05Intersection reveal — an element that appears once its visibility ratio crosses a threshold

<main class="stage">
<div class="ir is-demo" id="ir">
  <div class="ir__btns">
    <button class="ir__btn" id="irDown" type="button">Scroll down</button>
    <button class="ir__btn" id="irUp" type="button">Scroll up</button>
  </div>
  <div class="ir__pane" id="irPane">
    <div class="ir__spacer"></div>
    <div class="ir__card" id="irCard">
      <p class="ir__title">Reveal complete</p>
      <p class="ir__desc">Once revealed, it stays visible even if you scroll back up.</p>
    </div>
    <div class="ir__spacer"></div>
  </div>
</div>
</main>

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

// 인터섹션 리빌 — 카드는 가시 비율 값이 기준을 넘는 순간 한 번만 opacity·translateY 로 등장한다.
// 상태(.ir__card--in)는 JS 가 한 번 붙이면 다시 떼지 않는다 — 재실행되지 않는 계산의 결과가 그대로 클래스에 남는다.

$duration: $dur-enter;
$easing: $ease-spring;
$color: $subject-lilac;

@include stage($stage-ink);

.ir {
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: $sp-3;
  width: min(100%, 280px);
}

.ir__btns { display: flex; gap: $sp-2; }

.ir__btn {
  border: 0;
  border-radius: $r-pill;
  padding: $sp-1 $sp-4;
  font-family: inherit;
  font-weight: 700;
  font-size: 11px;
  color: $stage-ink;
  background: $color;
  cursor: pointer;
}

.ir__pane {
  width: 100%;
  height: 150px;
  overflow-y: auto;
  border-radius: $r-card;
  background: rgba(255, 255, 255, .06);
}

.ir__spacer { height: 90px; }

.ir__card {
  margin: 0 $sp-3;
  padding: $sp-4;
  border-radius: $r-card;
  background: $color;
  opacity: 0;
  transform: translateY(18px);
  transition: opacity $duration $easing, transform $duration $easing;
}

.ir__card--in { opacity: 1; transform: translateY(0); }

.ir__title {
  margin: 0 0 $sp-1;
  font-weight: 800;
  font-size: 12px;
  color: $subject-ink;
}

.ir__desc {
  margin: 0;
  font-weight: 600;
  font-size: 11px;
  color: rgba(23, 20, 26, .72);
}

@media (max-width: 340px) {
  .ir__pane { height: 130px; }
  .ir__spacer { height: 70px; }
}

@media (prefers-reduced-motion: reduce) {
  .ir__card { 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);
}

.ir {
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  gap: 12px;
  width: min(100%, 280px);
}

.ir__btns {
  display: flex;
  gap: 8px;
}

.ir__btn {
  border: 0;
  border-radius: 999px;
  padding: 4px 16px;
  font-family: inherit;
  font-weight: 700;
  font-size: 11px;
  color: #17141a;
  background: #b9a0ff;
  cursor: pointer;
}

.ir__pane {
  width: 100%;
  height: 150px;
  overflow-y: auto;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
}

.ir__spacer {
  height: 90px;
}

.ir__card {
  margin: 0 12px;
  padding: 16px;
  border-radius: 16px;
  background: #b9a0ff;
  opacity: 0;
  transform: translateY(18px);
  transition: opacity 600ms cubic-bezier(0.2, 0.8, 0.2, 1), transform 600ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.ir__card--in {
  opacity: 1;
  transform: translateY(0);
}

.ir__title {
  margin: 0 0 4px;
  font-weight: 800;
  font-size: 12px;
  color: #17141a;
}

.ir__desc {
  margin: 0;
  font-weight: 600;
  font-size: 11px;
  color: rgba(23, 20, 26, 0.72);
}

@media (max-width: 340px) {
  .ir__pane {
    height: 130px;
  }
  .ir__spacer {
    height: 70px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ir__card {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('ir');
  var pane = document.getElementById('irPane');
  var card = document.getElementById('irCard');
  var downBtn = document.getElementById('irDown');
  var upBtn = document.getElementById('irUp');
  var revealed = false;
  var autoplay = null;

