<div class="page-wrap">
  <header class="header">
    <h1>Header goes here</h1>
  </header>

  <div class=" sidebar left-sidebar">
    <nav class="side-nav">
      <h2>Header here</h2>
      <ul>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
        <li><a href="#">Lots of menu items</a></li>
      </ul>
    </nav>
  </div>
  <div class="content">
    <h2>Main Content</h2>
    <p>main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : </p>
    <p>main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : </p>
    <p>main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : </p>
    <p>main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : main content goes here : </p>

  </div>

  <footer class="footer">
    <p>Footer content goes here</p>
  </footer>

</div>

html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
  font-family: sans-serif;
}
html {
  box-sizing: border-box;
}
*,
*:before,
*:after {
  box-sizing: inherit;
}

.page-wrap {
  max-width: 2100px;
  min-height: 100%;
  background: #f9f9f9;
  padding: 10px;
  margin: auto;
  display: grid;
  gap: 1rem;
  grid-template-columns: 320px 1fr;
  grid-template-rows: auto 1fr auto;
  grid-template-areas:
    "fixed header"
    "fixed main"
    "fixed footer";
}
.header {
  grid-area: header;
}
.left-sidebar {
  grid-area: fixed;
}
.content {
  grid-area: main;
}
.footer {
  grid-area: footer;
}

.content,
.header,
.footer {
  background: #ccc;
  border: 5px solid red;
  padding: 10px;
}
.left-sidebar {
  padding: 0;
}

/* side nav */
.side-nav {
  position: fixed;
  bottom: 10px;
  top: 10px;
  width: 310px;
  z-index: 99;
  background: teal;
  overflow: auto;
  border: 5px solid red;
}

.side-nav h2 {
  position: -webkit-sticky; /* needed for ios*/
  position: sticky;
  top: 0;
  background: rgba(0, 0, 0, 0.8);
  color: #fff;
  padding: 1rem;
  font-size: 1rem;
  margin: 0;
  box-shadow: 0 0 20px rgba(0, 0, 0, 0.4);
}
.side-nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
}
.side-nav a {
  text-decoration: none;
  color: #eee;
  display: block;
  border-bottom: 1px solid #000;
  padding: 1rem;
}
.side-nav a:hover {
  background: #000;
  color: #fff;
}

@media screen and (max-width: 980px) {
  .page-wrap {
    grid-template-columns: auto;
    grid-template-areas:
      "header"
      "main"
      "footer";
  }
  .sidebar {
    display: none;
  }
}