The problem with z-index often stems from insufficient understanding of the stacked context. 1. z-index is only valid in the same stacking context, and the parent container level determines the upper limit of the child elements; 2. The way to create a new stacking context includes setting non-static position, non-auto z-index, transform, filter and other properties; 3. The pop-up window is blocked, which may be because the parent container has created an independent context, and it should be mounted to a higher level; 4. When multiple components are in different contexts, the hierarchy scope needs to be uniformly planned and managed with CSS variables; 5. To troubleshoot problems, you can view the computed style and temporarily add visual identity to assist in judgment.

The z-index property of CSS seems simple, but it is easy to have problems when it is really used in projects, especially when it involves multiple stacking contexts. You may encounter a situation where an element cannot be "out of" no matter how z-index is set. This is often not a problem with the attribute itself, but an inadequate understanding of the stacking context.

Here are some key points and practical suggestions that you must pay attention to when using z-index .

z-index is not a global hierarchical control

Many people think that as long as an element is given a higher z-index value, it will definitely be displayed at the top level. In fact, z-index is only valid within the same stacking context .

For example:
Parent container A has z-index: 100 , child element B has z-index: 9999 , and another sibling element C has z-index: 200 .
Although the value of B is much greater than C, because it belongs to the stacking context of A, the hierarchy of A itself is lower than C, so B will eventually be covered by C.

So when you find that an element "can't suppress" other elements, don't just look at its z-index , but also check the entire stacking context structure in which it is located.

Creating a new cascaded context is not difficult

As long as you set any of the following properties to an element, it will create a new cascade context:

• position not static
• z-index value is not auto
• transform , filter , opacity , <code>will-change and other properties
• Use isolation: isolate

This means that there may be a lot of nested cascaded contexts in the page, independent of each other. This is also why sometimes modifying a certain layer of z-index has little impact on the overall situation.

Several common pitfalls in actual development

Pop-up windows are always blocked?

This is one of the most common problems. If you place the pop-up window in a parent container with transform or filter set, the parent container has formed its own stacking context. Even if z-index of the pop-up window is very high, it can only "compete for the first place" within this parent container and cannot jump out of the parent to cover other components.

Solution:

• Hang the pop-up window directly to a node at body or higher level (for example, using React Portal)
• Check if there are unnecessary transform or filter that affect the cascade structure

Do multiple positioning elements overlap each other?

This situation is common in UI components such as navigation bar, floating button, drop-down menu, etc. If these components are in different stacking contexts, it is difficult to control the hierarchy relationships by z-index alone.

suggestion:

• Unified planning of the hierarchical structure and assign different hierarchical ranges to different types of components (for example, UI components are 100~200, and pop-up windows are 1000)
• Use CSS variables to manage z-index values ??for easy unified adjustment

How to quickly troubleshoot hierarchical issues?

You can view the actual cascade order of elements through the browser developer tools:

1. Select the target element in the Elements panel
2. Check the Computed style in the Styles panel to confirm whether a new stacking context has been formed
3. Look up the parent elements and see their z-index and position settings

You can also temporarily add borders or background colors to the elements to facilitate judgment of which elements are actually "on".

Basically that's it. z-index may seem simple, but once multiple context nestings are involved, a clearer sense of structure is required. Only by keeping the structure clear and the hierarchical order can we avoid the embarrassing situation of "crazy adjustment of z-index".

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