#main {
  transition: margin-left 0.3s;
  padding: 20px;
  overflow: hidden;
  width: 100%;
}

.mainopen {
  transition: margin-left 0.3s;
  margin-left: 0px;
}

.isOpen .mainopen {
  transition: margin-left 0.3s;
  padding: 20px;
  overflow: hidden;
  width: 100%;
  margin-left: 300px;
}