分段式滚动

在三幅作品之间滚动,松手后停靠到其中一节。

15 / 22Section snap打开示例

在画框内上下滚动,观察作品停靠的位置。

  1. 01 · 弧面
  2. 02 · 折面
  3. 03 · 切面
  1. 使用场景

    分章节浏览作品。例如让读者一次看清一幅作品,再继续滚到下一幅。

  2. 行为定义

    局部纵向滚动在章节边缘停靠,状态栏显示最接近的一节。到达两端后可以继续滚动正文。

  3. 实现要点

    用 CSS Scroll Snap 定义停靠点。原生监听滚动位置,GSAP 用 ScrollTrigger 读取同一容器的进度,两版都按最近停靠点更新章节。

原生 HTML / CSS / JS

完整示例,可另存为 HTML 直接打开。

打开示例

查看源码
<!doctype html>
<html lang="zh-CN">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>分段式滚动 · 原生示例</title>
<script>
  if (new URLSearchParams(location.search).get('theme') === 'dark') {
    document.documentElement.dataset.theme = 'dark';
  }
</script>
<style>
  :root {
    color-scheme: light;
    --page: #fbfaf7;
    --surface: #fff;
    --surface-2: #edf3f6;
    --text: #222c36;
    --text-2: #434f5d;
    --text-3: #5c6875;
    --line-hi: #b8c4cd;
    --line-strong: #82939f;
    --accent: #b8412c;
    --accent-text: #963324;
  }
  :root[data-theme="dark"] {
    color-scheme: dark;
    --page: #19222d;
    --surface: #222e3c;
    --surface-2: #2b3c4d;
    --text: #f6f5f1;
    --text-2: #d8dce1;
    --text-3: #b4becc;
    --line-hi: #576575;
    --line-strong: #8396a8;
    --accent: #ff9d7c;
    --accent-text: #ff9d7c;
  }
  * { box-sizing: border-box; }
  body { margin: 0; background: var(--page); color: var(--text); font: 16px/1.5 system-ui, sans-serif; }
  main { max-width: 48rem; margin-inline: auto; padding: clamp(1rem, 4vw, 2rem); }
  h1 { margin: 0 0 1rem; font-size: clamp(1.5rem, 4vw, 2rem); }
  .scroll-context { min-height: 60vh; color: var(--text-2); }
  .section-snap {
  --snap-height: clamp(11.5rem, 42vw, 23rem);
  max-inline-size: 54rem;
  margin-inline: auto;
  color: var(--text);
}

.section-snap-hint {
  margin-block: 0 .75rem;
  color: var(--text-2);
  font-size: .9375rem;
  line-height: 1.55;
}

.section-snap-viewport {
  position: relative;
  block-size: var(--snap-height);
  overflow-y: auto;
  scroll-snap-type: y mandatory;
  scrollbar-width: thin;
  scrollbar-color: var(--line-strong) var(--surface-2);
  border-radius: 12px;
  background: var(--surface-2);
}

.section-snap-viewport:focus-visible,
.section-snap-buttons button:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 3px;
}

.section-snap-chapters {
  margin: 0;
  padding: 0;
  list-style: none;
}

.section-snap-chapter {
  position: relative;
  block-size: var(--snap-height);
  scroll-snap-align: start;
}

.section-snap-chapter svg {
  display: block;
  inline-size: 100%;
  block-size: 100%;
}

.section-snap-label {
  position: absolute;
  inset-inline-start: .75rem;
  inset-block-end: .75rem;
  padding: .35rem .65rem;
  border: 1px solid var(--line-hi);
  border-radius: 6px;
  background: var(--surface);
  color: var(--text);
  font-size: .8125rem;
  font-variant-numeric: tabular-nums;
  line-height: 1.2;
}

.section-snap-controls {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: .5rem .75rem;
  margin-block-start: .75rem;
  padding: .6rem .75rem;
  border-radius: 12px;
  background: var(--surface-2);
}

.section-snap-controls[hidden] { display: none; }

.section-snap-status {
  margin: 0;
  color: var(--text);
  font-size: .875rem;
  font-variant-numeric: tabular-nums;
}

.section-snap-buttons {
  display: flex;
  flex-wrap: wrap;
  gap: .5rem;
}

.section-snap-buttons button {
  min-block-size: 44px;
  padding-inline: .75rem;
  border: 1px solid var(--line-strong);
  border-radius: 10px;
  background: var(--surface);
  color: var(--text);
  font: inherit;
  font-size: .875rem;
  cursor: pointer;
}

.section-snap-buttons button:not([aria-disabled="true"]):hover {
  border-color: var(--accent);
  color: var(--accent-text);
}

.section-snap-buttons button[aria-disabled="true"] {
  border-color: var(--line-hi);
  background: transparent;
  color: var(--text-3);
  cursor: default;
}

</style>
<style>.demo-guide {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: .5rem .9rem;
  min-inline-size: 0;
  margin-block: 0 .75rem;
  padding-block-end: .5rem;
  border-block-end: 1px solid var(--line-hi, #b8c4cd);
  color: var(--text-2, #434f5d);
  font-size: .8125rem;
  line-height: 1.5;
}

.trigger-stage > .demo-guide {
  margin: .35rem clamp(.85rem, 2.2vw, 1.4rem) 0;
}

.demo-guide [data-guide-caption] { margin: 0; }

.demo-guide > button[data-guide-replay] {
  flex: none;
  min-block-size: 44px;
  margin: 0;
  padding: .35rem .65rem;
  border: 1px solid var(--line-strong, #82939f);
  border-radius: 7px;
  background: var(--surface, #fff);
  color: var(--text, #222c36);
  font: inherit;
  font-weight: 550;
  cursor: pointer;
}

.demo-guide > button[data-guide-replay]:hover { border-color: var(--accent-text, #963324); }
.demo-guide > button[data-guide-replay]:focus-visible { outline: 2px solid var(--accent-text, #963324); outline-offset: 2px; }

