Add for example 10 elements to comboBox.
Set visible row count to 5.
Remove 9 elements.
Open popup. You may see, that 1 item is visible. And empty space for 4 items is left.
Seems, there should be space for the only item.

I look on visible row count as on limitation, but not as option of size.

In docs said: "Scroll Bar is being displayed when there are more items in the drop down list than the defined max. The maximum visible amount can be customized." so it is about maximum.