Why height=100% doesn't work?

Question

I use a third-party component that occupies all the available space, i.e. width=100% and height=100%. I don\'t have control over it.

I\'m t

Answer 1

Because .content haven't height (height = 0px) and .third-party-component have 100% of 0px. You can add propety height : calc (100% - <height of .header>) into .content

.container {
  display: flex;
  flex-direction: column;
  width: 200px;
  height: 100px;
}

.header {
  display: flex;
  background-color: rgba(255, 0, 0, 0.5);
}

.content {
  height: calc(100% - 18px);
  flex-grow: 1;
  background-color: rgba(0, 255, 0, 0.5);
}

.third-party-component {
  height: 100%;
  width: 100%;
  background-color: rgba(0, 0, 255, 0.5);
}

<div class="container">
  <div class="header">Header</div>
  <div class="content">
    <div class="third-party-component">
      Third party component
    </div>
  </div>
</div>

Answer 2

In general, for an element using percent on height to pick up its parent's height, the parent need a height other than auto or being positioned absolute, or the height will be computed as auto.

Based on those 2 options, and as you mentioned in a comment, your own header is dynamic in height, you are left with absolute positioning.

The problem with adding absolute to the content, it will be taken out of flow and stop behaving as a normal flowed flex item, the good news, one can add a wrapper set to absolute.

Stack snippet

.container {
  display: flex;
  flex-direction: column;
  width: 200px;
  height: 100px;
}

.header {
  display: flex;
  background-color: rgba(255, 0, 0, 0.5);
}

.content {
  position: relative;                               /*  added  */
  flex-grow: 1;
  background-color: rgba(0, 255, 0, 0.5);
}

.wrapper {
  position: absolute;                               /*  added  */
  left: 0;                                          /*  added  */
  top: 0;                                           /*  added  */
  right: 0;                                         /*  added  */
  bottom: 0;                                        /*  added  */
}

.third-party-component {
  height: 100%;
  width: 100%;
  background-color: rgba(0, 0, 255, 0.5);
}

<div class="container">
  <div class="header">Header</div>
  <div class="content">
    <div class="wrapper">
      <div class="third-party-component">
       Third party component
      </div>
    </div>
  </div>
</div>

---

Another option could be to update the Flexbox properties, to give the content a height, using flex: 1 1 100% and give header flex-shrink: 0; so it doesn't shrink (as content got 100%).

This might not work on Safari though, as I know it have had issues when the height property is not set, though can't test that now as I don't have access to Safari.

.container {
  display: flex;
  flex-direction: column;
  width: 200px;
  height: 100px;
}

.header {
  display: flex;
  flex-shrink: 0;
  background-color: rgba(255, 0, 0, 0.5);
}

.content {
  flex: 1 1 100%;
  background-color: rgba(0, 255, 0, 0.5);
}

.third-party-component {
  height: 100%;
  width: 100%;
  background-color: rgba(0, 0, 255, 0.5);
}

<div class="container">
  <div class="header">Header</div>
  <div class="content">
    <div class="third-party-component">
      Third party component
    </div>
  </div>
</div>

Answer 3

You can simply use another flex container in the .content element:

.container {
  display: flex;
  flex-direction: column;
  width: 200px;
  height: 100px;
}

.header {
  display: flex;
  background-color: rgba(255, 0, 0, 0.5);
}

.content {
  flex-grow: 1;
  display: flex;
  flex-direction: column;
  background-color: rgba(0, 255, 0, 0.5);
}

.third-party-component {
  flex-grow: 1;
  height: 100%;
  width: 100%;
  background-color: rgba(0, 0, 255, 0.5);
}

<div class="container">
  <div class="header">Header</div>
  <div class="content">
    <div class="third-party-component">
      Third party component
    </div>
  </div>
</div>