.background {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100vh;
  background-color: rgba(0, 0, 0, 0.3);
  z-index: 1000;
  
  /* 숨기기 */
  z-index: -1;
  opacity: 0;
}

.show {
  opacity: 1;
  z-index: 1000;
  transition: all .5s;
}

.window {
  position: relative;
  width: 100%;
  height: 100%;
}

.popup {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  background-color: #ffffff;
  box-shadow: 0 2px 7px rgba(0, 0, 0, 0.3);
  
  /* 임시 지정 */
  width: 100px;
  height: 100px;
  
  /* 초기에 약간 아래에 배치 */
  transform: translate(-50%, -40%);
}

.show .popup {
  transform: translate(-50%, -50%);
  transition: all .5s;
}