.textarea-container { 

  position: relative; 

} 

.textarea-container textarea { 

  width: 100%; 

  height: 100%; 

  box-sizing: border-box; 

} 

.textarea-container button { 

  position: absolute; 

  top: 0; 

  right: 0; 

}

<div class="textarea-container"> 

  <textarea name="foo">Some content here...</textarea> 

  <button>Menu</button> 

</div>