CodePen

HTML

            
              <div id="main">
  <div id="toolbar"></div>
  <textarea></textarea>
</div>
            
          
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↑ Insert the most common viewport meta tag

CSS

            
              #main {
  background-color: #ddd;
  height: 200px;
  width: 400px;
  position: relative;
}

#toolbar {
  background-color: #444;
  height: 40px;
  color: white;
  position: relative;
  z-index: 1;
}

textarea {
  outline: none; 
  border: none;
  width: 100%; height: 100%;
  border-top: 40px solid transparent;
  background: black;
  box-sizing: border-box;
  position: absolute;
  top: 0;
}
            
          
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JS

            
              
            
          
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