.demo-guide-cursor {
  position: fixed;
  z-index: 20;
  inset: 0 auto auto 0;
  inline-size: 20px;
  block-size: 24px;
  pointer-events: none;
}

.demo-guide-cursor svg { display: block; inline-size: 100%; block-size: 100%; }
.demo-guide-cursor::before {
  position: absolute;
  inset: -7px auto auto -7px;
  inline-size: 18px;
  block-size: 18px;
  border: 2px solid var(--accent-text, #963324);
  border-radius: 50%;
  content: '';
  opacity: 0;
}
.demo-guide-cursor[data-press]::before { opacity: 1; }
@media (prefers-reduced-motion: reduce) { .demo-guide-cursor { display: none; } }
</style>
</head>
<body>
<main>
  <h1>分段式滚动</h1>
  <section class="section-snap" data-section-snap aria-label="分段式滚动演示">
  <p class="section-snap-hint" id="section-snap-hint">在画框内上下滚动,观察作品停靠的位置。</p>
  <div class="section-snap-viewport" id="section-snap-viewport" data-snap-viewport tabindex="0" role="region" aria-label="三节作品展" aria-describedby="section-snap-hint">
    <ol class="section-snap-chapters">
      <li class="section-snap-chapter" data-snap-chapter data-caption="弧面">
        <svg viewBox="0 0 760 430" preserveAspectRatio="xMidYMid slice" aria-hidden="true" focusable="false"><path fill="#e8f3fa" d="M0 0h760v430H0z"/>
      <circle cx="380" cy="114" r="44" fill="#ef784d"/>
      <path fill="none" stroke="#39688b" stroke-width="21" d="M257 343v-96a123 123 0 0 1 246 0v96M303 343v-96a77 77 0 0 1 154 0v96"/>
      <path fill="#628ca8" d="M365 247h30v96h-30z"/></svg>
        <span class="section-snap-label">01 · 弧面</span>
      </li>
      <li class="section-snap-chapter" data-snap-chapter data-caption="折面">
        <svg viewBox="0 0 760 430" preserveAspectRatio="xMidYMid slice" aria-hidden="true" focusable="false"><path fill="#fffdf8" d="M0 0h760v430H0z"/>
      <path fill="#628ca8" d="m384 83 151 103-15 151-138-86-142 86-10-151z"/>
      <path fill="#e5a38a" d="m384 83-2 168-142 86-10-151z"/>
      <path fill="#f7c98b" d="m384 83 151 103-153 65-152-65z"/></svg>
        <span class="section-snap-label">02 · 折面</span>
      </li>
      <li class="section-snap-chapter" data-snap-chapter data-caption="切面">
        <svg viewBox="0 0 760 430" preserveAspectRatio="xMidYMid slice" aria-hidden="true" focusable="false"><path fill="#adcfdf" d="M0 0h760v430H0z"/>
      <circle cx="380" cy="215" r="128" fill="#39688b"/>
      <path fill="#ef784d" d="M380 87a128 128 0 0 1 0 256z"/>
      <ellipse cx="380" cy="215" rx="194" ry="54" transform="rotate(-22 380 215)" fill="none" stroke="#fffdf8" stroke-width="13"/></svg>
        <span class="section-snap-label">03 · 切面</span>
      </li>
    </ol>
  </div>
  <div class="section-snap-controls" data-snap-controls hidden>
    <p class="section-snap-status" data-snap-status role="status"></p>
    <div class="section-snap-buttons" role="group" aria-label="章节导航">
      <button type="button" data-snap-prev aria-controls="section-snap-viewport">上一节</button>
      <button type="button" data-snap-next aria-controls="section-snap-viewport">下一节</button>
    </div>
  </div>
</section>
  
  <p class="scroll-context">滚到画框的首节或末节后,可沿同一方向继续滚动页面。</p>
</main>

<script>
/**
 * @param {Element} root
 * @param {(viewport: Element, sync: () => void, measure: () => void) => void} [connectScrollTrigger]
 */
function bindSectionSnap(root, connectScrollTrigger) {
  const viewport = root.querySelector('[data-snap-viewport]');
  const chapters = [...root.querySelectorAll('[data-snap-chapter]')];
  const status = root.querySelector('[data-snap-status]');
  const controls = root.querySelector('[data-snap-controls]');
  const previous = root.querySelector('[data-snap-prev]');
  const next = root.querySelector('[data-snap-next]');
  if (!viewport || !chapters.length || !status || !controls || !previous || !next) return;

  const reducedMotion = matchMedia('(prefers-reduced-motion: reduce)');
  let stops = [];
  let active = -1;

  function syncChapter() {
    const top = viewport.scrollTop;
    const closest = stops.reduce((nearest, stop, index) => (
      Math.abs(stop - top) < Math.abs(stops[nearest] - top) ? index : nearest
    ), 0);
    if (closest === active) return;

    chapters[active]?.removeAttribute('aria-current');
    active = closest;
    chapters[active].setAttribute('aria-current', 'step');
    status.textContent = `第 ${active + 1} / ${chapters.length} 节 · ${chapters[active].dataset.caption}`;
    previous.setAttribute('aria-disabled', String(active === 0));
    next.setAttribute('aria-disabled', String(active === chapters.length - 1));
  }

  function measure() {
    stops = chapters.map((chapter) => chapter.offsetTop);
    syncChapter();
  }

  function goTo(offset) {
    const top = stops[active + offset];
    if (top === undefined) return;
    viewport.scrollTo({ top, behavior: reducedMotion.matches ? 'instant' : 'smooth' });
  }

  previous.addEventListener('click', () => goTo(-1));
  next.addEventListener('click', () => goTo(1));
  root.addEventListener('formtempo:guide-reset', () => {
    viewport.scrollTo({ top: stops[0], behavior: 'instant' });
    syncChapter();
  });
  root.addEventListener('formtempo:guide-cancel', () => {
    viewport.scrollTo({ top: viewport.scrollTop, behavior: 'instant' });
  });
  window.addEventListener('pageshow', measure);
  const resize = new ResizeObserver(measure);
  resize.observe(viewport);
  measure();
  controls.hidden = false;

