When you have a scroll pane or other scrollable control which contains other controls. You expect to be able to have mouse over that control and scroll it using the mouse wheel. What happens at the moment is if a child control passes under the mouse you stop scrolling. This is easily reproducible in Ensemble, on the main Samples icon page, try scrolling with the wheel, as soon as a icon is under the mouse the scrolling stops. This is the same on the bottom left panel in Chart Sampler.
RT-13995 for the temporary fix that was applied in Presidio.