Not 100% sure but it should be enough to give the sroll class a 100% height. the lower div will always stay on the floor of the fixed container.
<!doctype html>
<html>
<head>
<style>
.fixed {
position: fixed;
right: 0;
width: 150px;
height: 75%;
}
.scroll {
overflow: scroll;
height: 100%;
}
</style>
<title>test</title>
</head>
<body>
<div class="fixed">
<div class="scroll">
afsdfsadf asd
affdas
sadfsdfa
asdfsa
</div>
<div class="bottom">
this is my chatbar_description;
</div>
</div>
</body>
</html>