How make ScrollIntoView keep a ListItem fully in view?

I’m using ScrollIntoView to keep ListItem visible when moved up or down a list. This mostly works, i.e. the ListBox scrolls up or down. However, it seems always to be one row off where it needs to be! Can someone explain why is this, and how to ensure a ListItem is fully in view? 

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Gouranga Gupta :-)