"Saving and restoring a RecyclerView's position in Android" @ https://blog.jakelee.co.uk/saving-and-restoring-a-recyclerviews-position-in-android

ListPositioner.kt

interface ListPositioner {
    val recyclerScrollKey: String

    fun loadListPosition()

    fun saveListPosition()

    fun resetListPosition()
}

RecyclerFragment.kt

override val recyclerScrollKey = "uk.co.jakelee.scrollposition"

override fun saveListPosition() {
    val position = (myRecycler.layoutManager as LinearLayoutManager).findFirstCompletelyVisibleItemPosition()
    PreferenceManager.getDefaultSharedPreferences(activity!!)
        .edit()
        .putInt(recyclerScrollKey, position)
        .apply()
}

override fun loadListPosition() {
    var scrollPosition = PreferenceManager.getDefaultSharedPreferences(activity!!).getInt(recyclerScrollKey, 0)
    if (scrollPosition > 0 && scrollPosition < myRecycler.layoutManager.childCount) {
        scrollPosition++ // To offset the "completely visible" item under the action bar
    }
    myRecycler.scrollToPosition(scrollPosition)
}

override fun resetSavedListPosition(){
    PreferenceManager.getDefaultSharedPreferences(activity!!)
        .edit()
        .putInt(recyclerScrollKey, 0)
        .apply()
}