.a {
  position: relative;
}
.resizer {
  position: absolute;
  z-index: 1;
  visibility: hidden;
}
.a:hover{
  .resizer{
    visibility: visible;
  }
}
.top,.bottom{
  width: 100%;
  height: 1px;
  top: 0;
  left: 0;
  border-top: 1px dashed blue;
}
.bottom{
  top: unset;
  border: unset;
  bottom: 0;
  border-bottom: 1px dashed blue;
}
.left,.right{
  width: 1px;
  height: 100%;
  top: 0;
  left: 0;
  border-left: 1px dashed blue;
}
.right{
  border: unset;
  left: unset;
  right: 0;
  border-right: 1px dashed blue;
}