  if (connectScrollTrigger) {
    connectScrollTrigger(viewport, syncChapter, measure);
  } else {
    let frame = 0;
    viewport.addEventListener('scroll', () => {
      if (frame) return;
      frame = requestAnimationFrame(() => {
        frame = 0;
        syncChapter();
      });
    }, { passive: true });
  }
}


const stage = document.querySelector('[data-section-snap]');
bindSectionSnap(stage);
</script>
<script>
(() => {
/** @param {number} duration @param {AbortSignal} signal @param {(progress: number) => void} [render] */
function guideBeat(duration, signal, render) {
  if (signal.aborted) return Promise.resolve();
  if (duration === 0) {
    render?.(1);
    return Promise.resolve();
  }

  return new Promise((resolve) => {
    let timer = 0;
    let frame = 0;
    const done = () => {
      clearTimeout(timer);
      cancelAnimationFrame(frame);
      signal.removeEventListener('abort', done);
      resolve(undefined);
    };
    signal.addEventListener('abort', done, { once: true });
    if (!render) {
      timer = window.setTimeout(done, duration);
      return;
    }

    const start = performance.now();
    const tick = (now) => {
      const progress = Math.min(1, (now - start) / duration);
      render(progress);
      if (progress === 1) done();
      else if (!signal.aborted) frame = requestAnimationFrame(tick);
    };
    frame = requestAnimationFrame(tick);
  });
}

/** @param {HTMLElement} root @param {import('./demo-guide-cases').DemoGuide} guide */
function bindDemoGuide(root, guide) {
  const view = root.querySelector(guide.view);
  if (!view || root.dataset.guideState || guide.steps.some(({ target }) => !root.querySelector(target))) return;

  const dock = guide.dock ? root.querySelector(guide.dock) : null;
  const bar = document.createElement('div');
  bar.className = 'demo-guide';
  bar.innerHTML = '<p data-guide-caption></p><button type="button" data-guide-replay>重播导览</button>';
  const caption = bar.querySelector('[data-guide-caption]');
  const replay = bar.querySelector('button');
  const heading = root === document.body ? root.querySelector('h1') : null;
  if (dock) dock.prepend(bar);
  else if (heading) heading.after(bar);
  else root.prepend(bar);

  const cursor = document.createElement('div');
  cursor.className = 'demo-guide-cursor';
  cursor.setAttribute('aria-hidden', 'true');
  cursor.hidden = true;
  cursor.innerHTML = '<svg viewBox="0 0 20 24" focusable="false"><path d="M2 2v17l4.8-4.5L10 21l3.2-1.6-3.2-6.2 6.6-.7L2 2Z" fill="#222c36" stroke="#fff" stroke-width="1.5" stroke-linejoin="round"/></svg>';
  document.body.append(cursor);

  const reduced = matchMedia('(prefers-reduced-motion: reduce)');
  const threshold = .5;
  const idleCaption = '导览在画面进入视口后播放一次。';
  const reducedCaption = '可重播关键状态,或直接操作。';
  const reset = guide.reset ? (root.matches(guide.reset) ? root : root.querySelector(guide.reset)) : null;
  const releaseTarget = guide.release ? root.querySelector(guide.release) : null;
  let autoPending = !reduced.matches;
  let inView = false;
  let playbackScroll = window.scrollY;
  /** @type {AbortController | undefined} */
  let run;
  /** @type {[number, number] | undefined} */
  let cursorPosition;

  root.dataset.guideState = 'idle';
  caption.textContent = reduced.matches ? reducedCaption : idleCaption;

  function hideCursor() {
    cursor.hidden = true;
    delete cursor.dataset.press;
    cursorPosition = undefined;
  }

  /** @param {PointerEvent} [pointer] */
  function stop(pointer) {
    autoPending = false;
    if (!run) return;
    run.abort();
    run = undefined;
    hideCursor();
    if (releaseTarget) {
      // A real pointer inside the target keeps its hover when the guide yields.
      if (pointer) sendPointer(releaseTarget, 'pointerenter', [pointer.clientX, pointer.clientY]);
      else sendPointer(releaseTarget, 'pointerleave', [0, 0]);
    }
    reset?.dispatchEvent(new Event('formtempo:guide-cancel'));
    root.dataset.guideState = 'interrupted';
    caption.textContent = '导览已停止,可以继续手动操作。';
  }

  /** @param {[number, number]} position */
  function moveCursor(position) {
    cursor.hidden = false;
    cursor.style.transform = `translate3d(${position[0]}px, ${position[1]}px, 0)`;
    cursorPosition = position;
  }

  /** @param {Element} target @param {readonly [number, number]} point */
  function location(target, [x, y]) {
    const rect = target.getBoundingClientRect();
    return /** @type {[number, number]} */ ([rect.left + rect.width * x, rect.top + rect.height * y]);
  }

  /** @param {Element} target @param {string} type @param {readonly [number, number]} position */
  function sendPointer(target, type, [clientX, clientY]) {
    target.dispatchEvent(new PointerEvent(type, { pointerType: 'mouse', clientX, clientY }));
  }

  /** @param {HTMLInputElement} slider @param {number} value */
  function setProgress(slider, value) {
    const next = Math.round(value);
    if (slider.valueAsNumber === next) return;
    slider.value = String(next);
    slider.dispatchEvent(new Event('input', { bubbles: true }));
  }

  /** @param {number} value */
  function showReducedProgress(value) {
    const options = Array.from(root.querySelectorAll('button[data-driven-step], button[data-scale-step], button[data-progress-step], button[data-progress]'));
    const percent = (button) => button.hasAttribute('data-progress')
      ? Number(button.getAttribute('data-progress')) * 100
      : Number(button.getAttribute('data-driven-step') ?? button.getAttribute('data-scale-step') ?? button.getAttribute('data-progress-step'));
    options.sort((a, b) => Math.abs(percent(a) - value) - Math.abs(percent(b) - value));
    if (options[0] instanceof HTMLButtonElement) options[0].click();
  }

