Why Does Z-Index Behavior Differ in Internet Explorer 7?

IE7 Z-Index Layering Issues: A Comprehensive Understanding

Internet Explorer 7 (IE7) has a peculiar behavior regarding the z-index property, which is responsible for determining the stacking order of elements on a web page. This behavior can lead to unexpected layering issues, especially when working with multiple overlapping elements.

What's the Problem with Z-Index in IE7?

In IE7, the z-index property is relative to elements within the same stacking context. However, unlike most browsers, IE7 creates a new stacking context for any positioned element without an explicitly set z-index value. This means that an element with a lower z-index can overlap an element with a higher z-index if they belong to different stacking contexts.

A Practical Example

Consider the following example HTML and CSS code:

Input #1:

input {
    border: 1px solid #000;
}

div {
    border: 1px solid #00f;
}

ul {
    border: 1px solid #f00;
    background-color: #f00;
    list-style-type: none;
    margin: 0;
    padding-left: 0;
    z-index: 1000;
}

li {
    color: #fff;
    list-style-type: none;
    padding-left: 0;
    margin-left: 0;
}

span.envelope {
    position: relative;
}

span.envelope ul {
    position: absolute;
    top: 20px;
    left: 0;
    width: 150px;
}

In this example, we intended the menu (the

element) of the first envelope to overlap the second envelope. However, in IE7, the second envelope overlaps the menu because the positioned without an explicit z-index creates a new stacking context.

Possible Solutions

To address this issue, there are two main approaches:

1. Add an Explicit Z-Index to the Parent Element:

   • This forces all children (including the menu) of the parent element to overlap all siblings of the parent element. In our example, the following CSS would fix the issue:

   #envelope-1 {
       position: relative;
       z-index: 1;
   }

2. Remove the position: relative from the Parent Element:

   • If possible, remove the position: relative property from the parent element (the in our example) to avoid creating a new stacking context. This way, the menu will overlap the second envelope naturally. The modified HTML code would look like this:

   
       <title>Z-Index IE7 Test</title>
       <style type="text/css">
           ul {
               background-color: #f00;
               z-index: 1000;
               position: absolute;
               width: 150px;
           }
       </style>
   
   
       <div>
           <label>Input #1:</label> <input><br>
           <ul>
               <li>item</li>
               <li>item</li>
               <li>item</li>
               <li>item</li>
           </ul>
       </div>
   
       <div>
           <label>Input #2:</label> <input>
       </div>
   
   

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