07Dừng lại trên footer

<main class="stage is-demo">
  <div class="page-mock" aria-hidden="true">
    <span class="page-mock__line"></span>
    <span class="page-mock__line"></span>
  </div>
  <footer class="footer-mock" aria-hidden="true"></footer>
  <button type="button" class="totop" aria-label="Lên đầu trang">
    <svg class="totop__arrow" viewBox="0 0 24 24" aria-hidden="true"><polyline points="5,15 12,8 19,15"/></svg>
  </button>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: 220ms;          // 버튼이 들어 올려지는 실사용 전환 시간
$easing: $ease-out;
$color: $subject-ink;      // 화살표 색

@include stage($stage-orange);
.stage { position: relative; overflow: hidden; }

.page-mock {
  position: absolute;
  inset: $sp-6 $sp-10 auto $sp-6;
  display: flex;
  flex-direction: column;
  gap: $sp-4;
}
.page-mock__line {
  display: block;
  height: $sp-2;
  border-radius: $r-xs;
  background: rgba(#fff, .18);
}
.page-mock__line:nth-child(1) { width: 70%; }
.page-mock__line:nth-child(2) { width: 50%; }

.footer-mock {
  position: absolute;
  left: 0; right: 0; bottom: 0;
  height: 64px;
  background: $subject-ink;
  transform: translateY(100%);
}
// 데모 루프 — 푸터가 올라오고, 버튼은 겹치지 않게 같은 박자로 위로 비켜준다 (캡처 주기 2s 고정)
.stage.is-demo .footer-mock { animation: footer-rise 2s $easing infinite; }
@keyframes footer-rise {
  0%, 15%  { transform: translateY(100%); }
  55%, 100% { transform: translateY(0); }
}

.totop {
  position: absolute;
  right: $sp-6; bottom: $sp-6;
  width: 48px; height: 48px;
  border: 0;
  border-radius: $r-pill;
  background: $stage-paper;
  display: grid;
  place-items: center;
  box-shadow: $shadow-raised;
  cursor: pointer;
  // 실제 사용: --lift 는 JS 가 IntersectionObserver 로 채운다
  transform: translateY(calc(-1 * var(--lift, 0px)));
  transition: transform $duration $easing;
}
.stage.is-demo .totop { animation: totop-lift 2s $easing infinite; }
@keyframes totop-lift {
  0%, 15%   { transform: translateY(0); }
  55%, 100% { transform: translateY(-72px); }
}

.totop__arrow {
  width: 22px; height: 22px;
  fill: none;
  stroke: $color;
  stroke-width: 2.4;
  stroke-linecap: round;
  stroke-linejoin: round;
}

@media (prefers-reduced-motion: reduce) {
  .footer-mock, .totop, .stage.is-demo .footer-mock, .stage.is-demo .totop { animation: none; transition: none; }
}

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;
  overflow: hidden;
}

.page-mock {
  position: absolute;
  inset: 24px 40px auto 24px;
  display: flex;
  flex-direction: column;
  gap: 16px;
}

.page-mock__line {
  display: block;
  height: 8px;
  border-radius: 4px;
  background: rgba(255, 255, 255, 0.18);
}

.page-mock__line:nth-child(1) {
  width: 70%;
}

.page-mock__line:nth-child(2) {
  width: 50%;
}

.footer-mock {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 64px;
  background: #17141a;
  transform: translateY(100%);
}

.stage.is-demo .footer-mock {
  animation: footer-rise 2s ease-out infinite;
}

@keyframes footer-rise {
  0%, 15% {
    transform: translateY(100%);
  }
  55%, 100% {
    transform: translateY(0);
  }
}
.totop {
  position: absolute;
  right: 24px;
  bottom: 24px;
  width: 48px;
  height: 48px;
  border: 0;
  border-radius: 999px;
  background: #ffffff;
  display: grid;
  place-items: center;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  cursor: pointer;
  transform: translateY(calc(-1 * var(--lift, 0px)));
  transition: transform 220ms ease-out;
}

.stage.is-demo .totop {
  animation: totop-lift 2s ease-out infinite;
}

@keyframes totop-lift {
  0%, 15% {
    transform: translateY(0);
  }
  55%, 100% {
    transform: translateY(-72px);
  }
}
.totop__arrow {
  width: 22px;
  height: 22px;
  fill: none;
  stroke: #17141a;
  stroke-width: 2.4;
  stroke-linecap: round;
  stroke-linejoin: round;
}

@media (prefers-reduced-motion: reduce) {
  .footer-mock, .totop, .stage.is-demo .footer-mock, .stage.is-demo .totop {
    animation: none;
    transition: none;
  }
}

// 실제 사용: 푸터 센티널이 화면에 들어오면 버튼을 그만큼 translateY 로 밀어 올린다 (9줄)
var btn = document.querySelector('.totop');
var footer = document.querySelector('footer');
new IntersectionObserver(function (entries) {
  entries.forEach(function (e) {
    var overlap = Math.max(0, e.intersectionRect.height);
    btn.style.setProperty('--lift', overlap + 'px');
    btn.classList.toggle('is-near-footer', e.isIntersecting);
  });
}, { threshold: [0, 0.01, 1] }).observe(footer);

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

export type StopFooterButtonProps = {
  /** 겹치지 않게 감시할 푸터 요소를 가리키는 선택자 (기본 "footer") */
  footerSelector?: string;
  duration?: number;
  easing?: string;
};

type Vars = CSSProperties & { "--lift"?: string; "--duration"?: string; "--easing"?: string };

// 실제 사용: 푸터가 화면에 들어오면 겹친 높이만큼 translateY 로 버튼을 밀어 올린다
export default function StopFooterButton({
  footerSelector = "footer",
  duration = 2000,
  easing = "ease-out",
}: StopFooterButtonProps) {
  const [lift, setLift] = useState(0);
  const observed = useRef(false);

  useEffect(() => {
    const footer = document.querySelector(footerSelector);
    if (!footer || observed.current) return;
    observed.current = true;
    const io = new IntersectionObserver((entries) => {
      entries.forEach((e) => setLift(Math.max(0, e.intersectionRect.height)));
    }, { threshold: [0, 0.01, 1] });
    io.observe(footer);
    return () => io.disconnect();
  }, [footerSelector]);

  const vars: Vars = { "--lift": `${lift}px`, "--duration": `${duration}ms`, "--easing": easing };

  return (
    <button
      type="button"
      className={styles.totop}
      style={vars}
      aria-label="맨 위로"
      onClick={() => window.scrollTo({ top: 0, behavior: "smooth" })}
    >
      <svg className={styles.arrow} viewBox="0 0 24 24" aria-hidden="true">
        <polyline points="5,15 12,8 19,15" />
      </svg>
    </button>
  );
}