How can I fit a content element next to a sidebar?

Issue

I'm using this menu https://codepen.io/StephenScaff/pen/bVbEbJ and it works well, but I'm having an issue trying to add a div to cover the rest of the screen.

I'm trying to achieve this:

The div should resize with the screen.

When I add a div after the menu in the HTML I end up with some of it hidden behind the menu or all of it appearing off screen after the menu.

I've simply adding <div>Main Page</div>

Could some one explain what I've done wrong and how to do this?

Thanks

@charset "UTF-8";
@import url(https://fonts.googleapis.com/css?family=Source+Sans+Pro:400,300,600);
body {
  font-family: "Lato";
  font-size: 100%;
  overflow-y: scroll;
  font-family: sans-serif;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
  background-color: #fefefe;
}

a {
  text-decoration: none;
  -webkit-transition: all 0.6s ease;
  -moz-transition: all 0.6s ease;
  transition: all 0.6s ease;
}
a:hover {
  -webkit-transition: all 0.6s ease;
  -moz-transition: all 0.6s ease;
  transition: all 0.6s ease;
}

.app {
  height: 100vh;
}

/* -------------
Sidebar
----------------*/
.sidebar {
  position: absolute;
  width: 17em;
  height: 100%;
  top: 0;
  overflow: hidden;
  background-color: #19222a;
  -webkit-transform: translateZ(0);
  visibility: visible;
  -webkit-backface-visibility: hidden;
}
.sidebar header {
  background-color: #09f;
  width: 100%;
  display: block;
  padding: 0.75em 1em;
}

/* -------------
Sidebar Nav
----------------*/
.sidebar-nav {
  position: fixed;
  background-color: #19222a;
  height: 100%;
  font-weight: 400;
  font-size: 1.2em;
  overflow: auto;
  padding-bottom: 6em;
  z-index: 9;
  overflow: hidden;
  -webkit-overflow-scrolling: touch;
  /* -------------
  Chev elements
  ----------------*/
  /* -------------
  Nav-Flyout
  ----------------*/
  /* -------------
  Hover
  ----------------*/
}
.sidebar-nav ul {
  list-style: none;
  display: block;
  padding: 0;
  margin: 0;
}
.sidebar-nav ul li {
  margin-left: 0;
  padding-left: 0;
  display: inline-block;
  width: 100%;
  /* -------------
  Sidebar: icons
  ----------------*/
}
.sidebar-nav ul li a {
  color: rgba(255, 255, 255, 0.9);
  font-size: 0.75em;
  padding: 1.05em 1em;
  position: relative;
  display: block;
}
.sidebar-nav ul li a:hover {
  background-color: rgba(0, 0, 0, 0.9);
  -webkit-transition: all 0.6s ease;
  -moz-transition: all 0.6s ease;
  transition: all 0.6s ease;
}
.sidebar-nav ul li i {
  font-size: 1.8em;
  padding-right: 0.5em;
  width: 9em;
  display: inline;
  vertical-align: middle;
}
.sidebar-nav > ul > li > a:after {
  content: "";
  font-family: ionicons;
  font-size: 0.5em;
  width: 10px;
  color: #fff;
  position: absolute;
  right: 0.75em;
  top: 45%;
}
.sidebar-nav .nav-flyout {
  position: absolute;
  background-color: #080D11;
  z-index: 9;
  left: 2.5em;
  top: 0;
  height: 100vh;
  -webkit-transform: translateX(100%);
  -moz-transform: translateX(100%);
  -ms-transform: translateX(100%);
  -o-transform: translateX(100%);
  transform: translateX(100%);
  -webkit-transition: all 0.5s ease;
  -moz-transition: all 0.5s ease;
  transition: all 0.5s ease;
}
.sidebar-nav .nav-flyout a:hover {
  background-color: rgba(255, 255, 255, 0.05);
}
.sidebar-nav ul > li:hover .nav-flyout {
  -webkit-transform: translateX(0);
  -moz-transform: translateX(0);
  -ms-transform: translateX(0);
  -o-transform: translateX(0);
  transform: translateX(0);
  -webkit-transition: all 0.5s ease;
  -moz-transition: all 0.5s ease;
  transition: all 0.5s ease;
}

<section class="app">
  <aside class="sidebar">
    <header>
      Menu
    </header>
    <nav class="sidebar-nav">

