Messages section

A simple messaging section desktop layout, for a common social platform.
You can see the static web-implemented version on https://codepen.io/dimc/pen/WYKNeV.
I have used CSS Grid and Flexbox. I thought it would be nice to share it, because it includes some tricky CSS parts, related to the different relative and fixed areas combined with different z-index levels, while keeping the whole layout responsive (only for desktop though).
I hope that you find it useful!