.modal {
  position: 'fixed';
  z-index: 1040;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

.backdrop {
  position: fixed;
  background: rgba(0, 0, 0, 0.8);
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

.content {
  position: absolute;
  max-width: 640px;
  top: 100px;
  left: 0;
  right: 0;
  margin: 0 auto;
  background: #fff;
  border-radius: 5px;
  padding: 20px;
  outline: none;
}

.closeButton {
  position: absolute;
  top: 10px;
  right: 10px;
  font-size: 20px;
}

.modal {
  position: 'fixed';
  z-index: 1040;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

.backdrop {
  position: fixed;
  background: rgba(0, 0, 0, 0.8);
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

.content {
  position: fixed;
  max-width: 640px;
  top: 100px;
  left: 0;
  right: 0;
  margin: 0 auto;
  background: #fff;
  border-radius: 5px;
  padding: 20px;
  outline: none;
}

.closeButton {
  position: absolute;
  top: 10px;
  right: 10px;
  font-size: 20px;
}

Make modal content position fixed
Make modal content position fixed
