Stacking context is created in CSS with specific attributes, which determines the order in which elements stack on the z-axis. Common ways of creating include: 1. Set position to relative, absolute, fixed or sticky and cooperate with z-index; 2. Use opacity to be less than 1; 3. Use transform, filter, will-change and other properties; 4. Set isolation: isolate. Once created, the child elements will be stacked in the environment, and the levels between different stacking contexts cannot be interspersed. The level comparison requires consideration of the hierarchy depth of the stacking context, not just the z-index value. To avoid stacking problems, you should avoid abuse of high z-index, check whether the element is positioned, pay attention to whether the parent element has created a new stacking context, and use developer tools to view the hierarchy. Understanding the nesting and isolation mechanism of stacking context is key.
A stacking context determines the stacking order of elements in the z-axis direction in CSS. In other words, it controls which elements are displayed in front and which are displayed in back. This is not a problem that can be solved simply by z-index, because z-index is only valid within the same stacking context.

How is Stacking context created?
Not all elements are automatically in the same stacking environment. When you use certain CSS properties and values, the browser creates a new stacking context for this element.
Common ways of creating include:

- Set
position
torelative
,absolute
,fixed
, orsticky
, and cooperate withz-index
- Use
opacity
value less than 1 - Use
transform
,filter
,will-change
and other properties - Set
isolation: isolate
Once an element creates a new stacking context, its child elements are stacked in this new environment without affecting the external hierarchy.
How to compare the levels of different stacking contexts?
The levels between different stacking contexts cannot be interspersed with each other. You can think of it as a "box set box", each box has its own hierarchical relationship inside, but the entire box can only appear as a whole in the parent class.

For example:
Suppose you have two divs, A and B, which belong to different stacking contexts. A's z-index is 1000 and B's 2000, but if the stacking context of the parent container A is located is behind B, it will still be displayed below B even if A's z-index is larger.
Therefore, the levels not only look at the numerical size of z-index, but also the depth of the stacking context level they are in .
How to avoid common stacking problems?
Many times we set z-index but have no effect, or some pop-up windows on the page are blocked, which is usually because we do not understand the mechanism of stacking context.
Some practical suggestions:
- Avoid abuse of high z-index values (such as 9999), which can easily cause confusion.
- If you find that z-index does not work, check if the element is positioned (position is not static).
- Note whether the parent element creates a new stacking context, which affects the hierarchical performance of the child element.
- For complex layouts, you can use the browser developer tools to view the hierarchy structure and observe the stacking context in which each element is located.
Basically that's it. Understanding stacking context does not require rote memorization. The key is to understand how it is nested and isolated layer by layer, and ultimately determine the relationship between elements.
The above is the detailed content of What is a stacking context in CSS. For more information, please follow other related articles on the PHP Chinese website!

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