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CSS

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HTML 

              
                <div class="flex-master">

  <header class="page-header">
    This is a header
  </header>


  <div class="page-content">
    <nav class="sidebar-left">
      <h3>Sidebar</h3>
      <ul>
        <li>Item 1</li>
        <li>Item 2</li>
        <li>Item 3</li>
        <li>Item 4</li>
        <li>Item 5</li>
        <li>Item 6</li>
        <li>Item 7</li>
        <li>Item 8</li>
        <li>Item 9</li>
        <li>Item 10</li>
        <li>Item 11</li>
        <li>Item 12</li>
        <li>Item 13</li>
        <li>Item 14</li>
        <li>Item 15</li>
        <li>Item 16</li>
        <li>Item 17</li>
        <li>Item 19</li>
      </ul>
    </nav>

    <div class="splitter">
    </div>

    <div class="content-container">
      <article class="main-content">
        This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr /> This is my main content!
        <hr />
      </article>
      
      <aside class="page-ads">
        <div class="advert">
          Display Ad
        </div>

        <div class="advert">
          Ad
        </div>

        <div class="advert">
          Ad
        </div>

        <div class="advert">
          Ad
        </div>
      </aside>
    </div>
  </div>

  <footer>
    <small>&copy West Wind Technologies, 2016</small>
  </footer>

</div>
!

CSS 

              
                 html,
 body {
   height: 100%;
   padding: 0;
   margin: 0;
   font-family: Arial, Helvetica, sans-serif;
 }
 
 .flex-master {
   display: flex;
   flex-direction: column;
   flex-wrap: nowrap;   
   height: 100%;
   overflow-x: auto;
   overflow-y: hidden;
 }
 
 .page-header {
   /* fixed size */
   flex: none;
   
   padding: 15px;
   font-size: 3em;
   font-weight: bold;
   background: #535353;
   color: white;
 }
 
 .page-content { 
   /* resize in container */
   flex: 1; 
   
   /* IMPORTANT for FireFox */
   overflow-y: auto;
   overflow-x: hidden;
   
   /* new flex container for content */
   display: flex;
   flex-direction: row;
 }
 
 .sidebar-left {   
   flex: none;
   background: #838383;
   padding: 20px;
   width: 250px;
   min-width: 150px;
   overflow: auto;
   color: white;
   white-space: nowrap;   
 }
 
 .splitter {     
   flex: none;
   width: 17px;
   cursor: col-resize;
   background: #535353;
   background: url(https://raw.githubusercontent.com/RickStrahl/jquery-resizable/master/assets/vsizegrip.png) center center no-repeat #535353;
 }
 
 .content-container {
   flex: 1 1 auto;
   background: #eee;
   padding: 20px;
   width: 100%;
   min-width: 400px;
   overflow-y: auto;
   overflow-x: hidden;
   display: flex;
   /* new flex for content/ads*/
   flex-direction: row;
 }
 
 .main-content {
   flex: 1 1 auto;
   width: 100%;
   padding: 10px;
   min-width: 300px;
 }
 
 .page-ads {
   flex: none;
   border-left: 1px solid silver;
   margin-left: 10px;
   padding-left: 10px;
   width: 180px;
   overflow: hidden;
   
   display: flex;
   /* new flex for each ad*/
   flex-direction: column;
 }
 
 .advert {
   flex: none;
   align-self: center;
   padding: 5px;
   margin: 15px;
   width: 150px;
   height: 150px;
   background: lightblue;
   color: green;
   overflow: hidden;
   /* center ad text */
   display: flex;
   justify-content: center;
   align-items: center;
 }
 
 footer {
   flex: none;
   background: #535353;
   color: white;
   padding: 10px;
 }
@media(max-width: 900px) {
  .page-ads { display: none; }
}
!

JS 

              
                // jquery-resizable
//http://weblog.west-wind.com/posts/2015/Dec/21/A-small-jQuery-Resizable-Plugin
$(".sidebar-left").resizable({
   handleSelector: ".splitter",
   resizeHeight: false
 });
!
999px

Console