Why does the z-index property not work with PDF files in iframes in Internet Explorer?

z-index Issue with PDFs in iframes in Internet Explorer

Browsing the web, many developers encounter the challenge of positioning elements with the z-index property. In particular, when using iframes containing PDF files, the z-index property might fail to work in Internet Explorer. While the issue may not arise in other browsers like Google Chrome, it can cause obstacles in achieving desired visual arrangements.

Background: Windowed and Windowless Elements in Internet Explorer

To understand the root of the issue, it's crucial to know how Internet Explorer categorizes HTML elements. IE classifies elements into two types: windowed and windowless.

• Windowless elements, such as divs and inputs, are directly rendered by the browser and occupy the same plane, respecting their z-index values.
• Windowed elements, such as ActiveX controls and select elements, are rendered outside of the browser's main rendering engine and are drawn on top of windowless elements, regardless of z-index.

Iframe Behavior in Internet Explorer

Initially, iframes were considered windowed elements in IE5, but this changed in IE5.5, where they became windowless elements. However, for backward compatibility reasons, iframes still have the ability to draw over windowed elements with lower z-indexes.

The Issue

The issue arises when an iframe containing a PDF (a windowed element) is placed on a web page. Due to the unique behavior of iframes in IE, any windowless elements (e.g., regular text or buttons) positioned over the iframe will be hidden behind the PDF, regardless of their z-index values.

Solution: A Covering Iframe

To resolve this issue, an additional iframe, known as a "covering iframe," is introduced. This iframe is positioned between the PDF iframe and the windowless elements that need to be displayed on top. By setting the covering iframe's z-index to a negative value (e.g., -1), it is effectively placed behind all other elements, allowing the windowless elements to be visible over the PDF.

Implementation

The HTML code for implementing this solution would look something like this:

<code class="html"><div id="outer">
  <div id="inner">my text that should be on top</div>
  <iframe class="cover" src="about:blank"></iframe>
</div>

<iframe id="pdf" src="path/to/pdf.pdf" width="200" height="200"></iframe></code>

With the following CSS:

<code class="css">#outer {
  position: relative;
  left: 150px;
  top: 20px;
  width: 100px;
  z-index: 2;
}

#inner {
  background: red;
}

.cover {
  border: none;
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  z-index: -1;
}

#pdf {
  position: relative;
  z-index: 1;
}</code>

Browser Support

This solution has been tested and confirmed to work in Internet Explorer 7 through 9. It's important to note that the covering iframe approach may not be ideal for all cases or browsers, and alternative solutions may be more suitable depending on specific requirements.

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