html - Why display grid with 100% in grid-template-columns goes out of body?

Question

.parent {
  position:fixed;
  width:100%;
  left:0;
  top:14px;
  display:grid;
  grid-template-columns:40% 60%;
  grid-gap:5px;
  background:#eee;
}
.left {
  border:2px solid red;
}
.right {
  border:2px solid red;
}

<div class='parent'>
  <div class='left'>LEFT</div>
  <div class='right'>RIGHT</div>
</div>

Answer 1

The issue isn't with width:100% like you think. It is with grid-template that you made 40% 60% and you also have a grid-gap of 5px which will make the total more than 100%.

Instead rely on the fr unit to split the free space considering the gap:

.parent {
  position:fixed;
  width:100%;
  left:0;
  top:14px;
  display:grid;
  grid-template-columns:4fr 6fr;
  grid-gap:5px;
  background:#eee;
}
.left {
  border:2px solid red;
}
.right {
  border:2px solid red;
}

<div class='parent'>
  <div class='left'>LEFT</div>
  <div class='right'>RIGHT</div>
</div>