css - How can I have a position: fixed; behaviour for a flexbox sized element?

Question

I have a div called .side-el which I would like to have in a position: fixed; behavior, but as soon as I apply position fixed the width alternates from the right one. The right width would be the one set by flexbox. How can I achieve this goal?

.container {
  -webkit-align-content: flex-start;
  align-content: flex-start;
  -webkit-align-items: flex-start;
  align-items: flex-start;
  display: -webkit-flex;
  display: flex;
  -webkit-flex-direction: row;
  flex-direction: row;
  -webkit-flex-wrap: wrap;
  flex-wrap: wrap;
  -webkit-justify-content: flex-start;
  justify-content: flex-start;
  
  * {
    box-sizing: border-box;
    -webkit-flex-grow: 1;
    flex-grow: 1;
    -webkit-flex-shrink: 0;
    flex-shrink: 0;
  }
}


.main-el {
  box-sizing: border-box;
  padding:0 2em;
  width: 70%;
}

.side-el {
  box-sizing: border-box;
  width: 30%;
}

<div class="container" style="background-color: blue; height: 100px;">
  <div class="main-el">
    <div  style="background-color: red; height: 1000px;">content</div>
  </div>
  <div class="side-el" >
    <div style="background-color: red; height: 100px;">content</div>
  </div>
</div>

Answer 1

Here's a way to do this inspired by bootstrap:

.fixed-top {
  display: flex;
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
}

This gives your flex-box room to breathe and do it's flex-box thing. If your flex-direction is column, you could use top, left, bottom instead.

This works because when you give an element a fixed position and a left and right of 0 or a top and bottom of 0, the element is stretched to fill the space from left to right, or top to bottom. That in turn allows a flex-box to use the amount of space you would expect without position fixed.