JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="textarea">
    <textarea placeholder="textarea ..."></textarea>
  </div>
  <div class="footer">
    #footer
  </div>
</div>

CSS

.container{
  display:flex;
  height:100px;
  background-color:whitesmoke;
  flex-direction:column;
}

.textarea{
  flex-grow:1;
  display:flex;
}

.footer{
  background-color:lightseagreen;
  height:40px;
  color:white;
}

textarea{
   flex-grow: 1;
   border: none;
   resize: none;
   padding: 5px;
   font-size: 16px;
   margin: 0px;
   display: block;
}