LauncherActivity.kt

...
pager.registerOnPageChangeCallback(object : OnPageChangeCallback() {
    override fun onPageSelected(position: Int) {
        super.onPageSelected(position)
        
        // Clear the messages so that every time an item has changed
        // we reset our handler.
        handler.removeMessages(0)

        if (position == 0) btnBack.visibility = View.GONE
        else btnBack.visibility = View.VISIBLE

        val runnable = Runnable { pager.currentItem = position + 1 }

        if (position < pager.adapter?.itemCount ?: 0) {
            handler.postDelayed(runnable, timerDelay)
        }
    }
})
...