.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;
}

  
LEFT
  
RIGHT

if position is not fixed there is no problem, but if position is fixed - parent is not entire visible on the right side.