Fixed header but something is missing - html

Question

I want to make a fixed header so I follow the steps listed in W3School, but the content becomes shorter... I don't know how to say, please see the pictures.
This is the picture that I follow the code
.header {
overflow: hidden;
width: 100%;
height: 46px;
background-color: #2f4779;
position: fixed;
top: 0;
z-index: 100;
}
<div class="header">
<div class="logo">
123
</div>
<div class="nav">
<ul>
<li>Discover and Explore</li>
</ul>
</div>
</div>
And here is the picture that I didn't write the fixed header.
Without fixed header
My plan is to make everything inside the viewport. How can I solve the problem?

Answer 1

Generally what I do in this situation is to add padding to the top of the body so it works for every page, and also use a css variable to keep the sizing consistant.
:root {
--headerHeight: 46px;
}
header {
height: var(--headerHeight);
background-color: #2f4779;
position: fixed;
top: 0;
width: 100%;
z-index: 100;
}
body {
padding-top: var(--headerHeight);
}
section {
height: 200vh;
}
<header></header>
<section>
<h1>Test</h1>
</section>

Answer 2

Well, when you give an element a fixed (or absolute) position, it is removed from the normal flow, so you need to add a padding top of 46px (eaqual to header's height) to the section that follows the header.

Answer 3

You'd probably benefit from using CSS Grids or CSS Flexbox. grid-template-columns is a great property and it makes your life a lot easier.
#grid {
display: grid;
width: 100%;
grid-template-columns: 50px 1fr;
}
#areaA {
background-color: lime;
}
#areaB {
background-color: yellow;
}
<div id="grid">
<div id="areaA">A</div>
<div id="areaB">B</div>
</div>
Do give them a read :)