How to stretch a block to full width?

Question

css

Good afternoon, please help, I want the black rectangle to occupy all available space when the menu is resized

Here is a link to this page cardiohelp.me/ultra-sound-analyzer.html
Here is a page where this effect works cardiohelp.me/charts.html
What done wrong? thanks in advance

Answer 1

.viewport-containers {
background-color: black;
width: 80%
height: 80%;
color: red;
position: absolute;
right: 5px;
}
change the numbers to 100% and right 0