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<span style="font-size:30px;cursor:pointer" onclick="openNav()">☰ open</span>

body {
  font-family: "Lato", sans-serif;
}

.sidenav {
  height: 100%;
  width: 0;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: #111;
  overflow-x: hidden;
  transition: 0.5s;
  padding-top: 60px;
}

.sidenav a {
  padding: 8px 8px 8px 32px;
  text-decoration: none;
  font-size: 15px;
  color: #818181;
  display: block;
  transition: 0.3s;
}

.sidenav a:hover {
  color: #f1f1f1;
}

.sidenav .closebtn {
  position: absolute;
  top: 0;
  right: 25px;
  font-size: 36px;
  margin-left: 50px;
}

function openNav() {
  document.getElementById("mySidenav").style.width = "250px";
}

function closeNav() {
  document.getElementById("mySidenav").style.width = "0";
}

<div id="content">
    <div class="group">
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    </div>
    
  
</div>

#content {
    padding: 10px;
    background-color: #eee;
    display: flex;
    flex-grow: 1;
}

#content > .group {
    margin: 10px;
    padding: 10px;
    border: 1px solid #cfcfcf;
    background-color: #ddd;
    flex: 1 1 auto;
}

#content > .group:first-child {
    columns: 10em;
    flex-grow: 2;    
}

#content > .group .item {
    margin: 10px;
    padding: 10px;
    background-color: #aaa;
    break-inside: avoid;
}

#content > .group .item:first-child {
    margin-top: 0;
}