Why Does My CSS Grid Overflow When Using Percentage Gap?

Understanding CSS Grid Gap Percentage Overflow Issue

When using CSS grids, specifying the grid-gap property as a percentage can sometimes lead to unexpected results, particularly overflowing content. In this article, we'll delve into the reasons behind this overflow and provide a solution to resolve it.

The Problem: Percentage Gaps and Content Size

Initially, the percentage value for grid-gap is calculated relative to the height of the grid container. However, this approach can lead to inconsistencies in different browsers' behavior. Browsers first calculate the grid's height based on its content, effectively ignoring the percentage gap. This causes the content to fill the entire grid space, resulting in an overflow outside the grid.

Example:

Consider the following code:

.grid {
  display: grid;
  grid-gap: 50%;
  background-color: blue;
}

.grid-1 {
  background-color: red;
}

<div class="grid">
  <div class="grid-1">
    test
  </div>
  <div class="grid-1">
    test
  </div>
  <div class="grid-1">
    test
  </div>
</div>

Solution: Adjusting Grid Height

To resolve this issue, we can adjust the grid's height explicitly. One approach is to use the height property, ensuring that it is larger than the total height of the content:

.grid {
  display: grid;
  grid-gap: 50%;
  background-color: blue;
  height: calc(100% + 50%);
}

Alternatively, we can use the calc() function to define the grid's height based on the percentage gap:

.grid {
  display: grid;
  grid-gap: 50%;
  background-color: blue;
  height: calc(100% + 25% * 2);  // 50% divided by 2 for each gap
}

By explicitly defining the grid's height, we ensure that it accommodates both the content and the percentage-based gap, preventing any overflowing content.

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