  /** @param {import('./demo-guide-cases').DemoGuide['steps'][number]} step @param {AbortSignal} signal */
  async function playStep(step, signal) {
    const target = root.querySelector(step.target);
    if (!target) return;
    caption.textContent = step.text;
    const { action, value = 0, point = [.5, .5] } = step;

    if (action === 'progress' && target instanceof HTMLInputElement) {
      if (reduced.matches) {
        showReducedProgress(value);
      } else {
        const from = target.valueAsNumber;
        await guideBeat(step.duration ?? 0, signal, (progress) => {
          const current = from + (value - from) * progress;
          setProgress(target, current);
          moveCursor(location(target, [.04 + .92 * current / 100, .65]));
        });
      }
    } else {
      if (!reduced.matches) {
        const to = location(target, point);
        const [x, y] = point;
        const entry = x < .25 ? [-.08, y] : x > .75 ? [1.08, y] : [x, y < .5 ? -.08 : 1.08];
        const from = action === 'enter' ? location(target, /** @type {[number, number]} */ (entry))
          : cursorPosition ?? [to[0] + 26, to[1] + 19];
        const duration = action === 'leave' ? 200 : 340;
        await guideBeat(duration, signal, (progress) => {
          const t = 1 - (1 - progress) ** 3;
          const end = location(target, point);
          const position = /** @type {[number, number]} */ ([
            from[0] + (end[0] - from[0]) * t,
            from[1] + (end[1] - from[1]) * t,
          ]);
          moveCursor(position);
          if (action === 'move') sendPointer(target, 'pointermove', position);
        });
      }
      if (signal.aborted) return;

      if (action === 'click' && target instanceof HTMLElement) {
        if (!reduced.matches) {
          cursor.dataset.press = '';
          await guideBeat(125, signal);
          if (signal.aborted) return;
        }
        target.click();
        delete cursor.dataset.press;
      } else if (action === 'enter' || action === 'leave') {
        sendPointer(target, `pointer${action}`, location(target, point));
      }
    }

    await guideBeat(reduced.matches ? 600 : step.hold ?? 500, signal);
  }

  function bringViewIntoFrame() {
    const rect = view.getBoundingClientRect();
    const visibleHeight = Math.min(rect.bottom, window.innerHeight) - Math.max(rect.top, 0);
    if (visibleHeight < rect.height * threshold) view.scrollIntoView({ block: 'center', behavior: 'instant' });
  }

  async function start(replaying = false) {
    stop();
    const controller = new AbortController();
    const { signal } = controller;
    run = controller;
    root.dataset.guideState = 'playing';

    if (guide.ready) {
      caption.textContent = '正在等待视频就绪…';
      const deadline = performance.now() + 4000;
      while (!signal.aborted && !root.querySelector(guide.ready) && performance.now() < deadline &&
        !root.querySelector('[data-video-state="error"]')) {
        await guideBeat(90, signal);
      }
      if (signal.aborted) return;
      if (!root.querySelector(guide.ready)) {
        run = undefined;
        root.dataset.guideState = 'unavailable';
        caption.textContent = '视频暂未就绪,可以查看关键帧或重播。';
        return;
      }
    }

    reset?.dispatchEvent(new Event('formtempo:guide-reset'));
    if (replaying) bringViewIntoFrame();
    playbackScroll = window.scrollY;
    const steps = (reduced.matches && guide.reducedSteps) || guide.steps;
    for (const step of steps) {
      if (signal.aborted) return;
      await playStep(step, signal);
    }
    if (signal.aborted) return;

    run = undefined;
    hideCursor();
    root.dataset.guideState = 'complete';
    caption.textContent = '导览结束,可以继续手动操作。';
  }

  function tryAutoplay() {
    if (autoPending && inView && !document.hidden) void start();
  }

  if (typeof IntersectionObserver === 'function') {
    const observer = new IntersectionObserver(([entry]) => {
      inView = entry.intersectionRatio >= threshold;
      if (!entry.isIntersecting && run) stop();
      tryAutoplay();
    }, { threshold: [0, threshold] });
    observer.observe(view);
  }

  replay.addEventListener('click', () => {
    bringViewIntoFrame();
    void start(true);
  });

  const takeOver = (event) => {
    if (!event.isTrusted || replay.contains(event.target)) return;
    if (!run) {
      if (event.type === 'wheel' || !(event.target instanceof Element)) return;
      if (!event.target.closest('button, input') && !view.contains(event.target)) return;
    }
    const pointer = event instanceof PointerEvent && event.pointerType === 'mouse' && releaseTarget?.contains(event.target)
      ? event : undefined;
    stop(pointer);
  };
  for (const event of ['pointerdown', 'keydown', 'focusin', 'input', 'click', 'wheel']) {
    root.addEventListener(event, takeOver, { capture: true, passive: true });
  }
  if (guide.steps.some(({ action }) => action === 'enter' || action === 'move')) {
    view.addEventListener('pointermove', takeOver, { capture: true, passive: true });
  }
  if (guide.pageScrollInput) {
    window.addEventListener('scroll', () => {
      if (run && window.scrollY !== playbackScroll) stop();
    }, { passive: true });
  }
  document.addEventListener('visibilitychange', () => {
    if (document.hidden && run) stop();
    else tryAutoplay();
  });
  reduced.addEventListener('change', () => {
    if (!reduced.matches) return;
    const wasIdle = root.dataset.guideState === 'idle';
    stop();
    if (wasIdle) caption.textContent = reducedCaption;
  });
}

bindDemoGuide(document.body, {"view":"[data-snap-viewport]","reset":"[data-section-snap]","steps":[{"target":"[data-snap-next]","action":"click","text":"向下浏览,画面停靠在第二节。","hold":650},{"target":"[data-snap-next]","action":"click","text":"继续向下,停靠在第三节。","hold":650},{"target":"[data-snap-prev]","action":"click","text":"反向浏览,回到上一节的停靠点。","hold":650}]});
})();
</script>
</body>
</html>

