button inside textarea

.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>