JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<div class="container">
    <div class="grid">

      <div class="col1">

        <div class="col1content">
          <div class="col1title">
            Connected users
          </div>
          <div class="nameform">
            <form action="">
              <div class="col-md-9 inputdiv">
                <input class="nameinput" placeholder="What's your name?" />
              </div>
              <div class="col-md-3 btndiv">
                <button class="btn btn-success namebtn" type="button">OK</button>
              </div>
            </form>
          </div>

        </div>
      </div>
      <div class="col2" id="chatbox">
            <div class="chat" id="chat">Welcome.</div>
            <div class="msgbox" id="msgbox">
              <form id="chatform" action="">
                <input class="writebox" id="msg" placeholder="Type your message here..." autocomplete="off">
                <button class="btn btn-primary chatbtn" id="chatbutton" type="button">Send</button>
              </form>
            </div>

      </div>

    </div>
  </div>

CSS

body, html {
  height: 100%;
}


.grid {
  display: flex;
  flex-flow: row;
  height: 83%;
  position: relative;
  width: 100%;
  margin:5px;
}
.col1 {
  position: relative;
  background-color: #f2efe8;
  width:50%;
  height: 100%;
  float: left;
  margin-right: 5px;
  font-family: "Segoe UI";

}
.col1content {
  margin: 5px 10px;
}

.nameform {
  position: absolute;
  bottom: 5;
  right: 5;
  height: 10%;
}
.nameinput {
  height:100%;
  max-width: 100%;
  width: 100%;
}
.inputdiv {
  padding-right: 5px;
  padding-left: 5px;
}
.btndiv {
  padding-right: 5px;
  padding-left: 2.5px;
  height: 100%;
}
.namebtn {
  max-width: 100%;
  width: 100%;
  height: 100%;
}
.col2{
  position: relative;
  background-color: #1d2120;
  width:100%;
  float: right;
  margin-left: 5px;
  color: white;
  font-family: "Segoe UI";
  border: medium solid #1d2120;

}
.chat {
width:100%;
height:87%;
overflow:auto;

}
.msgbox {
height:10%;
  display:block;
  background-color:#ffffff;
  height: 10%;
  width:100%;
  position:absolute;
  bottom: 0;
  color: #000000;
  border-radius: 2px;
}

.chatbtn {
  position: absolute;
  top: 1;
  right: 2;
  bottom: 1;
  width: 15%;
  display: inline-block;
}

.writebox {
  background: transparent;
  display:inline-block;
  border: 0;
  height:100%;
  width: 83%;
  margin-left: 5;
}

.separator{
    position:absolute;
    left:34%;
    top:25%;
    bottom:10%;
    height: 70%;
    border-left:1px solid #8c8b8b;
}

.container {
  display: flex;
  align-items: stretch;
  flex-flow: column;
  background-color: #feffff;
  border-radius: 10px;
  box-shadow: 0px 0px 10px black;
  position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    width: 70%;
    height: 70%;
    margin: auto;

}

centered {
  display: flex;
  justify-content: center;
}