How to make a flex item not fill the height of the flex container? [duplicate]

The align-items, or respectively align-content attribute controls this behaviour.

align-items defines the items’ positioning perpendicularly to flex-direction.

The default flex-direction is row, therfore vertical placement can be controlled with align-items.

There is also the align-self attribute to control the alignment on a per item basis.

#a {
  display:flex;

  align-items:flex-start;
  align-content:flex-start;
  }

#a > div {
  
  background-color:red;
  padding:5px;
  margin:2px;
  }
 #a > #c {
  align-self:stretch;
 }

<div id="a">
  
  <div id="b">left</div>
  <div id="c">middle</div>
  <div>right<br>right<br>right<br>right<br>right<br></div>
  
</div>

css-tricks has an excellent article on the topic. I recommend reading it a couple of times.