!Rant
Being changing apartment this days, I had to continue my personal project on the notebook but soon I realized that I can't, a project that requires full stack programming isn't bearable on a 1440p 14" screen, I need at least 2 screens 😫.

On your side, have you ever stopped working because you needed more screens?

It's CSS quick maffs time! Consider the following code:

<div class='container flex'>
<nav class='menu flex'>
<a href='#'>Menu item 1</a>
(arbitrary amount of links)
</nav>
<button type='button'>Sign in</button>
</div>

You want the layout to look like a horizontally scrolling, single line menu with a Sign in button to the right. Both container and menu are flex containers. So, here's the code for the menu:

.menu {
overflow: auto;
}

The problem is, as there is no flex-wrap, menu will not be wrapped, and it will occupy all the space it's needed to accommodate all the elements, breaking its container. Pesky horizontal scroll appears on the whole body.

Boubas will set menu's width to some fixed value like 800px, and this is a bouba approach because bye-bye responsiveness.

Here's what you should do:

.menu {
overflow: auto;
min-width: 0;
}

.menu * {
flex-shrink: 0;
}

This way, menu will occupy exactly the width of an empty div. In flexbox, its width will be equal to all free space that is not occupied by the Sign in button. Setting flex-shrink is needed for items to preserve their original width. We don't care about making those items narrower on narrower screens, because we now have infinite amount of horizontal real estate. Pure, inherent responsiveness achieved without filthy media queries, yay!

The menu will scroll horizontally just like you wanted.

aight bye