In Qt example "sliders", or any other Qt example with a combo box:
1. Click an edit field somewhere so that the edit field has keyboard focus.
2. Position your cursor over a combo box
3. Use your mouse scroll wheel to scroll up or down.
BUG: combo box's selected item changes as you scroll through values.

Compare with Apple-authored TextEdit.app:
1. Open TextEdit.app
2. Preferences
3. Open and Save tab
4. Position your cursor over a combo box
5. Use your mouse scroll wheel to scroll up or down.
Correct: combo box's selected item does not change.
Even if you tab the keyboard xfocus into an Apple-authored combo box, the combo box ignores scroll wheel events. Only arrow keys can change its selected item.