CSS to make the container DIV to cover full page - css

Question

I am trying to convert a chat widget template so that:
The height and width of the container div are 100%.
A vertical scroll appears if the member list exceeds the screen size so that the user can scroll through the rest of the member list.
The textarea to type messages is visible all the time. Currently, the user has to scroll down to see it.
I have managed to achieve the first thing on my list but not the second and third. How would you do it?
Here is where I am at so far:
#import url(https://fonts.googleapis.com/css?family=Lato:400,700);
*, *:before, *:after {
box-sizing: border-box;
}
body {
background: #C5DDEB;
font: 14px/20px "Lato", Arial, sans-serif;
padding: 0;
color: white;
}
.container {
margin: 0 auto;
width: auto;
background: #444753;
border-radius: 5px;
}
.people-list {
width: 260px;
float: left;
}
.people-list .search {
padding: 20px;
}
.people-list input {
border-radius: 3px;
border: none;
padding: 14px;
color: white;
background: #6A6C75;
width: 90%;
font-size: 14px;
}
.people-list .fa-search {
position: relative;
left: -25px;
}
.people-list ul {
padding: 20px;
height: 770px;
}
.people-list ul li {
padding-bottom: 20px;
}
.people-list img {
float: left;
}
.people-list .about {
float: left;
margin-top: 8px;
}
.people-list .about {
padding-left: 8px;
}
.people-list .status {
color: #92959E;
}
.chat {
width: auto;
float: left;
background: #F2F5F8;
border-top-right-radius: 5px;
border-bottom-right-radius: 5px;
color: #434651;
}
.chat .chat-header {
padding: 20px;
border-bottom: 2px solid white;
}
.chat .chat-header img {
float: left;
}
.chat .chat-header .chat-about {
float: left;
padding-left: 10px;
margin-top: 6px;
}
.chat .chat-header .chat-with {
font-weight: bold;
font-size: 16px;
}
.chat .chat-header .chat-num-messages {
color: #92959E;
}
.chat .chat-header .fa-star {
float: right;
color: #D8DADF;
font-size: 20px;
margin-top: 12px;
}
.chat .chat-history {
padding: 30px 30px 20px;
border-bottom: 2px solid white;
overflow-y: scroll;
height: 575px;
}
.chat .chat-history .message-data {
margin-bottom: 15px;
}
.chat .chat-history .message-data-time {
color: #a8aab1;
padding-left: 6px;
}
.chat .chat-history .message {
color: white;
padding: 18px 20px;
line-height: 26px;
font-size: 16px;
border-radius: 7px;
margin-bottom: 30px;
width: 90%;
position: relative;
}
.chat .chat-history .message:after {
bottom: 100%;
left: 7%;
border: solid transparent;
content: " ";
height: 0;
width: 0;
position: absolute;
pointer-events: none;
border-bottom-color: #86BB71;
border-width: 10px;
margin-left: -10px;
}
.chat .chat-history .my-message {
background: #86BB71;
}
.chat .chat-history .other-message {
background: #94C2ED;
}
.chat .chat-history .other-message:after {
border-bottom-color: #94C2ED;
left: 93%;
}
.chat .chat-message {
padding: 30px;
}
.chat .chat-message textarea {
width: 100%;
border: none;
padding: 10px 20px;
font: 14px/22px "Lato", Arial, sans-serif;
margin-bottom: 10px;
border-radius: 5px;
resize: none;
}
.chat .chat-message .fa-file-o, .chat .chat-message .fa-file-image-o {
font-size: 16px;
color: gray;
cursor: pointer;
}
.chat .chat-message button {
float: right;
color: #94C2ED;
font-size: 16px;
text-transform: uppercase;
border: none;
cursor: pointer;
font-weight: bold;
background: #F2F5F8;
}
.chat .chat-message button:hover {
color: #75b1e8;
}
.online, .offline, .me {
margin-right: 3px;
font-size: 10px;
}
.online {
color: #86BB71;
}
.offline {
color: #cccccc;
}
.busy {
color: #E38968;
}
.me {
color: #94C2ED;
}
.align-left {
text-align: left;
}
.align-right {
text-align: right;
}
.float-right {
float: right;
}
.clearfix:after {
visibility: hidden;
display: block;
font-size: 0;
content: " ";
clear: both;
height: 0;
}

Answer 1

This will do:
.chat .chat-history {
height: calc(100vh - 278px);
overflow-y: auto;
}
.people-list {
height: 100vh;
overflow-y: auto;
}
Updated codepen: https://codepen.io/anon/pen/BRxXmV