如何用CSS让子div撑满带滚动溢出的父容器全高度（含隐藏区域）

问题原因 #

当前.container设置了position: relative，绝对定位的.layer继承的是.container的固定可视高度300px，无法跟随容器内部的滚动内容高度自适应，因此滚动后超出可视区的部分没有覆盖层。

解决方案 #

方案一：无需修改HTML结构（推荐）

仅修改CSS即可实现，利用Grid布局的特性让覆盖层自动匹配内容总高度：

.body{
 height: 200px; 
 overflow-y: auto;  
}
.container{
  /* 移除原有position: relative属性 */
  display: grid;
  width: 200px;
  height: 300px;  
  overflow-y: auto;  
}
.layer{
  /* 移除原有position: absolute、top: 0、width: inherit、height: inherit属性 */
  grid-area: 1 / 1 / -1 / -1;
  z-index: 1;
  background: #aaccff99;     
}

实现原理：Grid布局下容器的所有直接子元素会默认放置在同一个网格单元中，grid-area: 1 / 1 / -1 / -1设置让layer占满网格的所有行和列，大小自动匹配容器的实际内容总高度，完全保留容器的滚动能力，同时可以正常阻止下方元素的交互。

方案二：保留absolute定位写法

需要微调HTML结构，新增内容包裹层作为绝对定位的参照物：

1. 修改后的HTML代码：

<div class="body">
<div class="container">
  <div class="content-wrapper">
    <div class="layer"></div>
    <!-- 原有文本内容保持不变 -->
    Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old. Richard McClintock, a Latin professor at Hampden-Sydney College in Virginia, looked up one of the more obscure Latin words, consectetur, from a Lorem Ipsum passage, and going through the cites of the word in classical literature, discovered the undoubtable source. Lorem Ipsum comes from sections 1.10.32 and 1.10.33 of "de Finibus Bonorum et Malorum" (The Extremes of Good and Evil) by Cicero, written in 45 BC. This book is a treatise on the theory of ethics, very popular during the Renaissance. The first line of Lorem Ipsum, "Lorem ipsum dolor sit amet..", comes from a line in section 1.10.32.

    The standard chunk of Lorem Ipsum used since the 1500s is reproduced below for those interested. Sections 1.10.32 and 1.10.33 from "de Finibus Bonorum et Malorum" by Cicero are also reproduced in their exact original form, accompanied by English versions from the 1914 translation by H. Rackham.
  </div>
</div>
</div>

2. 修改后的CSS代码：

.body{
 height: 200px; 
 overflow-y: auto;  
}
.container{
  /* 移除原有position: relative属性 */
  width: 200px;
  height: 300px;  
  overflow-y: auto;  
}
.content-wrapper {
  position: relative;
}
.layer{
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: #aaccff99;     
}

实现原理：内容包裹层的高度等于实际内容总高度，作为绝对定位的参照物后，layer的100%宽高会匹配包裹层的尺寸，从而撑满整个滚动区域。

内容的提问来源于stack exchange，提问作者Nolesh。