GSAP 3.15.0

完整示例,从 CDN 加载指定版本。

打开示例

查看源码
<!doctype html>
<html lang="zh-CN">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>分段式滚动 · GSAP示例</title>
<script>
  if (new URLSearchParams(location.search).get('theme') === 'dark') {
    document.documentElement.dataset.theme = 'dark';
  }
</script>
<style>
  :root {
    color-scheme: light;
    --page: #fbfaf7;
    --surface: #fff;
    --surface-2: #edf3f6;
    --text: #222c36;
    --text-2: #434f5d;
    --text-3: #5c6875;
    --line-hi: #b8c4cd;
    --line-strong: #82939f;
    --accent: #b8412c;
    --accent-text: #963324;
  }
  :root[data-theme="dark"] {
    color-scheme: dark;
    --page: #19222d;
    --surface: #222e3c;
    --surface-2: #2b3c4d;
    --text: #f6f5f1;
    --text-2: #d8dce1;
    --text-3: #b4becc;
    --line-hi: #576575;
    --line-strong: #8396a8;
    --accent: #ff9d7c;
    --accent-text: #ff9d7c;
  }
  * { box-sizing: border-box; }
  body { margin: 0; background: var(--page); color: var(--text); font: 16px/1.5 system-ui, sans-serif; }
  main { max-width: 48rem; margin-inline: auto; padding: clamp(1rem, 4vw, 2rem); }
  h1 { margin: 0 0 1rem; font-size: clamp(1.5rem, 4vw, 2rem); }
  .scroll-context { min-height: 60vh; color: var(--text-2); }
  .section-snap {
  --snap-height: clamp(11.5rem, 42vw, 23rem);
  max-inline-size: 54rem;
  margin-inline: auto;
  color: var(--text);
}

.section-snap-hint {
  margin-block: 0 .75rem;
  color: var(--text-2);
  font-size: .9375rem;
  line-height: 1.55;
}

.section-snap-viewport {
  position: relative;
  block-size: var(--snap-height);
  overflow-y: auto;
  scroll-snap-type: y mandatory;
  scrollbar-width: thin;
  scrollbar-color: var(--line-strong) var(--surface-2);
  border-radius: 12px;
  background: var(--surface-2);
}

.section-snap-viewport:focus-visible,
.section-snap-buttons button:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 3px;
}

.section-snap-chapters {
  margin: 0;
  padding: 0;
  list-style: none;
}

.section-snap-chapter {
  position: relative;
  block-size: var(--snap-height);
  scroll-snap-align: start;
}

.section-snap-chapter svg {
  display: block;
  inline-size: 100%;
  block-size: 100%;
}

.section-snap-label {
  position: absolute;
  inset-inline-start: .75rem;
  inset-block-end: .75rem;
  padding: .35rem .65rem;
  border: 1px solid var(--line-hi);
  border-radius: 6px;
  background: var(--surface);
  color: var(--text);
  font-size: .8125rem;
  font-variant-numeric: tabular-nums;
  line-height: 1.2;
}

.section-snap-controls {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: .5rem .75rem;
  margin-block-start: .75rem;
  padding: .6rem .75rem;
  border-radius: 12px;
  background: var(--surface-2);
}

.section-snap-controls[hidden] { display: none; }

.section-snap-status {
  margin: 0;
  color: var(--text);
  font-size: .875rem;
  font-variant-numeric: tabular-nums;
}

.section-snap-buttons {
  display: flex;
  flex-wrap: wrap;
  gap: .5rem;
}

.section-snap-buttons button {
  min-block-size: 44px;
  padding-inline: .75rem;
  border: 1px solid var(--line-strong);
  border-radius: 10px;
  background: var(--surface);
  color: var(--text);
  font: inherit;
  font-size: .875rem;
  cursor: pointer;
}

.section-snap-buttons button:not([aria-disabled="true"]):hover {
  border-color: var(--accent);
  color: var(--accent-text);
}

.section-snap-buttons button[aria-disabled="true"] {
  border-color: var(--line-hi);
  background: transparent;
  color: var(--text-3);
  cursor: default;
}

</style>
<style>.demo-guide {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: .5rem .9rem;
  min-inline-size: 0;
  margin-block: 0 .75rem;
  padding-block-end: .5rem;
  border-block-end: 1px solid var(--line-hi, #b8c4cd);
  color: var(--text-2, #434f5d);
  font-size: .8125rem;
  line-height: 1.5;
}

.trigger-stage > .demo-guide {
  margin: .35rem clamp(.85rem, 2.2vw, 1.4rem) 0;
}

.demo-guide [data-guide-caption] { margin: 0; }

.demo-guide > button[data-guide-replay] {
  flex: none;
  min-block-size: 44px;
  margin: 0;
  padding: .35rem .65rem;
  border: 1px solid var(--line-strong, #82939f);
  border-radius: 7px;
  background: var(--surface, #fff);
  color: var(--text, #222c36);
  font: inherit;
  font-weight: 550;
  cursor: pointer;
}

.demo-guide > button[data-guide-replay]:hover { border-color: var(--accent-text, #963324); }
.demo-guide > button[data-guide-replay]:focus-visible { outline: 2px solid var(--accent-text, #963324); outline-offset: 2px; }

.demo-guide-cursor {
  position: fixed;
  z-index: 20;
  inset: 0 auto auto 0;
  inline-size: 20px;
  block-size: 24px;
  pointer-events: none;
}

