z-index在CSS中依賴定位和堆疊上下文工作，不能單獨(dú)生效。元素必須設(shè)置relative、absolute、fixed或sticky定位才能使用z-index；1. 不同堆疊上下文中的元素不會(huì)直接比較z-index值；2. 堆疊上下文由根元素、設(shè)置了z-index的定位元素等創(chuàng)建；3. 常見(jiàn)問(wèn)題如菜單被遮擋可通過(guò)提升整個(gè)容器的z-index解決；4. 模態(tài)框應(yīng)放在body下避免嵌套在低層級(jí)上下文中；5. 應(yīng)規(guī)劃層級(jí)范圍而非濫用高數(shù)值，如UI覆蓋層1000、模態(tài)框2000、提示3000、通知4000。理解這些規(guī)則能有效減少混亂。

The z-index property in CSS might seem simple at first glance, but it can get tricky when elements are layered. It doesn’t work on its own—it depends heavily on positioning and something called a stacking context. Let’s break it down.

How z-index Actually Works

Contrary to what some beginners think, setting a high z-index value like 9999 doesn’t always bring an element to the front. For z-index to take effect, the element must have a position value of relative, absolute, fixed, or sticky.

For example:

.box {
  position: relative;
  z-index: 5;
}

This .box will appear above elements with lower z-index values within the same stacking context. If another box is positioned and has a z-index: 10, it’ll sit above this one—assuming both live in the same context.

But if they’re in different stacking contexts (more on that next), their z-index values won’t directly compete.

What Is a Stacking Context?

A stacking context is like a self-contained layer in your webpage. Elements inside it are stacked relative to each other, not to elements outside of it.

Stacking contexts form under certain conditions:

• The root element () creates the initial stacking context.
• Any element with:
  • position set to relative, absolute, or fixed a z-index value (even 0)
  • display: contents (in some cases)
  • filter, opacity , <code>transform, mix-blend-mode, etc.
  • isolation: isolate
  • will-change targeting certain properties

Once a new stacking context forms, everything inside it is treated as a group. So even if you give a child element a super high z-index, it won’t escape that parent context.

Common Pitfalls and Fixes

Here are some real-world issues people run into—and how to fix them:

• Dropdown menus hidden behind other content:
  This usually happens because the dropdown lives in a stacking context with a lower base level than another container. Try increasing the z-index of the entire dropdown container—not just the menu itself.

• Modals not showing up on top:
  Make sure the modal sits directly under the and not nested inside a component that creates its own stacking context with a low z-index baseline.

• Overusing high z-index values:
  Some developers throw in z-index: 99999 to force visibility. But this leads to messy layering later. Instead, plan ahead with consistent layers:

  • UI overlays: 1000
  • Modals: 2000
  • Tooltips: 3000
  • Notifications: 4000

That way, you avoid unnecessary conflicts.

Final Thoughts

Understanding z-index means understanding stacking contexts. They define how layers relate to each other. Once you realize that z-index only works within these boundaries, a lot of confusion goes away.

So remember:

• Position matters before z-index
• Stacking contexts isolate layers
• Don’t go overboard with numbers

基本上就這些。

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