Repro Steps:

1) Create a Project (and a MainPage.xaml if not created automatically)
2) Navigate to the Interactivity Panel (Ctrl+4).
3) Create many Triggers for the Page, enough so that the Scrollbar appears within the top panel where all of the Triggers are listed. These can be Triggers of any type. In my case, I created a bunch of DataTriggers on the Page.
4) Hover the Mouse over the Triggers list, and MouseWheel Up/Down.

Observe how the list doesn't scroll.

5) Hover the Mouse over the Scrollbar, and MouseWheel Up/Down.

Observe how the list scrolls.

Expected Results: MouseWheeling over scrollable lists, allows them to scroll.