08Below-fold Section Where a Scroll Value Computes a Threshold via Lazy Loading

<main class="stage">
<div class="lsr is-demo" id="lsr">
  <div class="lsr__view" id="lsrView">
    <div class="lsr__pad"></div>
    <p class="lsr__mark">— scroll threshold —</p>
    <div class="lsr__section" id="lsrSection">
      <div class="lsr__empty" id="lsrEmpty">Empty slot</div>
      <div class="lsr__content" id="lsrContent">
        <div class="lsr__row"></div>
        <div class="lsr__row"></div>
        <div class="lsr__row"></div>
      </div>
    </div>
  </div>
  <p class="lsr__hint">Scroll to fill the section below</p>
</div>
</main>

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

// 스크롤 컨테이너의 scrollTop 값을 정해 둔 임계치(px)와 비교해 넘어선
// 순간에만 아래 섹션이 빈 자리에서 실제 내용으로 바뀐다.

$duration: $dur-enter;
$easing: $ease-out;
$color: $subject-sky;

@include stage($stage-paper);

.lsr {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-2;
}

.lsr__view {
  width: 220px;
  height: 130px;
  overflow-y: auto;
  border-radius: $r-card;
  background: $stage-ink;
  scroll-behavior: auto;
}

.lsr__pad {
  height: 90px;
}

.lsr__mark {
  margin: 0;
  padding: $sp-1 0;
  text-align: center;
  font-size: 9px;
  color: rgba(255, 255, 255, .4);
}

.lsr__section {
  position: relative;
  height: 160px;
}

.lsr__empty {
  position: absolute;
  inset: $sp-2;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 1px dashed rgba(255, 255, 255, .25);
  border-radius: $r-sm;
  color: rgba(255, 255, 255, .35);
  font-size: 11px;
  transition: opacity $dur-quick $ease-out;
}

.lsr__content {
  position: absolute;
  inset: $sp-2;
  display: flex;
  flex-direction: column;
  justify-content: center;
  gap: $sp-2;
  opacity: 0;
  transform: translateY(10px);
  transition: opacity $duration $easing, transform $duration $easing;
  pointer-events: none;
}

.lsr__row {
  height: 20px;
  border-radius: $r-sm;
  background: linear-gradient(90deg, $subject-sky, $subject-blue-light);
}

.lsr__row:nth-child(2) { transition-delay: 60ms; opacity: .85; }
.lsr__row:nth-child(3) { transition-delay: 120ms; opacity: .7; }

.lsr__section--filled .lsr__empty { opacity: 0; }
.lsr__section--filled .lsr__content {
  opacity: 1;
  transform: translateY(0);
}

.lsr__hint {
  margin: 0;
  color: rgba(23, 20, 26, .55);
  font-size: 11px;
}

@media (max-width: 340px) {
  .lsr__view { width: 180px; height: 110px; }
  .lsr__pad { height: 70px; }
  .lsr__section { height: 130px; }
}

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

.lsr {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
}

.lsr__view {
  width: 220px;
  height: 130px;
  overflow-y: auto;
  border-radius: 16px;
  background: #17141a;
  scroll-behavior: auto;
}

.lsr__pad {
  height: 90px;
}

.lsr__mark {
  margin: 0;
  padding: 4px 0;
  text-align: center;
  font-size: 9px;
  color: rgba(255, 255, 255, 0.4);
}

.lsr__section {
  position: relative;
  height: 160px;
}

.lsr__empty {
  position: absolute;
  inset: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 1px dashed rgba(255, 255, 255, 0.25);
  border-radius: 8px;
  color: rgba(255, 255, 255, 0.35);
  font-size: 11px;
  transition: opacity 220ms ease-out;
}

.lsr__content {
  position: absolute;
  inset: 8px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  gap: 8px;
  opacity: 0;
  transform: translateY(10px);
  transition: opacity 600ms ease-out, transform 600ms ease-out;
  pointer-events: none;
}