.demo-guide-cursor svg { display: block; inline-size: 100%; block-size: 100%; }
.demo-guide-cursor::before {
  position: absolute;
  inset: -7px auto auto -7px;
  inline-size: 18px;
  block-size: 18px;
  border: 2px solid var(--accent-text, #963324);
  border-radius: 50%;
  content: '';
  opacity: 0;
}
.demo-guide-cursor[data-press]::before { opacity: 1; }
@media (prefers-reduced-motion: reduce) { .demo-guide-cursor { display: none; } }
</style>
</head>
<body>
<main>
  <h1>分段式滚动</h1>
  <section class="section-snap" data-section-snap aria-label="分段式滚动演示">
  <p class="section-snap-hint" id="section-snap-hint">在画框内上下滚动,观察作品停靠的位置。</p>
  <div class="section-snap-viewport" id="section-snap-viewport" data-snap-viewport tabindex="0" role="region" aria-label="三节作品展" aria-describedby="section-snap-hint">
    <ol class="section-snap-chapters">
      <li class="section-snap-chapter" data-snap-chapter data-caption="弧面">
        <svg viewBox="0 0 760 430" preserveAspectRatio="xMidYMid slice" aria-hidden="true" focusable="false"><path fill="#e8f3fa" d="M0 0h760v430H0z"/>
      <circle cx="380" cy="114" r="44" fill="#ef784d"/>
      <path fill="none" stroke="#39688b" stroke-width="21" d="M257 343v-96a123 123 0 0 1 246 0v96M303 343v-96a77 77 0 0 1 154 0v96"/>
      <path fill="#628ca8" d="M365 247h30v96h-30z"/></svg>
        <span class="section-snap-label">01 · 弧面</span>
      </li>
      <li class="section-snap-chapter" data-snap-chapter data-caption="折面">
        <svg viewBox="0 0 760 430" preserveAspectRatio="xMidYMid slice" aria-hidden="true" focusable="false"><path fill="#fffdf8" d="M0 0h760v430H0z"/>
      <path fill="#628ca8" d="m384 83 151 103-15 151-138-86-142 86-10-151z"/>
      <path fill="#e5a38a" d="m384 83-2 168-142 86-10-151z"/>
      <path fill="#f7c98b" d="m384 83 151 103-153 65-152-65z"/></svg>
        <span class="section-snap-label">02 · 折面</span>
      </li>
      <li class="section-snap-chapter" data-snap-chapter data-caption="切面">
        <svg viewBox="0 0 760 430" preserveAspectRatio="xMidYMid slice" aria-hidden="true" focusable="false"><path fill="#adcfdf" d="M0 0h760v430H0z"/>
      <circle cx="380" cy="215" r="128" fill="#39688b"/>
      <path fill="#ef784d" d="M380 87a128 128 0 0 1 0 256z"/>
      <ellipse cx="380" cy="215" rx="194" ry="54" transform="rotate(-22 380 215)" fill="none" stroke="#fffdf8" stroke-width="13"/></svg>
        <span class="section-snap-label">03 · 切面</span>
      </li>
    </ol>
  </div>
  <div class="section-snap-controls" data-snap-controls hidden>
    <p class="section-snap-status" data-snap-status role="status"></p>
    <div class="section-snap-buttons" role="group" aria-label="章节导航">
      <button type="button" data-snap-prev aria-controls="section-snap-viewport">上一节</button>
      <button type="button" data-snap-next aria-controls="section-snap-viewport">下一节</button>
    </div>
  </div>
</section>
  <p data-snap-fallback role="status" hidden>GSAP 未加载,当前使用原生章节定位。</p>
  <p class="scroll-context">滚到画框的首节或末节后,可沿同一方向继续滚动页面。</p>
</main>
<script src="https://cdn.jsdelivr.net/npm/gsap@3.15.0/dist/gsap.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/gsap@3.15.0/dist/ScrollTrigger.min.js"></script>
<script>
/**
 * @param {Element} root
 * @param {(viewport: Element, sync: () => void, measure: () => void) => void} [connectScrollTrigger]
 */
function bindSectionSnap(root, connectScrollTrigger) {
  const viewport = root.querySelector('[data-snap-viewport]');
  const chapters = [...root.querySelectorAll('[data-snap-chapter]')];
  const status = root.querySelector('[data-snap-status]');
  const controls = root.querySelector('[data-snap-controls]');
  const previous = root.querySelector('[data-snap-prev]');
  const next = root.querySelector('[data-snap-next]');
  if (!viewport || !chapters.length || !status || !controls || !previous || !next) return;

  const reducedMotion = matchMedia('(prefers-reduced-motion: reduce)');
  let stops = [];
  let active = -1;

  function syncChapter() {
    const top = viewport.scrollTop;
    const closest = stops.reduce((nearest, stop, index) => (
      Math.abs(stop - top) < Math.abs(stops[nearest] - top) ? index : nearest
    ), 0);
    if (closest === active) return;

    chapters[active]?.removeAttribute('aria-current');
    active = closest;
    chapters[active].setAttribute('aria-current', 'step');
    status.textContent = `第 ${active + 1} / ${chapters.length} 节 · ${chapters[active].dataset.caption}`;
    previous.setAttribute('aria-disabled', String(active === 0));
    next.setAttribute('aria-disabled', String(active === chapters.length - 1));
  }

  function measure() {
    stops = chapters.map((chapter) => chapter.offsetTop);
    syncChapter();
  }

  function goTo(offset) {
    const top = stops[active + offset];
    if (top === undefined) return;
    viewport.scrollTo({ top, behavior: reducedMotion.matches ? 'instant' : 'smooth' });
  }

  previous.addEventListener('click', () => goTo(-1));
  next.addEventListener('click', () => goTo(1));
  root.addEventListener('formtempo:guide-reset', () => {
    viewport.scrollTo({ top: stops[0], behavior: 'instant' });
    syncChapter();
  });
  root.addEventListener('formtempo:guide-cancel', () => {
    viewport.scrollTo({ top: viewport.scrollTop, behavior: 'instant' });
  });
  window.addEventListener('pageshow', measure);
  const resize = new ResizeObserver(measure);
  resize.observe(viewport);
  measure();
  controls.hidden = false;

  if (connectScrollTrigger) {
    connectScrollTrigger(viewport, syncChapter, measure);
  } else {
    let frame = 0;
    viewport.addEventListener('scroll', () => {
      if (frame) return;
      frame = requestAnimationFrame(() => {
        frame = 0;
        syncChapter();
      });
    }, { passive: true });
  }
}


