CodePen

HTML

            
              <div id="main">
  <div id="toolbar"></div>
  <textarea resize></textarea>
</div>
            
          
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CSS

            
              #main {
  background-color: #ddd;
  width: 400px; height: 200px; 
  padding: 0;
}

div #toolbar {
  background: #444;
  width: 100%; height: 40px;
}

textarea {
  margin: 0; 
  width: 100%; height: 100%;
}
            
          
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JS

            
              
            
          
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