Why Doesn\'t My CSS Grid Item Height Double the Parent Container\'s Height?

CSS Grid: Element Height Calculation Confusion

You're encountering an issue with the height of a CSS grid element, specifically the .gridItem, not being calculated as expected. In this grid setup:

.gridContainer {
  height: 100px;
  grid-template-rows: 1fr;
}

.gridItem {
  height: 200%;
}

The intention is for .gridItem to be twice the height of its parent, .gridContainer. However, the result seems to consider the child element's height instead.

Understanding Parent-Child Relationship in CSS Grid

When working with CSS grids, it's crucial to understand the parent-child relationship. In this case, the parent element for the .gridItem is not the .gridContainer directly but rather the grid track that contains it.

When specifying grid-template-rows: 1fr;, you create a track with a flexible height that takes up 1 fractional unit of the parent's available space. Since the parent is a fixed height of 100px, the track's height becomes 100px.

Therefore, even though .gridItem has height: 200%, it refers to the height of its parent track, which is 100px. This explains why the height of .gridItem is effectively twice the height of the track instead of the parent container.

Fix: Setting Explicit Heights on Both Container and Track

To resolve this issue, you need to specify a fixed height on both the .gridContainer and the grid track. This establishes a clear parent-child relationship and allows the percentage height of .gridItem to function correctly.

.gridContainer {
  height: 100px;
  grid-template-rows: 100px;
}

.gridItem {
  height: 200%;
}

With this adjustment, the height of .gridItem should now be twice the height of .gridContainer, as expected.

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