Ensure selected index is visible by adjusting scrollTop.
If the item is above the viewport, scroll up to show it at the top.
If the item is below the viewport, scroll down to show it at the bottom.
If the item is already visible, return current scrollTop unchanged.
Ensure selected index is visible by adjusting scrollTop.
If the item is above the viewport, scroll up to show it at the top. If the item is below the viewport, scroll down to show it at the bottom. If the item is already visible, return current scrollTop unchanged.