Box shadow on a round element causes a thin gap

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Summary

The issue of a thin gap appearing when applying box-shadow to a round element is a common problem in web development. This gap occurs due to the way border-radius and box-shadow interact with each other. In this article, we will explore the root cause of this issue, why it happens in real systems, and how to fix it.

Root Cause

The root cause of this issue is the way box-shadow is calculated and applied to elements with border-radius. When an element has border-radius applied, the box-shadow is calculated based on the element’s bounding box, not its visual shape. This means that the box-shadow will always be applied as a rectangle, even if the element is visually a circle or an ellipse. The resulting gap is due to the difference between the bounding box and the visual shape of the element.

Why This Happens in Real Systems

This issue happens in real systems because of the following reasons:

  • Border-radius is commonly used to create rounded corners and circular elements
  • Box-shadow is often used to add depth and visual interest to elements
  • The interaction between border-radius and box-shadow can lead to unexpected visual results
  • The bounding box calculation can cause the box-shadow to be applied incorrectly

Real-World Impact

The real-world impact of this issue can be significant, including:

  • Visual inconsistencies: The gap can create an inconsistent look and feel across different elements and browsers
  • Design limitations: The issue can limit the design possibilities and creativity of developers and designers
  • User experience: The gap can be distracting and affect the overall user experience

Example or Code

div {
  margin: 1rem;
  width: 100px;
  height: 100px;
  background-color: black;
  border-radius: 100%;
  box-shadow: 0 0 0 10px black;
  outline: 10px solid black; /* add this line to fix the gap */
}

How Senior Engineers Fix It

Senior engineers fix this issue by using various techniques, including:

  • Adding an outline: Adding an outline with the same color and width as the box-shadow can help fill the gap
  • Using a wrapper element: Wrapping the element in a container and applying the box-shadow to the container can help avoid the issue
  • Using SVG: Using SVG elements can provide more control over the visual shape and box-shadow application

Why Juniors Miss It

Juniors may miss this issue because:

  • Lack of experience: Limited experience with border-radius and box-shadow can make it difficult to anticipate the issue
  • Insufficient testing: Not testing the element in different browsers and scenarios can lead to overlooking the issue
  • Incomplete understanding: Not fully understanding the bounding box calculation and its impact on box-shadow application can make it hard to identify the root cause

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