How to make a fixed width column with a container-fluid?

A fixed-fluid layout is possible in Bootstrap 3, but now that Bootstrap 4 is flexbox this layout is much easier. You just need to enable flexbox, and make a few simple adjustments to set the width of your fixed side column flex: 0 0 300px;, and then flex: 1; on your main column to fill remaining width...

Bootstrap 4 Alpha 2 http://codeply.com/go/eAYKvDkiGw

Here's a simpler update for the latest Bootstrap 4 which is flexbox by default:

Bootstrap 4 Alpha 6 or Beta http://codeply.com/go/V9UQrvLWwz

 @media (min-width: 576px) {
     .sidebar {
          max-width: 280px;
     }
 }

<div class="container-fluid">
    <div class="row flex-nowrap">
        <div class="col-md-4 col-12 sidebar">

        </div>
        <div class="col-md col-12 main">
            <h2>Main (fluid width...)</h2>
        </div>
    </div>
</div>

Also see: How to build a 2 Column (Fixed - Fluid) Layout with Twitter Bootstrap?

Update 2018 Bootstrap 4.0.0 example

Try including your container-fluid in a div with margin-left: 200px (where 200px is your sidebar's width).

Then position your sidebar with position: absolute for instance