Sidebar

Try to scroll the main content!
<div class="container">
<!-- Sidebar -->
<aside class="container__sidebar">
...
</aside>
<!-- Main -->
<main class="container__main">
...
</main>
</div>
.container {
display: flex;
}
.container__sidebar {
width: 30%;
}
.container__main {
/* Take the remaining width */
flex: 1;
/* Make it scrollable */
overflow: auto;
}