01Fade in on scroll

<main class="stage">
  <div class="page-mock" aria-hidden="true">
    <span class="page-mock__line"></span>
    <span class="page-mock__line"></span>
    <span class="page-mock__line"></span>
  </div>
  <button type="button" class="totop is-demo" aria-label="Back to top">
    <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: 260ms;          // opacity·translateY 전환 시간 (실사용 값)
$easing: $ease-out;
$color: $subject-ink;      // 화살표 색

@include stage($stage-ink);
.stage { position: relative; }

.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, .14);
}
.page-mock__line:nth-child(1) { width: 72%; }
.page-mock__line:nth-child(2) { width: 88%; }
.page-mock__line:nth-child(3) { width: 56%; }

.totop {
  position: absolute;
  right: $sp-6; bottom: $sp-6;
  width: 48px; height: 48px;
  border: 0;
  border-radius: $r-pill;
  background: $stage-yellow;
  display: grid;
  place-items: center;
  box-shadow: $shadow-raised;
  cursor: pointer;
  opacity: 0;
  transform: translateY($sp-4);
  transition: opacity $duration $easing, transform $duration $easing;
}
.totop.is-visible { opacity: 1; transform: translateY(0); }

// 데모 루프 — 실제 스크롤 대신 자동으로 나타났다 사라진다 (실사용 트리거와는 별도 규칙, 캡처 주기 2s 고정)
.totop.is-demo { animation: totop-loop 2s $easing infinite; }
@keyframes totop-loop {
  0%, 100% { opacity: 0; transform: translateY($sp-4); }
  20%, 80% { opacity: 1; transform: translateY(0); }
}

.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) {
  .totop, .totop.is-demo { transition: none; animation: none; opacity: 1; transform: translateY(0); }
}

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;
}

.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, 0.14);
}

.page-mock__line:nth-child(1) {
  width: 72%;
}

.page-mock__line:nth-child(2) {
  width: 88%;
}

.page-mock__line:nth-child(3) {
  width: 56%;
}

.totop {
  position: absolute;
  right: 24px;
  bottom: 24px;
  width: 48px;
  height: 48px;
  border: 0;
  border-radius: 999px;
  background: #ffd23f;
  display: grid;
  place-items: center;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  cursor: pointer;
  opacity: 0;
  transform: translateY(16px);
  transition: opacity 260ms ease-out, transform 260ms ease-out;
}

.totop.is-visible {
  opacity: 1;
  transform: translateY(0);
}

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

@keyframes totop-loop {
  0%, 100% {
    opacity: 0;
    transform: translateY(16px);
  }
  20%, 80% {
    opacity: 1;
    transform: translateY(0);
  }
}
.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) {
  .totop, .totop.is-demo {
    transition: none;
    animation: none;
    opacity: 1;
    transform: translateY(0);
  }
}

// 실제 사용: 스크롤이 임계값을 넘으면 .is-visible 을 더해 opacity·translateY 전환이 켜진다 (7줄)
var btn = document.querySelector('.totop');
var threshold = 240;
function sync() { btn.classList.toggle('is-visible', window.scrollY > threshold); }
window.addEventListener('scroll', sync, { passive: true });
sync();

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

export type FadeScrollButtonProps = {
  /** 몇 px 내려가야 나타나는가 (기본 240) */
  threshold?: number;
  /** opacity·translateY 전환 시간 (기본 300ms) */
  duration?: number;
  easing?: string;
};

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

// 실제 사용: 스크롤이 threshold 를 넘으면 opacity·translateY 전환으로 떠오른다
export default function FadeScrollButton({
  threshold = 240,
  duration = 260,
  easing = "ease-out",
}: FadeScrollButtonProps) {
  const [visible, setVisible] = useState(false);

  useEffect(() => {
    const sync = () => setVisible(window.scrollY > threshold);
    sync();
    window.addEventListener("scroll", sync, { passive: true });
    return () => window.removeEventListener("scroll", sync);
  }, [threshold]);

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

  return (
    <button
      type="button"
      className={`${styles.totop} ${visible ? styles.visible : ""}`}
      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>
  );
}