Why is align-self: stretch not working on a flex item?

Question

I am trying to stretch the .side-bar div so that it takes up the height of the entire window.

I added flex to the flex container and specified the flex item widths and heights but the side-bar's height is displaying the same as the flex items.

Is it because of the order of my CSS classes?

* {
  margin: 0;
}

.flex-container {
  display: flex;
  height: 500px;
  border: 1px solid blue;
}

.flex-items {
  width: 100px;
  height: 250px;
  border: 1px solid red;
}

.side-bar {
  width: 400px;
  align-self: stretch;
}

<div class="flex-container">
  <div class="flex-items side-bar"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
</div>

Answer 1

You have specified a height on your flex items.

.flex-items {
   width: 100px;
   height: 250px;
   border: 1px solid red;
}

This height rule overrides stretch on align-items and align-self (here's why).

Also, adding align-self: stretch to .side-bar would do nothing anyway because stretch is already applied – it's the default value.

Try this instead:

* {
  margin: 0;
}

.flex-container {
  display: flex;
  height: 500px;
  border: 1px solid blue;
}

.flex-items:not(.side-bar) { /* modified selector */
  width: 100px;
  height: 250px;
  border: 1px solid red;
}

.side-bar {
  width: 400px;
  background-color: yellow;
  border: 1px solid red;
}

<div class="flex-container">
  <div class="flex-items side-bar"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
  <div class="flex-items"></div>
</div>