Fixed width sidebar on bootstrap 4 where main area disappears on mobile - css

Question

There are a lot of examples for how to have a fixed width sidebar on Bootstrap 4, but I haven't found one that will cause the main content to disappear on mobile.
I've tried the following:
<div class="row no-gutters flex-nowrap">
<div class="col-md col-12 d-none d-xs-block d-sm-block ">
Main area
</div>
<div class="col-md-4 col-12 sidebar">
Sidebar
</div>
</div>
.sidebar {
max-width: 600px; min-width: 600px
}
And this mostly works, but what happens is that between 768 to 576 pixels the sidebar disappears entirely.
What I'm after is for the sidebar to be a fixed width with the main area shrinking and eventually disappearing.

Answer 1

With few lines of CSS you can achieve this with Flexbox. Please have a look in snippet I added with this answer.
To know more about Flexbox you can checkout those links.
https://css-tricks.com/snippets/css/a-guide-to-flexbox/
https://yoksel.github.io/flex-cheatsheet/
.wrapper {
display: flex;
height: 100vh;
}
.sidebar {
min-width: 150px;
max-width: 150px;
height: 100%;
background: grey;
padding: 20px;
}
.content {
padding: 20px;
height: 100%;
background: lightgrey;
}
<div class="wrapper">
<div class="sidebar">
sidebar
</div>
<div class="content">
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</div>
</div>

Answer 2

I believe you're looking for this:
#media (min-width:768px) {
.sidebar {
max-width: 600px; min-width: 600px;
}
}
.sidebar {
background-color: #f5f5f5;
}
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="stylesheet"/>
<div class="row no-gutters flex-nowrap">
<div class="col d-none d-md-block">
Main area
</div>
<div class="col sidebar">
Sidebar
</div>
</div>
I wrapped the fixed size rule in a #media query condition only applying on md and up and I revised classes applied to columns accordingly.
Since you're setting the custom width of your sidebar anyway, you don't need col-* classes on your columns.