f I have a QComboBox that where I add 100 items in code, the aren't all displayed on screen and I see the arrows on the top and bottom of the popup (for navigating up and down the items).
This is all as expected.

If I know clear the QComboBox and add only a few items (even 1), the arrows are still displayed.
If I populate the QComboBox directly with the few items, the arrows are not displayed.

You can see it in the attached project.
If you click 10 items, all is good.
If you click 100 items, again all is good (with arrows displayed).
If you click 10 items again, 9 items are visible and the arrows to show the other one.