      <ul>
        <li>
          <a href="#"><i class="ion-bag"></i> <span>Shop</span></a>
          <ul class="nav-flyout">
            <li>
              <a href="#"><i class="ion-ios-color-filter-outline"></i>Derps</a>
            </li>
            <li>
              <a href="#"><i class="ion-heart-broken"></i>Beat</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#"><i class="ion-ios-settings"></i> <span class="">Controls</span></a>
          <ul class="nav-flyout">
            <li>
              <a href="#"><i class="ion-ios-alarm-outline"></i>Watch</a>
            </li>
            <li>
              <a href="#"><i class="ion-ios-cog-outline"></i>Grinder</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#"><i class="ion-ios-briefcase-outline"></i> <span class="">Folio</span></a>
          <ul class="nav-flyout">
            <li>
              <a href="#"><i class="ion-ios-flame-outline"></i>Burn</a>
            </li>
            <li>
              <a href="#"><i class="ion-ios-navigate-outline"></i>Ghostface</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#"><i class="ion-ios-analytics-outline"></i> <span class="">Graphicals</span></a>
          <ul class="nav-flyout">
            <li>
              <a href="#"><i class="ion-ios-timer-outline"></i>Timers</a>
            </li>
            <li>
              <a href="#"><i class="ion-ios-game-controller-a-outline"></i>Dork Mfer</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#"><i class="ion-ios-paper-outline"></i> <span class="">Papers</span></a>
          <ul class="nav-flyout">
            <li>
              <a href="#"><i class="ion-ios-filing-outline"></i>File Cab</a>
            </li>
            <li>
              <a href="#"><i class="ion-android-star-outline"></i>Shop</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#"><i class="ion-ios-navigate-outline"></i> <span class="">Ass Finder</span></a>
          <ul class="nav-flyout">
            <li>
              <a href="#"><i class="ion-ios-flame-outline"></i>Burn</a>
            </li>
            <li>
              <a href="#"><i class="ion-ios-navigate-outline"></i>Ghostface</a>
            </li>
          </ul>
        </li>
        <li>
          <a href="#"><i class="ion-ios-medical-outline"></i> <span class="">Cocaine</span></a>
        </li>
      </ul>
    </nav>
  </aside>
</section>

<div style='border: 1px solid red'>Main Page</div>

Solution

A simple flexbox row should get you started.

• https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Flexbox
• https://css-tricks.com/snippets/css/a-guide-to-flexbox/

.app {
  display: flex;
  align-items: stretch;
}

.content {
  flex: auto;
  border: 1px solid red;
}

.sidebar {
  /* position: absolute; (removed below) */
}

body {
  font-family: "Lato";
  font-size: 100%;
  overflow-y: scroll;
  font-family: sans-serif;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
  background-color: #fefefe;
}

a {
  text-decoration: none;
  -webkit-transition: all 0.6s ease;
  -moz-transition: all 0.6s ease;
  transition: all 0.6s ease;
}

a:hover {
  -webkit-transition: all 0.6s ease;
  -moz-transition: all 0.6s ease;
  transition: all 0.6s ease;
}

.app {
  height: 100vh;
}


/* -------------
Sidebar
----------------*/

.sidebar {
  width: 17em;
  height: 100%;
  top: 0;
  overflow: hidden;
  background-color: #19222a;
  -webkit-transform: translateZ(0);
  visibility: visible;
  -webkit-backface-visibility: hidden;
}

.sidebar header {
  background-color: #09f;
  width: 100%;
  display: block;
  padding: 0.75em 1em;
}


/* -------------
Sidebar Nav
----------------*/

.sidebar-nav {
  position: fixed;
  background-color: #19222a;
  height: 100%;
  font-weight: 400;
  font-size: 1.2em;
  overflow: auto;
  padding-bottom: 6em;
  z-index: 9;
  overflow: hidden;
  -webkit-overflow-scrolling: touch;
  /* -------------
  Chev elements
  ----------------*/
  /* -------------
  Nav-Flyout
  ----------------*/
  /* -------------
  Hover
  ----------------*/
}

.sidebar-nav ul {
  list-style: none;
  display: block;
  padding: 0;
  margin: 0;
}

.sidebar-nav ul li {
  margin-left: 0;
  padding-left: 0;
  display: inline-block;
  width: 100%;
  /* -------------
  Sidebar: icons
  ----------------*/
}

.sidebar-nav ul li a {
  color: rgba(255, 255, 255, 0.9);
  font-size: 0.75em;
  padding: 1.05em 1em;
  position: relative;
  display: block;
}

.sidebar-nav ul li a:hover {
  background-color: rgba(0, 0, 0, 0.9);
  -webkit-transition: all 0.6s ease;
  -moz-transition: all 0.6s ease;
  transition: all 0.6s ease;
}

.sidebar-nav ul li i {
  font-size: 1.8em;
  padding-right: 0.5em;
  width: 9em;
  display: inline;
  vertical-align: middle;
}

.sidebar-nav>ul>li>a:after {
  content: "";
  font-family: ionicons;
  font-size: 0.5em;
  width: 10px;
  color: #fff;
  position: absolute;
  right: 0.75em;
  top: 45%;
}

.sidebar-nav .nav-flyout {
  position: absolute;
  background-color: #080D11;
  z-index: 9;
  left: 2.5em;
  top: 0;
  height: 100vh;
  -webkit-transform: translateX(100%);
  -moz-transform: translateX(100%);
  -ms-transform: translateX(100%);
  -o-transform: translateX(100%);
  transform: translateX(100%);
  -webkit-transition: all 0.5s ease;
  -moz-transition: all 0.5s ease;
  transition: all 0.5s ease;
}

.sidebar-nav .nav-flyout a:hover {
  background-color: rgba(255, 255, 255, 0.05);
}

.sidebar-nav ul>li:hover .nav-flyout {
  -webkit-transform: translateX(0);
  -moz-transform: translateX(0);
  -ms-transform: translateX(0);
  -o-transform: translateX(0);
  transform: translateX(0);
  -webkit-transition: all 0.5s ease;
  -moz-transition: all 0.5s ease;
  transition: all 0.5s ease;
}

<section class="app">
  <aside class="sidebar">
    <header>
      Menu
    </header>

    <nav class="sidebar-nav">
      <ul>
        <li>menu</li>
      </ul>
    </nav>
  </aside>

  <div class="content">Main Page</div>
</section>

Answered By - isherwood