.lsr__row {
  height: 20px;
  border-radius: 8px;
  background: linear-gradient(90deg, #6ec8ff, #5b8cf8);
}

.lsr__row:nth-child(2) {
  transition-delay: 60ms;
  opacity: 0.85;
}

.lsr__row:nth-child(3) {
  transition-delay: 120ms;
  opacity: 0.7;
}

.lsr__section--filled .lsr__empty {
  opacity: 0;
}

.lsr__section--filled .lsr__content {
  opacity: 1;
  transform: translateY(0);
}

.lsr__hint {
  margin: 0;
  color: rgba(23, 20, 26, 0.55);
  font-size: 11px;
}

@media (max-width: 340px) {
  .lsr__view {
    width: 180px;
    height: 110px;
  }
  .lsr__pad {
    height: 70px;
  }
  .lsr__section {
    height: 130px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .lsr__empty, .lsr__content {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('lsr');
  var view = document.getElementById('lsrView');
  var section = document.getElementById('lsrSection');
  var SCROLL_THRESHOLD_PX = 86;
  var AUTOSCROLL_STEP_PX = 2.2;
  var CYCLE_GAP_MS = 1400;

  var filled = false;
  var userActive = false;

  // 계산 겹: 스크롤 컨테이너의 scrollTop 값을 정해 둔 임계치(px)와 비교해,
  // 넘어선 순간에만 filled 상태로 바뀐다(넘기 전엔 Empty slot 유지).
  function checkThreshold() {
    var over = view.scrollTop >= SCROLL_THRESHOLD_PX;
    if (over && !filled) {
      filled = true;
      section.classList.add('lsr__section--filled');
    } else if (!over && filled) {
      filled = false;
      section.classList.remove('lsr__section--filled');
    }
  }

  view.addEventListener('scroll', checkThreshold);

  function takeover() {
    if (userActive) { return; }
    userActive = true;
    root.classList.remove('is-demo');
  }
  view.addEventListener('wheel', takeover, { passive: true });
  view.addEventListener('touchstart', takeover, { passive: true });

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var raf = null;
  var loopTimer = null;

  function autoScrollTo(target, onDone) {
    function step() {
      if (userActive) { return; }
      var cur = view.scrollTop;
      var next = cur + (target > cur ? AUTOSCROLL_STEP_PX : -AUTOSCROLL_STEP_PX);
      if ((target > cur && next >= target) || (target < cur && next <= target)) {
        view.scrollTop = target;
        checkThreshold();
        onDone();
        return;
      }
      view.scrollTop = next;
      checkThreshold();
      raf = requestAnimationFrame(step);
    }
    raf = requestAnimationFrame(step);
  }

  if (reduce) {
    root.classList.remove('is-demo');
    view.scrollTop = SCROLL_THRESHOLD_PX + 20;
    checkThreshold();
  } else {
    (function loop() {
      if (userActive) { return; }
      view.scrollTop = 0;
      checkThreshold();
      loopTimer = setTimeout(function () {
        if (userActive) { return; }
        autoScrollTo(140, function () {
          loopTimer = setTimeout(function () {
            if (!userActive && root.classList.contains('is-demo')) { loop(); }
          }, CYCLE_GAP_MS);
        });
      }, 500);
    })();
  }
})();

