Sidebar

HTML

<div class="sidebar">
    <!-- Sidebar -->
    <aside class="sidebar__sidebar">...</aside>

    <!-- Main -->
    <main class="sidebar__main">...</main>
</div>

CSS

.sidebar {
    display: flex;
}

.sidebar__sidebar {
    width: 30%;
}

.sidebar__main {
    /* Take the remaining width */
    flex: 1;

    /* Make it scrollable */
    overflow: auto;
}