Why Does Fixed Positioning with Overflow: Hidden Fail on Nested Elements, and How Can `clip` Be Used as a Solution?

Parent and Child Elements with Fixed Positioning and Overflow: Hidden Issues

Fixed positioning is a useful CSS property that allows an element to be positioned at a specific location regardless of the scrolling behavior of its parent. However, when both the parent and child elements are positioned fixed and the parent has an overflow: hidden property, a peculiar issue arises.

Consider the following example:

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

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

In this scenario, the child element should be contained within the parent, with any overflow hidden by the parent's overflow property. However, this does not occur due to a limitation in CSS rendering.

Solution: Using CSS clip

To overcome this issue, the CSS clip property can be used instead of overflow: hidden. The clip property allows the parent to restrict the visibility of its child elements to a specific rectangular area.

.parent {
  position: fixed;
  clip: rect(0, 300px, 300px, 0);  /* Clip the parent to its own dimensions */
}

By setting the clip property to the parent element, the child element will be clipped to the parent's dimensions, effectively hiding any overflow.

Considerations

While the clip property is a viable solution, it comes with some caveats:

• The parent's position cannot be static or relative.
• Rect coordinates do not support percentages.
• Positioning and transforms of child elements may be limited.

To mitigate these limitations, the use of backface-visibility and an absolutely positioned parent can be considered.

Implementation

.parent {
  position: absolute; /* Use absolute positioning for the parent */
  clip: rect(0, 300px, 300px, 0);
  -webkit-backface-visibility: hidden;
  -moz-backface-visibility: hidden;
  backface-visibility: hidden;
}

This implementation provides a workaround for positioning and transform issues in child elements and ensures that the clipping behavior is consistent across browsers.

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