Why Does `overflow:hidden` Fail on a Fixed-Positioned Parent with Fixed-Positioned Children?

Parent and Child Elements with Position Fixed: Understanding the Overflow:hidden Bug

Introduction

In certain scenarios involving parent and child elements with fixed positioning, the overflow:hidden property on the parent element fails to function as expected. This article explores the reason behind this behavior and presents a potential solution.

Issue Overview

Consider the following CSS and HTML code:

.parent {
  position: fixed;
  overflow: hidden;
  width: 300px;
  height: 300px;
  background: #555;
}

.children {
  position: fixed;
  top: 200px;
  left: 200px;
  width: 150px;
  height: 150px;
  background: #333;
}

<div class="parent">
  <div class="children">
  </div>
</div>

In this scenario, the .parent element is positioned fixed and set to overflow:hidden, but the .children element appears to extend beyond the bounds of the parent, despite the overflow property.

Reason for the Bug

The issue arises because the parent element is positioned fixed, meaning it is removed from the normal document flow and is instead positioned relative to the viewport. Consequently, the overflow:hidden property is only applied within the fixed element's own coordinate system, which does not affect the child element outside of that coordinate system.

Potential Solution: CSS Clip Property

Since overflow:hidden does not work as expected in this scenario, an alternative approach is to use the CSS clip property on the parent element. The clip property allows you to create a clipping region, constraining the element's content within the specified boundaries:

.parent {
  position: fixed;
  clip: rect(0px, 300px, 300px, 0px);  /* Top, Right, Bottom, Left */
  width: 300px;
  height: 300px;
  background: #555;
}

Using clip: rect(), you can define a clipping region that aligns with the boundaries of the parent element, effectively hiding any child content that extends outside of those boundaries.

Considerations for Using CSS Clip Property

While the CSS clip property provides a solution to the overflow:hidden bug, it is important to note that there are some limitations and considerations to be aware of:

• The clip property has limited support in older browsers.
• It may require careful adjustment to ensure compatibility across different browsers.
• Child elements positioned relative or absolutely within the clipped parent may experience some positioning issues.

Conclusion

Understanding the limitations of overflow:hidden with fixed positioned elements is crucial for creating effective CSS layouts. By using alternative methods such as the CSS clip property, you can achieve the desired clipping behavior and avoid potential display issues. It is important to weigh the benefits and limitations of different techniques, ensuring that they align with the specific requirements of your design.

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