I am now using KDE 5 from OpenSUSE 42.1. When the window is small and you want to use mouse wheel to scroll it, the scroll event will be captured by some controls (like combobox) if it is just scrolled to the position under your mouse. The action here will be changing the content of that combobox, instead of continuing scroll the panel.

I have noticed that this is because when you move your mouse over the control, it will immediately get the focus (it's border begin glowing) so any latter wheel event will be captured by the control.

Reproducible: Always

Steps to Reproduce:
1. Open KDE System Settings and click any subpage that have many controls inside, for example "Window Behavior".
2. Adjust the window height of the System Settings, let the vertical scroll bar of right panel appears, so you can use your mouse wheel.
3. Put the mouse to any blank place in this panel, then scroll your mouse wheel.

Actual Results:  
If one of the controls on in "Window Behavior" just goes under your mouse during scrolling, it will be focused, and its value will be immediately modified. The scroll of the whole page is interrupted.

Expected Results:  
Scrolling of the whole page is not interrupted even when any control goes under your mouse during the process.

Maybe it is useful to add a delay for controls getting focus when hovered. It will be focused only when you have hover your mouse on it for enough time, like 500ms.