Short Answer
Because the width of the columns plus the width of the gaps is greater than 100%.
Explanation
You have a 3-column grid container (.body):
grid-template-columns: 25% 50% 25%
The total width of those columns is 100%.
You’re then adding gutters between the columns (and rows):
grid-gap: 10px
which is shorthand for:
grid-column-gap: 10px;
grid-row-gap: 10px;
So this becomes the width calculation:
25% + 50% + 25% + 10px + 10px
Hence,
100% + 20px > 100%, which results in an overflow condition
Note that the grid-*-gap properties apply only between grid items – never between items and the container. That’s why we calculate two grid gaps, not four.
As a solution, instead of percentage units, try using fr units, which apply only to free space. This means that fr lengths are calculated after any grid-gap lengths are applied.
grid-template-columns: 1fr 2fr 1fr
div:not(.header):not(.body):not(.row) {
border: 1px solid grey;
}
.header {
margin-top: 20px;
display: grid;
grid-gap: 10px;
grid-template-areas: "header-left header-right-up" "header-left header-right-down";
grid-template-rows: 40px 40px;
grid-template-columns: minmax(50px, 200px) auto;
}
.header-left {
grid-area: header-left;
}
.header-right-up {
grid-area: header-right-up;
}
.header-right-down {
grid-area: header-right-down;
}
.body {
margin-top: 20px;
display: grid;
grid-template-columns: 1fr 2fr 1fr; /* ADJUSTMENT */
grid-auto-rows: 80px;
grid-gap: 10px;
}
.row-left {}
.row-center {}
.row-right {}<div class="header">
<div class="header-left">image</div>
<div class="header-right-up">content</div>
<div class="header-right-down">long content</div>
</div>
<div class="body">
<div class="row-left"></div>
<div class="row-center"></div>
<div class="row-right"></div>
<div class="row-left"></div>
<div class="row-center"></div>
<div class="row-right"></div>
<div class="row-left"></div>
<div class="row-center"></div>
<div class="row-right"></div>
</div>revised codepen demo
More details here: The difference between percentage and fr units