The xamComboEditor contains a WPF ComboBox when in edit mode. The ComboBox always handles the MouseWheel and in looking at its code I don't see anyway of preventing it from doing so. When it has the input focus and its not dropped down, it uses this event to change the selected item. When it is droppeddown they probably want to prevent the containing scrollviewer from scrolling the control which I believe is what a WinForms/OS combobox would do. Note, readonly in terms of the ComboBox really just controls whether the edit area can be typed into and not preventing the value from being changed.