const stage = document.querySelector('[data-section-snap]');

if (window.gsap && window.ScrollTrigger) {
  gsap.registerPlugin(ScrollTrigger);
  bindSectionSnap(stage, (viewport, sync, measure) => {
    ScrollTrigger.create({
      scroller: viewport,
      start: 0,
      end: () => viewport.scrollHeight - viewport.clientHeight,
      onUpdate: sync,
      onRefresh: measure,
    });
  });
} else {
  document.querySelector('[data-snap-fallback]').hidden = false;
  bindSectionSnap(stage);
}
</script>
<script>
(() => {
/** @param {number} duration @param {AbortSignal} signal @param {(progress: number) => void} [render] */
function guideBeat(duration, signal, render) {
  if (signal.aborted) return Promise.resolve();
  if (duration === 0) {
    render?.(1);
    return Promise.resolve();
  }

  return new Promise((resolve) => {
    let timer = 0;
    let frame = 0;
    const done = () => {
      clearTimeout(timer);
      cancelAnimationFrame(frame);
      signal.removeEventListener('abort', done);
      resolve(undefined);
    };
    signal.addEventListener('abort', done, { once: true });
    if (!render) {
      timer = window.setTimeout(done, duration);
      return;
    }

    const start = performance.now();
    const tick = (now) => {
      const progress = Math.min(1, (now - start) / duration);
      render(progress);
      if (progress === 1) done();
      else if (!signal.aborted) frame = requestAnimationFrame(tick);
    };
    frame = requestAnimationFrame(tick);
  });
}

/** @param {HTMLElement} root @param {import('./demo-guide-cases').DemoGuide} guide */
function bindDemoGuide(root, guide) {
  const view = root.querySelector(guide.view);
  if (!view || root.dataset.guideState || guide.steps.some(({ target }) => !root.querySelector(target))) return;

  const dock = guide.dock ? root.querySelector(guide.dock) : null;
  const bar = document.createElement('div');
  bar.className = 'demo-guide';
  bar.innerHTML = '<p data-guide-caption></p><button type="button" data-guide-replay>重播导览</button>';
  const caption = bar.querySelector('[data-guide-caption]');
  const replay = bar.querySelector('button');
  const heading = root === document.body ? root.querySelector('h1') : null;
  if (dock) dock.prepend(bar);
  else if (heading) heading.after(bar);
  else root.prepend(bar);

  const cursor = document.createElement('div');
  cursor.className = 'demo-guide-cursor';
  cursor.setAttribute('aria-hidden', 'true');
  cursor.hidden = true;
  cursor.innerHTML = '<svg viewBox="0 0 20 24" focusable="false"><path d="M2 2v17l4.8-4.5L10 21l3.2-1.6-3.2-6.2 6.6-.7L2 2Z" fill="#222c36" stroke="#fff" stroke-width="1.5" stroke-linejoin="round"/></svg>';
  document.body.append(cursor);

  const reduced = matchMedia('(prefers-reduced-motion: reduce)');
  const threshold = .5;
  const idleCaption = '导览在画面进入视口后播放一次。';
  const reducedCaption = '可重播关键状态,或直接操作。';
  const reset = guide.reset ? (root.matches(guide.reset) ? root : root.querySelector(guide.reset)) : null;
  const releaseTarget = guide.release ? root.querySelector(guide.release) : null;
  let autoPending = !reduced.matches;
  let inView = false;
  let playbackScroll = window.scrollY;
  /** @type {AbortController | undefined} */
  let run;
  /** @type {[number, number] | undefined} */
  let cursorPosition;

  root.dataset.guideState = 'idle';
  caption.textContent = reduced.matches ? reducedCaption : idleCaption;

  function hideCursor() {
    cursor.hidden = true;
    delete cursor.dataset.press;
    cursorPosition = undefined;
  }

  /** @param {PointerEvent} [pointer] */
  function stop(pointer) {
    autoPending = false;
    if (!run) return;
    run.abort();
    run = undefined;
    hideCursor();
    if (releaseTarget) {
      // A real pointer inside the target keeps its hover when the guide yields.
      if (pointer) sendPointer(releaseTarget, 'pointerenter', [pointer.clientX, pointer.clientY]);
      else sendPointer(releaseTarget, 'pointerleave', [0, 0]);
    }
    reset?.dispatchEvent(new Event('formtempo:guide-cancel'));
    root.dataset.guideState = 'interrupted';
    caption.textContent = '导览已停止,可以继续手动操作。';
  }

  /** @param {[number, number]} position */
  function moveCursor(position) {
    cursor.hidden = false;
    cursor.style.transform = `translate3d(${position[0]}px, ${position[1]}px, 0)`;
    cursorPosition = position;
  }

  /** @param {Element} target @param {readonly [number, number]} point */
  function location(target, [x, y]) {
    const rect = target.getBoundingClientRect();
    return /** @type {[number, number]} */ ([rect.left + rect.width * x, rect.top + rect.height * y]);
  }

  /** @param {Element} target @param {string} type @param {readonly [number, number]} position */
  function sendPointer(target, type, [clientX, clientY]) {
    target.dispatchEvent(new PointerEvent(type, { pointerType: 'mouse', clientX, clientY }));
  }

  /** @param {HTMLInputElement} slider @param {number} value */
  function setProgress(slider, value) {
    const next = Math.round(value);
    if (slider.valueAsNumber === next) return;
    slider.value = String(next);
    slider.dispatchEvent(new Event('input', { bubbles: true }));
  }

  /** @param {number} value */
  function showReducedProgress(value) {
    const options = Array.from(root.querySelectorAll('button[data-driven-step], button[data-scale-step], button[data-progress-step], button[data-progress]'));
    const percent = (button) => button.hasAttribute('data-progress')
      ? Number(button.getAttribute('data-progress')) * 100
      : Number(button.getAttribute('data-driven-step') ?? button.getAttribute('data-scale-step') ?? button.getAttribute('data-progress-step'));
    options.sort((a, b) => Math.abs(percent(a) - value) - Math.abs(percent(b) - value));
    if (options[0] instanceof HTMLButtonElement) options[0].click();
  }

