반영한다.

$duration: $dur-quick;
$easing: $ease-out;
$color: $subject-ink;

@include stage($stage-orange);

.stage { position: relative; }

.cscr__panel {
  width: min(240px, 70vw);
  height: 190px;
  display: flex;
  flex-direction: column;
  border-radius: $r-card;
  overflow: hidden;
  background: $subject-cream;
}

.cscr__scroll {
  flex: 1 1 auto;
  overflow-y: auto;
  padding: $sp-3;
}

.cscr__list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
}

.cscr__row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: $sp-3;
  border-radius: $r-sm;
  background: rgba(23, 20, 26, .05);
  color: $color;
  font-weight: 600;
  font-size: 12px;
}

.cscr__bar {
  position: relative;
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: $sp-3 $sp-4;
  background: $subject-cream;
  color: $color;
  font-weight: 700;
  font-size: 14px;
  --shadow-op: 0;
  will-change: transform;
}

.cscr__bar::before {
  content: '';
  position: absolute;
  left: 0;
  right: 0;
  bottom: 100%;
  height: 14px;
  box-shadow: 0 -10px 18px rgba(23, 20, 26, .4);
  opacity: var(--shadow-op);
  pointer-events: none;
}

@media (max-width: 340px) {
  .cscr__panel { width: min(190px, 66vw); height: 160px; }
}

@media (prefers-reduced-motion: reduce) {
  .cscr__bar { 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: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

.cscr__panel {
  width: min(240px, 70vw);
  height: 190px;
  display: flex;
  flex-direction: column;
  border-radius: 16px;
  overflow: hidden;
  background: #fff7e6;
}

.cscr__scroll {
  flex: 1 1 auto;
  overflow-y: auto;
  padding: 12px;
}

.cscr__list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.cscr__row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px;
  border-radius: 8px;
  background: rgba(23, 20, 26, 0.05);
  color: #17141a;
  font-weight: 600;
  font-size: 12px;
}

.cscr__bar {
  position: relative;
  flex: 0 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px 16px;
  background: #fff7e6;
  color: #17141a;
  font-weight: 700;
  font-size: 14px;
  --shadow-op: 0;
  will-change: transform;
}

.cscr__bar::before {
  content: "";
  position: absolute;
  left: 0;
  right: 0;
  bottom: 100%;
  height: 14px;
  box-shadow: 0 -10px 18px rgba(23, 20, 26, 0.4);
  opacity: var(--shadow-op);
  pointer-events: none;
}

@media (max-width: 340px) {
  .cscr__panel {
    width: min(190px, 66vw);
    height: 160px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .cscr__bar {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('cscr');
  var scrollEl = document.getElementById('cscrScroll');
  var bar = document.getElementById('cscrBar');

  // 계산 겹: 스크롤 진행률(0~1)을 매 프레임 lerp() 로 그림자 강도·bar 높이 값에 보간한다 —
  // 스크롤 위치를 그대로 쓰지 않고 목표 값 쪽으로 매 프레임 조금씩 다가가는 관성 보간이다.
  function lerp(a, b, t) { return a + (b - a) * t; }

  var maxScroll = 1;
  function computeMax() {
    maxScroll = Math.max(1, scrollEl.scrollHeight - scrollEl.clientHeight);
  }

  var shadow = 0;
  var raf = null;
  function tick() {
    var target = Math.min(1, scrollEl.scrollTop / maxScroll);
    shadow = lerp(shadow, target, 0.18);
    bar.style.setProperty('--shadow-op', shadow.toFixed(3));
    bar.style.transform = 'translateY(' + (-shadow * 2).toFixed(2) + 'px)';
    if (Math.abs(target - shadow) > 0.002) {
      raf = requestAnimationFrame(tick);
    } else {
      raf = null;
    }
  }
  function kick() {
    if (!raf) { raf = requestAnimationFrame(tick); }
  }

  function stopOnRealInput() {
    root.classList.remove('is-demo');
    stopAutoplay();
  }
  scrollEl.addEventListener('wheel', stopOnRealInput, { passive: true });
  scrollEl.addEventListener('pointerdown', stopOnRealInput);
  scrollEl.addEventListener('scroll', kick);

  var autoplayRaf = null;
  function stopAutoplay() {
    if (autoplayRaf) { cancelAnimationFrame(autoplayRaf); autoplayRaf = null; }
  }

  computeMax();
  window.addEventListener('resize', computeMax);

  var CYCLE_MS = 1600;
  function autoLoop(now) {
    var p = (now % CYCLE_MS) / CYCLE_MS;
    var tri = p < 0.5 ? p * 2 : 2 - p * 2;
    scrollEl.scrollTop = tri * maxScroll;
    var target = Math.min(1, scrollEl.scrollTop / maxScroll);
    shadow = lerp(shadow, target, 0.18);
    bar.style.setProperty('--shadow-op', shadow.toFixed(3));
    bar.style.transform = 'translateY(' + (-shadow * 2).toFixed(2) + 'px)';
    autoplayRaf = requestAnimationFrame(autoLoop);
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) {
    autoplayRaf = requestAnimationFrame(autoLoop);
  }
})();

import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./CartDrawerStickyCheckoutReveal.module.scss";

