flex child is growing out of parent

How to force the green boxes to be contained in the red one without setting a static height value and no absolute position?

I want to shrink the content to fit into the parent.

The content (video in this case) is allowed to shrink and scrollbars are allowed.

.my-box {
  height: 300px;
  width: 600px;
  background: red;
  padding: 5px;
}
.content-box {
  background: blue;
}
.col {
  display: flex;
  flex-direction: column;
  justify-content: space-between
}
.box-shrink {
  flex: 0 1 auto;
  background: green;
  padding: 5px;
  margin: 5px;
}
.box-grow {
  flex: 1 0 auto;
  background: green;
  padding: 5px;
  margin: 5px;
}
video {
  max-height: 100%;
  max-width: 100%;
  margin: auto;
  display: block;
}

<div class="my-box col">
  <div class="box-shrink">
    small sized static content
  </div>
  <div class="content-box box-grow">
    <video controls>
      <source src="http://techslides.com/demos/sample-videos/small.webm" type="video/webm">
    </video>
  </div>
  <div class="box-shrink">
    small sized static content
  </div>
</div>

Solution #1 - Without Scroll

Instead of flex: 1 0 auto on the video container, just use flex: 1. This sizes the item based on available space, not the intrinsic height of the content.

Then, because flex items cannot be smaller than the size of their content – min-height: auto is the default – add min-height: 0 to allow the item to shrink to fit inside the container.

.box-grow {
  flex: 1; /* formerly flex: 1 0 auto; */
  background: green;
  padding: 5px;
  margin: 5px;
  min-height: 0; /* new */
}

.my-box {
  height: 300px;
  width: 600px;
  background: red;
  padding: 5px;
}
.content-box {
  background: blue;
}
.col {
  display: flex;
  flex-direction: column;
  justify-content: space-between
}
.box-shrink {
  flex: 0 1 auto;
  background: green;
  padding: 5px;
  margin: 5px;
}
.box-grow {
  flex: 1; /* formerly flex: 1 0 auto; */
  background: green;
  padding: 5px;
  margin: 5px;
  min-height: 0; /* new */
}
video {
  max-height: 100%;
  max-width: 100%;
  margin: auto;
  display: block;
}

<div class="my-box col">
  <div class="box-shrink">
    small sized static content
  </div>
  <div class="content-box box-grow">
    <video controls>
      <source src="http://techslides.com/demos/sample-videos/small.webm" type="video/webm">
    </video>
  </div>
  <div class="box-shrink">
    small sized static content
  </div>
</div>

Solution #2 - With Scroll

Alternatively, give the video container overflow: auto, which does the same as above, except it keeps the video full-width. You need to enable flex-shrink for this to work.

.box-grow {
  flex: 1 1 auto; /* formerly flex: 1 0 auto; */
  background: green;
  padding: 5px;
  margin: 5px;
  overflow: auto; /* new */
}

.my-box {
  height: 300px;
  width: 600px;
  background: red;
  padding: 5px;
}
.content-box {
  background: blue;
}
.col {
  display: flex;
  flex-direction: column;
  justify-content: space-between
}
.box-shrink {
  flex: 0 1 auto;
  background: green;
  padding: 5px;
  margin: 5px;
}
.box-grow {
  flex: 1 1 auto; /* formerly flex: 1 0 auto; */
  background: green;
  padding: 5px;
  margin: 5px;
  overflow: auto; /* new */
}
video {
  max-height: 100%;
  max-width: 100%;
  margin: auto;
  display: block;
}

<div class="my-box col">
  <div class="box-shrink">
    small sized static content
  </div>
  <div class="content-box box-grow">
    <video controls>
      <source src="http://techslides.com/demos/sample-videos/small.webm" type="video/webm">
    </video>
  </div>
  <div class="box-shrink">
    small sized static content
  </div>
</div>

Both solutions are explained in more detail here:

• https://stackoverflow.com/q/36247140/3597276

As you said, the content (a video in this case) is allowed to shrink and scrollbars are allowed. How about putting overflow:auto; on the .box-grow class and set flex-shrink: 1; like, flex: 1 1 auto; Or if you set the flex: 1 1 100%; the video will fit center on the .box-grow class also overflow:auto will be needed.

.my-box {
    height: 300px;
    width: 600px;
    background: red;
    padding: 5px;
}

.content-box {
    background: blue;
            
}

.col {
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    
}

.box-shrink {
    flex: 0 1 auto;
    background: green;
    padding: 5px;
    margin: 5px;
}

.box-grow {
    flex: 1 1 auto;  /* Set the shrik 1 which is by default */
    background: green;
    padding: 5px;
    margin: 5px; 
    overflow:auto; /* Overflow will be needed if you set flex:1 1 100%*/
}

video {
    max-height: 100%;
    max-width: 100%;
    margin: auto;
    display: block;
}

<div class="my-box col">
    <div class="box-shrink">
        small sized static content
    </div>
    <div class="content-box box-grow">
        <video controls>
            <source src="http://techslides.com/demos/sample-videos/small.webm" type="video/webm">
        </video>
    </div>
    <div class="box-shrink">
        small sized static content
    </div>
</div>

In my situation I had a max-width on the child and then deep inside the leaf node had width: 100%, somehow this was taking the outer browser width rather than the inner width

the solution was to give max-width to the intermediate flex child also.