How to create a fixed sidebar layout with Bootstrap 4?

Question

I'm trying to create a layout like the screenshot using Bootstrap 4 but I'm having some problems with making the sidebar fixed and achieving this layout at the same time.

Going with a basic example:

<div class="container">
  <div class="row">
    <div class="col-m-4" id="sticky-sidebar">
      Sidebar
    </div>
    <div class="col-m-8" id="main">
      Main Area
    </div>
  </div>
</div>

It's possible to get this layout but things get tricky once I declare:

.sidebar {
position: fixed // or absolute
}

Once I make the sidebar sticky, the main div starts appearing behind the sidebar instead of next to it. Of course it's possible to declare some margin and push it back to it's original position but it makes things complicated for responsiveness.

I feel like I'm missing something, I read the Bootstrap 4 documentation but I couldn't find a simple way to achieve this layout.

Answer 1

Updated 2020

Here's an updated answer for the latest Bootstrap 4.0.0. This version has classes that will help you create a sticky or fixed sidebar without the extra CSS....

Use sticky-top:

<div class="container">
    <div class="row py-3">
        <div class="col-3 order-2" id="sticky-sidebar">
            <div class="sticky-top">
                ...
            </div>
        </div>
        <div class="col" id="main">
            <h1>Main Area</h1>
            ...   
        </div>
    </div>
</div>

Demo: https://codeply.com/go/O9GMYBer4l

or, use position-fixed:

<div class="container-fluid">
    <div class="row">
        <div class="col-3 px-1 bg-dark position-fixed" id="sticky-sidebar">
            ...
        </div>
        <div class="col offset-3" id="main">
            <h1>Main Area</h1>
            ...
        </div>
    </div>
</div>

Demo: https://codeply.com/p/0Co95QlZsH

Also see:
Fixed and scrollable column in Bootstrap 4 flexbox
Bootstrap col fixed position
How to use CSS position sticky to keep a sidebar visible with Bootstrap 4
Create a responsive navbar sidebar "drawer" in Bootstrap 4?