How to stretch flex child to fill height of the container?

If I understood correctly, you are looking for the align-items property, which defines the layout along the cross axis, in this case vertically.

You can find a good overview here: https://css-tricks.com/snippets/css/a-guide-to-flexbox/

.flexbox {
  display: flex;
  flex-direction: row;
  justify-content: center;
  align-items: stretch; // <-- stretch vertical (default value, can be omitted here)
  align-content: center;
}

.box {
    padding: 1em;
    text-align: center;
    margin: 1em;
}

.box--yellow {
  background-color: yellow;
}

.box--blue {
  background-color: blue;
  color: white;
}

<div class="flexbox">
  <div class="box box--yellow">
    yellow
  </div>
  <div class="box box--blue">
    some<br>
    cool<br>
    text
  </div>
</div>

Remove

height:100%;

and instead do something like this example :

<div style="display:flex; flex-direction:column">
<div style="flex-grow:1;"> Item 1 </div>
<div style="flex-grow:1;"> Item 2 </div>
</div>

above example will show you what you want.

When using Flexbox, the major mistake is to start using height: 100% all over.

We are used to that in many cases, though when it comes to Flexbox, it often breaks it instead.

The solution is simple, just remove the height: 100%, and it will work automatically.

The reason it does, is for flex item in row direction (the default), the align-items control the vertical behavior, and as its default is stretch this just work as is.

Stack snippet

<div style='display: flex'>
  <div style='background-color: yellow; width: 20px'>
  </div>
  <div style='background-color: blue'>
    some<br> cool<br> text
  </div>
</div>