Child with position absolute has full parent height - css

Question

Trying to recreate this layout:
The problem is the white box on top of the black. I don't know the height of the black box. How can I create the white box on top of the others?
This is my code:
https://codepen.io/olefrankjensen/pen/oQyZpX?editors=1100
HTML
<div class="container">
<div class="formbox">[login form markup here...]</div>
<div class="box">
<h1>Don't have an account</h1>
<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Accusamus dicta cumque harum.</p>
<button>SIGN UP</button>
</div>
<div class="box">
<h1>Have an account</h1>
<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Accusamus dicta cumque harum.</p>
<button>LOGIN</button>
</div>
</div>
CSS
// reset
h1, h2, h3, h4, h5, h6, p, button, div {
margin: 0;
padding: 0;
line-height: 1;
}
// font
#import url('https://fonts.googleapis.com/css?family=Roboto');
// colors
$white: rgba(255,255,255,.8);
:root {
font-size: 16px;
}
body {
background-image: url(
https://picsum.photos/1200/800?);
display: flex;
justify-content: center;
align-items: center;
font-family: 'Roboto', sans-serif;
color: $white;
}
.container {
width: 900px;
margin: 50px auto;
background: rgba(0,0,0,.5);
position: relative;
display: flex;
}
.box {
padding: 5rem 3rem;
float: left;
h1, p {
margin-bottom: 1rem;
}
button {
padding: 1rem 2rem;
background: transparent;
border: 2px solid $white;
border-radius: 6px;
color: $white;
text-align: center;
font-size: 1rem;
cursor: pointer;
text-transform: uppercase;
}
}
.formbox {
width: 500px;
background: white;
margin-left: -50px;
margin-top: -25px;
box-shadow: 0 0 50px black;
display: block;
position: absolute;
}
I have to absolute position the .formbox to not make it "interfere" with the other flex items in the flex container .container. But as a consequence it has no height.
How can I give the Login overlay full height of it's parent?

Answer 1

When position is absolute, top, bottom, right and left properties work relative to the first parent with position: relative.
So, you don't really need the margin-top: -25px or margin-left: -50px, you can just add top: -25px; bottom: -25px; left: -25px to .formbox and you are done. If you need to move it to the right, just add right: XXpx and remove left (or set a positive value like left: 50%).
https://codepen.io/anon/pen/gQKWNq?editors=1100

Answer 2

I could go with adding to formbox class a height of 100% + 50px (margin -25px what you have took x 2). You can do this with calc function: https://developer.mozilla.org/en-US/docs/Web/CSS/calc
.formbox {
width: 500px;
background: white;
margin-left: -50px;
margin-top: -25px;
box-shadow: 0 0 50px black;
display: block;
position: absolute;
height: calc(100% + 50px);
}