可滚动性功能提示

概览

阴影或渐变等视觉提示有助于用户了解他们可以滚动浏览更多内容。本指南介绍了如何使用 CSS container-scroll-state-queries 构建这些提示，以便根据容器的可滚动状态为元素设置样式，而无需依赖 JavaScript 滚动监听器或观察器。

实现

1. 建立滚动容器

滚动容器必须声明为滚动状态查询容器。

.scroller {
  overflow-y: auto;
  /* Establish this element as a scroll-state query container */
  container-type: scroll-state;
  position: relative;
}

2. 设置指示器的样式

将指示器元素（例如阴影、渐变或箭头）放置在容器内并设置其样式。默认情况下，这些控件不应显示。显示时，应通过设置 pointer-events: none 使其不可交互。

.indicator-top, .indicator-bottom {
  position: sticky;
  left: 0;
  right: 0;
  height: 20px;
  opacity: 0;
  transition: opacity 0.2s;
  pointer-events: none; /* Let clicks pass through */
}

.indicator-top {
  top: 0;
  background: linear-gradient(to bottom, rgba(0,0,0,0.2), transparent); /* Example: Shadow */
}

.indicator-bottom {
  bottom: 0;
  background: linear-gradient(to top, rgba(0,0,0,0.2), transparent); /* Example: Shadow */
}

3. 查询滚动状态

将 @container 规则与 scroll-state 函数搭配使用。检查容器是否可向上或向下滚动，以显示相应的指示器。

/* Show top indicator when the user can scroll up */
@container scroll-state(scrollable: top) {
  .indicator-top {
    opacity: 1;
  }
}

/* Show bottom indicator when the user can scroll down */
@container scroll-state(scrollable: bottom) {
  .indicator-bottom {
    opacity: 1;
  }
}

后备策略

基本后备

如果不支持该功能，指示器将保持不可见状态。由于这些只是提示，对功能并不重要，因此在不支持的浏览器中省略它们是可以接受的。

高级后备（Intersection Observer）

如果需要提示，请使用 IntersectionObserver 在滚动条顶部和底部的哨兵元素移入和移出滚动条时切换类。

<!-- Sentinel elements placed at the ends of the scroller -->
<div class="sentinel-top"></div>
<!-- Content goes here -->
<div class="sentinel-bottom"></div>

/* Marker styling to ensure it does not affect layout */
.sentinel-top, .sentinel-bottom {
  height: 0;
  width: 0;
  visibility: hidden;
}

.scroller.scrolled-down .indicator-top {
  opacity: 1;
}

.scroller.can-scroll-down .indicator-bottom {
  opacity: 1;
}

if (!CSS.supports('container-type', 'scroll-state')) {
  const topSentinel = document.querySelector('.sentinel-top');
  const bottomSentinel = document.querySelector('.sentinel-bottom');
  const scroller = document.querySelector('.scroller');

  const observer = new IntersectionObserver((entries) => {
    entries.forEach(entry => {
      if (entry.target === topSentinel) {
        // If top sentinel is not intersecting, we have scrolled down
        scroller.classList.toggle('scrolled-down', !entry.isIntersecting);
      }
      if (entry.target === bottomSentinel) {
        // If bottom sentinel is intersecting, we reached the bottom
        scroller.classList.toggle('can-scroll-down', !entry.isIntersecting);
      }
    });
  }, { root: scroller });

  observer.observe(topSentinel);
  observer.observe(bottomSentinel);
}