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

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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?

This is my HTML:

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

This is my CSS:

#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;
}

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


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;
}