  // 계산 겹: 가시 비율 값이 기준(.4)을 넘는 순간 한 번만 상태를 뒤집고 즉시 관찰을 끊는다.
  // 이후 비율 값이 다시 낮아져도(스크롤을 올려도) 상태 값은 그대로 유지된다 — 재실행 없음.
  var io = new IntersectionObserver(function (entries) {
    var entry = entries[0];
    if (!revealed && entry.intersectionRatio >= 0.4) {
      revealed = true;
      card.classList.add('ir__card--in');
      io.unobserve(card);
    }
  }, { root: pane, threshold: [0, .4, .8] });
  io.observe(card);

  downBtn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    pane.scrollTop = pane.scrollHeight;
  });
  upBtn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    pane.scrollTop = 0;
  });

  function stopAutoplay() {
    if (autoplay) { clearInterval(autoplay); autoplay = null; }
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) {
    var down = true;
    autoplay = setInterval(function () {
      pane.scrollTop = down ? pane.scrollHeight : 0;
      down = !down;
    }, 520);
  }
})();

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

// 인터섹션 리빌 — 카드는 가시 비율 값이 기준을 넘는 순간 한 번만 opacity·translateY 로 등장한다.
// 계산 겹: 상태를 한 번 뒤집으면 다시 관찰을 끊어 재실행되지 않는다(비율이 다시 낮아져도 유지).
export default function IntersectionReveal() {
  const rootRef = useRef<HTMLDivElement>(null);
  const paneRef = useRef<HTMLDivElement>(null);
  const cardRef = useRef<HTMLDivElement>(null);
  const [revealed, setRevealed] = useState(false);
  const revealedRef = useRef(false);
  const autoplayRef = useRef<number | null>(null);

  function stopAutoplay() {
    if (autoplayRef.current) {
      window.clearInterval(autoplayRef.current);
      autoplayRef.current = null;
    }
  }

  useEffect(() => {
    const pane = paneRef.current;
    const card = cardRef.current;
    if (!pane || !card) return;
    const io = new IntersectionObserver((entries) => {
      const entry = entries[0];
      if (!revealedRef.current && entry.intersectionRatio >= 0.4) {
        revealedRef.current = true;
        setRevealed(true);
        io.unobserve(card);
      }
    }, { root: pane, threshold: [0, .4, .8] });
    io.observe(card);

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) {
      let down = true;
      autoplayRef.current = window.setInterval(() => {
        pane.scrollTop = down ? pane.scrollHeight : 0;
        down = !down;
      }, 520);
    }
    return () => { io.disconnect(); stopAutoplay(); };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div ref={rootRef} className={`${styles.ir} ${styles["is-demo"]}`}>
        <div className={styles.ir__btns}>
          <button
            className={styles.ir__btn}
            type="button"
            onClick={() => {
              rootRef.current?.classList.remove(styles["is-demo"]);
              stopAutoplay();
              if (paneRef.current) paneRef.current.scrollTop = paneRef.current.scrollHeight;
            }}
          >
            아래로
          </button>
          <button
            className={styles.ir__btn}
            type="button"
            onClick={() => {
              rootRef.current?.classList.remove(styles["is-demo"]);
              stopAutoplay();
              if (paneRef.current) paneRef.current.scrollTop = 0;
            }}
          >
            위로
          </button>
        </div>
        <div ref={paneRef} className={styles.ir__pane}>
          <div className={styles.ir__spacer} />
          <div ref={cardRef} className={`${styles.ir__card} ${revealed ? styles["ir__card--in"] : ""}`}>
            <p className={styles.ir__title}>등장 완료</p>
            <p className={styles.ir__desc}>한 번 나타나면 다시 스크롤을 올려도 상태가 유지됩니다.</p>
          </div>
          <div className={styles.ir__spacer} />
        </div>
      </div>
    </main>
  );
}