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

// 스크롤 컨테이너의 scrollTop 값을 정해 둔 임계치(px)와 비교해 넘어선
// 순간에만 아래 섹션이 빈 자리에서 실제 내용으로 바뀐다.
const SCROLL_THRESHOLD_PX = 86;
const AUTOSCROLL_STEP_PX = 2.2;
const CYCLE_GAP_MS = 1400;

type Box<T> = { current: T };
function useBox<T>(initial: T): Box<T> {
  return useRef(initial) as unknown as Box<T>;
}

export default function SectionRevealBelowFold() {
  const rootRef = useRef<HTMLDivElement>(null);
  const viewRef = useRef<HTMLDivElement>(null);
  const [demo, setDemo] = useState(true);
  const [filled, setFilled] = useState(false);
  const userActiveRef = useBox<boolean>(false);
  const rafRef = useBox<number | null>(null);

  const checkThreshold = useCallback(() => {
    const view = viewRef.current;
    if (!view) { return; }
    const over = view.scrollTop >= SCROLL_THRESHOLD_PX;
    setFilled((prev) => (prev === over ? prev : over));
  }, []);

  const takeover = useCallback(() => {
    if (userActiveRef.current) { return; }
    userActiveRef.current = true;
    setDemo(false);
  }, [userActiveRef]);

  const autoScrollTo = useCallback((target: number, onDone: () => void) => {
    function step() {
      const view = viewRef.current;
      if (userActiveRef.current || !view) { return; }
      const cur = view.scrollTop;
      const next = cur + (target > cur ? AUTOSCROLL_STEP_PX : -AUTOSCROLL_STEP_PX);
      if ((target > cur && next >= target) || (target < cur && next <= target)) {
        view.scrollTop = target;
        checkThreshold();
        onDone();
        return;
      }
      view.scrollTop = next;
      checkThreshold();
      rafRef.current = requestAnimationFrame(step);
    }
    rafRef.current = requestAnimationFrame(step);
  }, [checkThreshold, rafRef, userActiveRef]);

  useEffect(() => {
    const view = viewRef.current;
    view?.addEventListener("scroll", checkThreshold);
    view?.addEventListener("wheel", takeover, { passive: true });
    view?.addEventListener("touchstart", takeover, { passive: true });
    return () => {
      view?.removeEventListener("scroll", checkThreshold);
      view?.removeEventListener("wheel", takeover);
      view?.removeEventListener("touchstart", takeover);
    };
  }, [checkThreshold, takeover]);

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const view = viewRef.current;
    if (reduce) {
      setDemo(false);
      if (view) { view.scrollTop = SCROLL_THRESHOLD_PX + 20; }
      checkThreshold();
      return;
    }
    let cancelled = false;
    let t1: number | null = null;
    let t2: number | null = null;
    function loop() {
      if (userActiveRef.current || cancelled) { return; }
      if (view) { view.scrollTop = 0; }
      checkThreshold();
      t1 = window.setTimeout(() => {
        if (userActiveRef.current || cancelled) { return; }
        autoScrollTo(140, () => {
          t2 = window.setTimeout(() => {
            if (!userActiveRef.current && !cancelled && rootRef.current?.classList.contains(styles["is-demo"])) { loop(); }
          }, CYCLE_GAP_MS);
        });
      }, 500);
    }
    loop();
    return () => {
      cancelled = true;
      if (t1 !== null) { window.clearTimeout(t1); }
      if (t2 !== null) { window.clearTimeout(t2); }
      if (rafRef.current !== null) { cancelAnimationFrame(rafRef.current); }
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div ref={rootRef} className={`${styles.lsr} ${demo ? styles["is-demo"] : ""}`}>
        <div ref={viewRef} className={styles["lsr__view"]}>
          <div className={styles["lsr__pad"]} />
          <p className={styles["lsr__mark"]}>{"— 스크롤 임계선 —"}</p>
          <div className={`${styles["lsr__section"]} ${filled ? styles["lsr__section--filled"] : ""}`}>
            <div className={styles["lsr__empty"]}>빈 자리</div>
            <div className={styles["lsr__content"]}>
              <div className={styles["lsr__row"]} />
              <div className={styles["lsr__row"]} />
              <div className={styles["lsr__row"]} />
            </div>
          </div>
        </div>
        <p className={styles["lsr__hint"]}>스크롤해서 아래 섹션을 채워보세요</p>
      </div>
    </main>
  );
}