How to avoid overlapping between two divs positioning absolute inside a div positioning relative?

Ok, I got an equivalent result by changing approach.

CSS (JSFiddle):

#div-chatroom {
    position: relative;
    height: calc(100% - 70px); /* IE9+ and future browsers */
    height: -moz-calc(100% - 70px); /* Firefox */
    height: -webkit-calc(100% - 70px); /* Chrome, Safari */
    padding: 0;
    text-align: center;
    margin: 0;
    border-right: 2px solid #333333;
    background-color: #ffffff;
    overflow: auto;
}

#div-messages {
    position: relative;
    margin: 0;
    padding: 0;    
    min-height: 200px;
    height: calc(100% - 100px); /* IE9+ and future browsers */
    height: -moz-calc(100% - 100px); /* Firefox */
    height: -webkit-calc(100% - 100px); /* Chrome, Safari */
    background-color: green;
    overflow: auto;
}

#div-sending {
    position: relative;
    margin: 0;
    padding: 0;    
    height: 100px;
    background-color: red;
}