CSS stacking order and stacking context

Introduction

　　Taking into account that two elements may overlap, the stacking order determines which element is in the front and which element is in the back, which is for ordinary elements. The stacking context is basically the same as the block-level formatting context (BFC), which is basically created by some CSS properties. It is used as a system alone, that is to say, the stacking order of the content is based on the stacking order of the container. Regardless of the stacking order of external elements, the creation of stacking contexts can be divided into the following three categories:

(1) The root element of the page is born with a stacking context, which is called the root stacking context

(2) The positioning element whose "z-index" is a value has its own stacking context. The larger the value, the higher the stacking order.

(3) Some CSS3 properties also have their own stacking context

stacking order of elements

　　as the picture shows:

(1) Elements with larger stacking order will cover smaller elements

(2) When the stacking order is the same, the later elements in the DOM flow will cover the earlier elements

(3) "background/border" refers specifically to the background and border of the stacking context element. If an element is a normal element, no separate stacking context is created, and its children have a negative "z-index", then this child will be hidden behind the background because its children are determined relative to the root stacking context.

Example 1:

<!-- html -->
<div class="box">
  <div class="order1"></div>
</div>

//css
.box {
  width: 200px;height: 200px;
  background: lightblue;
}
.order1 {
  width: 150px;height: 150px;
  background: red;
  position: relative;
  z-index: -1;
}

operation result:

Example 2:

//css
.box {
  width: 200px;height: 200px;
  background: lightblue;
  position: relative;
  z-index: 1;
}
.order1 {
  width: 150px;height: 150px;
  background: red;
  position: relative;
  z-index: -1;
}

运行结果：

(4) "z-index:auto" 和 "z-index:0" 虽然层叠顺序一样，也就是遵循规则 (2) 中 "后来者居上" 的原则，但是它们的区别在于后者会创建单独的层叠上下文。

示例1：

<!-- html -->
<div class="box">
  <div class="order1"><div class="child1"></div></div>
  <div class="order2"><div class="child2"></div></div>
</div>

//css
.box {
  height: 300px;
  position: relative;
  z-index: 1;
}
.order1 {
  width: 250px;height: 250px;
  background: red;
  position: absolute;
  z-index: auto;
}
.order1 .child1 {
  width: 50px;height: 50px;
  background: yellow;
  position: relative;
  z-index: 2;
}
.order2 {
  width: 200px;height: 200px;
  background: green;
  position: absolute;
  z-index: auto;
}
.order2 .child2 {
  width: 100px;height: 100px;
  background: blue;
  position: relative;
  z-index: 1;
}

运行结果：

由于 "order1" 和 "order2" 的 "z-index:auto"，虽然后者覆盖了前者，但是没有创建新的层叠上下文，因此它们的子代的层叠顺序其实是相对于同一个 "根层叠上下文"，所以 "z-index" 值大的元素位于前面。

示例2：

　　将 "order1" 和 "order2" 设置为 "z-index:0"

运行结果：

同理，后者覆盖了前者，同时 "z-index:0" 创建了新的层叠上下文，子代层叠顺序是相对于父容器决定的，而 "order1" 已经被 "order2" 覆盖，其子代无论将 "z-index" 设置为多大都无法影响父容器外部的层叠顺序。

(5) 定位元素的 "z-index" 属性默认生效，并且值为 "auto"，这也是为什么定位元素的层叠顺序要高于块级、浮动和内联元素的原因

(6) CSS3 中某些新属性会创建新的层叠上下文，层叠顺序与 "z-index:auto" 相同，例如："flex" 布局的元素，"opacity" 不为1的元素，"transform" 不是1的元素...

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CSS stacking order and stacking context

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