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

Question

The following code works if the page has enough space to host all divs, but if I resize at minimum the page the two divs positioning absolute overlap. How can I avoid that?

#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;
  overflow: auto;
}

#div-messages {
  position: absolute;
  top: 10px;
  bottom: 110px;
  left: 10px;
  right: 10px;
  min-height: 200px;
  overflow: auto;
}

#div-sending {
  position: absolute;
  bottom: 10px;
  left: 10px;
  right: 10px;
  height: 100px;
}

<div id="div-chatroom">
  <div id="div-messages">messages here</div>
  <div id="div-sending">sending tools here</div>
</div>

UPDATE #1
As required the code on JSFiddle (but if the two divs have position: absolute it doesn't seem to work).

Answer 1

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;
}