<div class="app-layout">
  <div class="teams box">Teams</div>
  <div class="channels box">Channels</div>
  <div class="header box">Header</div>
  <div class="messages box">
    <ul class="message-list">
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
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      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
      <li></li>
    </ul>
  </div>
  <div class="input box">
    <input type="text" placeholder="Have you tried the CSS Grid Layout Module?">
  </div>
</div>
*,
*:before,
*:after {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: Arial;
  background-color: #f2f2f2;
}

.box {
  color: #fff;
  padding: 1em;
  text-align: center;
}

input {
  width: 100%;
  padding: 1em;
  outline: none;
  border: 1px solid #f2f2f2;
}

.message-list {
  margin: 0;
  padding: 0;
}

.message-list li {
  padding: .2em;
  margin-top: 1em;
  margin-bottom: 1em;
  background-color: #f2f2f2;
}

/* Our Grid Code */

.app-layout {
  display: grid;
  height: 100vh;
  grid-template-columns: 100px 250px 1fr;
  grid-template-rows: auto 1fr auto;
}

.header     { background-color: #ffffff; }
.teams      { background-color: #362233; }
.channels   { background-color: #52364E; }
.messages   { background-color: #ffffff; }
.write      { background-color: #f2f2f2; }

.teams {
  grid-column: 1;
  grid-row: 1 / 4;
}

.channels {
  grid-column: 2;
  grid-row: 1 / 4;
}

.header {
  grid-column: 3;
  grid-row: 1;
  color: #333;
  border-bottom: 1px solid #f2f2f2;
}

.messages {
  grid-column: 3;
  grid-row: 2;
  padding: 0 1em;
  overflow-y: scroll;
}

.input {
  grid-column: 3;
  grid-row: 3;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

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