Flexbox grandchild height

Shows difference in behavior between Chrome and Firefox

HTML

<div id="flexy">
  <div id="flexy1">
    <div id="insideFlexy">
      I am inside
    </div>
  </div>
  <div id="flexy2">
  </div>
</div>

CSS

#flexy {
  display: flex;
  flex-direction: row;
  width: 500px;
  background-color: black;
  padding-right: 5px;
}

#flexy1 {
  width: 100px;
  background-color: blue;
  display: flex;
  align-items: stretch;  
}

#insideFlexy {
  background-color: cyan;
}

#flexy2 {
  flex-grow: 1;
  height: 300px;
  background-color: white;
}