html - 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 trying to fit it in the following layout, but its height=100% doesn't work (I expect the third-party component to occupy all the green space).

Why? How would you fix that?

.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;
  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 1

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>