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

// 계산 겹: 스크롤 진행률(0~1)을 매 프레임 lerp() 로 그림자 강도·bar 높이 값에 보간한다 —
// 스크롤 위치를 그대로 쓰지 않고 목표 값 쪽으로 매 프레임 조금씩 다가가는 관성 보간이다.
function lerp(a: number, b: number, t: number): number {
  return a + (b - a) * t;
}

export default function CartDrawerStickyCheckoutReveal({
  color = "#17141a",
}: CartDrawerStickyCheckoutRevealProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const scrollRef = useRef<HTMLDivElement | null>(null);
  const barRef = useRef<HTMLElement | null>(null);
  const shadowRef = useRef(0);
  const maxScrollRef = useRef(1);
  const rafRef = useRef<number | null>(null);
  const autoplayRafRef = useRef<number | null>(null);

  useEffect(() => {
    const scrollEl = scrollRef.current;
    const bar = barRef.current;
    if (!scrollEl || !bar) return;

    function computeMax() {
      maxScrollRef.current = Math.max(1, (scrollEl as HTMLDivElement).scrollHeight - (scrollEl as HTMLDivElement).clientHeight);
    }

    function tick() {
      const target = Math.min(1, scrollEl!.scrollTop / (maxScrollRef.current ?? 1));
      const shadow = lerp(shadowRef.current ?? 0, target, 0.18);
      shadowRef.current = shadow;
      bar!.style.setProperty("--shadow-op", shadow.toFixed(3));
      bar!.style.transform = `translateY(${(-shadow * 2).toFixed(2)}px)`;
      if (Math.abs(target - shadow) > 0.002) {
        rafRef.current = requestAnimationFrame(tick);
      } else {
        rafRef.current = null;
      }
    }
    function kick() {
      if (!rafRef.current) rafRef.current = requestAnimationFrame(tick);
    }

    function stopAutoplay() {
      if (autoplayRafRef.current) {
        cancelAnimationFrame(autoplayRafRef.current);
        autoplayRafRef.current = null;
      }
    }

    function stopOnRealInput() {
      rootRef.current?.classList.remove(styles.isDemo);
      stopAutoplay();
    }

    scrollEl.addEventListener("wheel", stopOnRealInput, { passive: true });
    scrollEl.addEventListener("pointerdown", stopOnRealInput);
    scrollEl.addEventListener("scroll", kick);
    window.addEventListener("resize", computeMax);

    computeMax();

    const CYCLE_MS = 1600;
    function autoLoop(now: number) {
      const p = (now % CYCLE_MS) / CYCLE_MS;
      const tri = p < 0.5 ? p * 2 : 2 - p * 2;
      const maxScroll = maxScrollRef.current ?? 1;
      scrollEl!.scrollTop = tri * maxScroll;
      const target = Math.min(1, scrollEl!.scrollTop / maxScroll);
      const shadow = lerp(shadowRef.current ?? 0, target, 0.18);
      shadowRef.current = shadow;
      bar!.style.setProperty("--shadow-op", shadow.toFixed(3));
      bar!.style.transform = `translateY(${(-shadow * 2).toFixed(2)}px)`;
      autoplayRafRef.current = requestAnimationFrame(autoLoop);
    }

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) {
      autoplayRafRef.current = requestAnimationFrame(autoLoop);
    }

    return () => {
      stopAutoplay();
      if (rafRef.current) cancelAnimationFrame(rafRef.current);
      scrollEl.removeEventListener("wheel", stopOnRealInput);
      scrollEl.removeEventListener("pointerdown", stopOnRealInput);
      scrollEl.removeEventListener("scroll", kick);
      window.removeEventListener("resize", computeMax);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.isDemo}`} style={{ "--color": color } as CSSProperties}>
      <div className={styles.panel}>
        <div ref={scrollRef} className={styles.scroll}>
          <ul className={styles.list}>
            <li className={styles.row}><span>텀블러</span><b>$16</b></li>
            <li className={styles.row}><span>노트</span><b>$8</b></li>
            <li className={styles.row}><span>파우치</span><b>$14</b></li>
            <li className={styles.row}><span>키링</span><b>$6</b></li>
            <li className={styles.row}><span>양초</span><b>$18</b></li>
          </ul>
        </div>
        <footer ref={barRef} className={styles.bar}>
          <span>합계</span>
          <b>$62</b>
        </footer>
      </div>
    </div>
  );
}