.btn-mouseover:hover + .mouseover__box{
  display: block;
}
.mouseover__box:hover{
  display: block;
}
.mouseover__box{
  display: none;
  position: absolute;
  top:0; 
  left:50;
  width: 250px;
  padding: 15px;
  font-size: 14px;
  background-color: #fff;
  border: 1px solid #ccc;
  box-sizing: border-box;
}