LauncherActivity.kt

override fun onCreate(savedInstanceState: Bundle?) {
  ...
  btnContinue.setOnClickListener {
    if (pager.currentItem < pager.adapter?.itemCount!! - 1) {
      handler.removeMessages(0)
      pager.currentItem = ++pager.currentItem
    }
  }
  
  // I expect the back button to be hidden when ViewPager2 is at position 0
  btnBack.setOnClickListener { onBackPressed() }
  ...
}

override fun onBackPressed() {
  if (pager.currentItem == 0) super.onBackPressed()
  else {
    handler.removeMessages(0)
    pager.currentItem = --pager.currentItem
  }
}