  /** @param {import('./demo-guide-cases').DemoGuide['steps'][number]} step @param {AbortSignal} signal */
  async function playStep(step, signal) {
    const target = root.querySelector(step.target);
    if (!target) return;
    caption.textContent = step.text;
    const { action, value = 0, point = [.5, .5] } = step;

    if (action === 'progress' && target instanceof HTMLInputElement) {
      if (reduced.matches) {
        showReducedProgress(value);
      } else {
        const from = target.valueAsNumber;
        await guideBeat(step.duration ?? 0, signal, (progress) => {
          const current = from + (value - from) * progress;
          setProgress(target, current);
          moveCursor(location(target, [.04 + .92 * current / 100, .65]));
        });
      }
    } else {
      if (!reduced.matches) {
        const to = location(target, point);
        const [x, y] = point;
        const entry = x < .25 ? [-.08, y] : x > .75 ? [1.08, y] : [x, y < .5 ? -.08 : 1.08];
        const from = action === 'enter' ? location(target, /** @type {[number, number]} */ (entry))
          : cursorPosition ?? [to[0] + 26, to[1] + 19];
        const duration = action === 'leave' ? 200 : 340;
        await guideBeat(duration, signal, (progress) => {
          const t = 1 - (1 - progress) ** 3;
          const end = location(target, point);
          const position = /** @type {[number, number]} */ ([
            from[0] + (end[0] - from[0]) * t,
            from[1] + (end[1] - from[1]) * t,
          ]);
          moveCursor(position);
          if (action === 'move') sendPointer(target, 'pointermove', position);
        });
      }
      if (signal.aborted) return;

      if (action === 'click' && target instanceof HTMLElement) {
        if (!reduced.matches) {
          cursor.dataset.press = '';
          await guideBeat(125, signal);
          if (signal.aborted) return;
        }
        target.click();
        delete cursor.dataset.press;
      } else if (action === 'enter' || action === 'leave') {
        sendPointer(target, `pointer${action}`, location(target, point));
      }
    }

    await guideBeat(reduced.matches ? 600 : step.hold ?? 500, signal);
  }

  function bringViewIntoFrame() {
    const rect = view.getBoundingClientRect();
    const visibleHeight = Math.min(rect.bottom, window.innerHeight) - Math.max(rect.top, 0);
    if (visibleHeight < rect.height * threshold) view.scrollIntoView({ block: 'center', behavior: 'instant' });
  }

  async function start(replaying = false) {
    stop();
    const controller = new AbortController();
    const { signal } = controller;
    run = controller;
    root.dataset.guideState = 'playing';

    if (guide.ready) {
      caption.textContent = '正在等待视频就绪…';
      const deadline = performance.now() + 4000;
      while (!signal.aborted && !root.querySelector(guide.ready) && performance.now() < deadline &&
        !root.querySelector('[data-video-state="error"]')) {
        await guideBeat(90, signal);
      }
      if (signal.aborted) return;
      if (!root.querySelector(guide.ready)) {
        run = undefined;
        root.dataset.guideState = 'unavailable';
        caption.textContent = '视频暂未就绪,可以查看关键帧或重播。';
        return;
      }
    }

    reset?.dispatchEvent(new Event('formtempo:guide-reset'));
    if (replaying) bringViewIntoFrame();
    playbackScroll = window.scrollY;
    const steps = (reduced.matches && guide.reducedSteps) || guide.steps;
    for (const step of steps) {
      if (signal.aborted) return;
      await playStep(step, signal);
    }
    if (signal.aborted) return;

    run = undefined;
    hideCursor();
    root.dataset.guideState = 'complete';
    caption.textContent = '导览结束,可以继续手动操作。';
  }

  function tryAutoplay() {
    if (autoPending && inView && !document.hidden) void start();
  }

  if (typeof IntersectionObserver === 'function') {
    const observer = new IntersectionObserver(([entry]) => {
      inView = entry.intersectionRatio >= threshold;
      if (!entry.isIntersecting && run) stop();
      tryAutoplay();
    }, { threshold: [0, threshold] });
    observer.observe(view);
  }

  replay.addEventListener('click', () => {
    bringViewIntoFrame();
    void start(true);
  });

  const takeOver = (event) => {
    if (!event.isTrusted || replay.contains(event.target)) return;
    if (!run) {
      if (event.type === 'wheel' || !(event.target instanceof Element)) return;
      if (!event.target.closest('button, input') && !view.contains(event.target)) return;
    }
    const pointer = event instanceof PointerEvent && event.pointerType === 'mouse' && releaseTarget?.contains(event.target)
      ? event : undefined;
    stop(pointer);
  };
  for (const event of ['pointerdown', 'keydown', 'focusin', 'input', 'click', 'wheel']) {
    root.addEventListener(event, takeOver, { capture: true, passive: true });
  }
  if (guide.steps.some(({ action }) => action === 'enter' || action === 'move')) {
    view.addEventListener('pointermove', takeOver, { capture: true, passive: true });
  }
  if (guide.pageScrollInput) {
    window.addEventListener('scroll', () => {
      if (run && window.scrollY !== playbackScroll) stop();
    }, { passive: true });
  }
  document.addEventListener('visibilitychange', () => {
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bindDemoGuide(document.body, {"view":"[data-snap-viewport]","reset":"[data-section-snap]","steps":[{"target":"[data-snap-next]","action":"click","text":"向下浏览,画面停靠在第二节。","hold":650},{"target":"[data-snap-next]","action":"click","text":"继续向下,停靠在第三节。","hold":650},{"target":"[data-snap-prev]","action":"click","text":"反向浏览,回到上一节的停靠点。","hold":650}]});
})();
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