In this setup:
html,
body {
height: 100%;
}
.app {
display: flex;
}
.menu {
background-color: red;
width: 100px;
}
.content {
flex: 1;
border: 1px solid blue;
}
<div class="app">
<div class="menu">menu</div>
<div class="content">
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
<div>content</div>
</div>
</div>
See Question&Answers more detail:os