The children of a row-flexbox container automatically fill the container's vertical space.
Specify flex: 1;
for a child if you want it to fill the remaining horizontal space:
.wrapper {_x000D_
display: flex;_x000D_
flex-direction: row;_x000D_
align-items: stretch;_x000D_
width: 100%;_x000D_
height: 5em;_x000D_
background: #ccc;_x000D_
}_x000D_
.wrapper > .left_x000D_
{_x000D_
background: #fcc;_x000D_
}_x000D_
.wrapper > .right_x000D_
{_x000D_
background: #ccf;_x000D_
flex: 1; _x000D_
}
_x000D_
<div class="wrapper">_x000D_
<div class="left">Left</div>_x000D_
<div class="right">Right</div>_x000D_
</div>
_x000D_
flex: 1;
for both children if you want them to fill equal amounts of the horizontal space: .wrapper {_x000D_
display: flex;_x000D_
flex-direction: row;_x000D_
align-items: stretch;_x000D_
width: 100%;_x000D_
height: 5em;_x000D_
background: #ccc;_x000D_
}_x000D_
.wrapper > div _x000D_
{_x000D_
flex: 1; _x000D_
}_x000D_
.wrapper > .left_x000D_
{_x000D_
background: #fcc;_x000D_
}_x000D_
.wrapper > .right_x000D_
{_x000D_
background: #ccf;_x000D_
}
_x000D_
<div class="wrapper">_x000D_
<div class="left">Left</div>_x000D_
<div class="right">Right</div>_x000D_
</div>
_x000D_