Use transparent mask when resizing side-panels to prevent mouse events from bleeding into iframe(s)

The openUI branch introduced iframes in the right-side-panel, which asked for a more generic fix for bleeding mouse-events than we had at that point.

This was fixed by adding a transparent mask which in turn triggered some refactoring and better test-coverage. We realized this change should also be merged into mainline as it will pave the way for fixing CHANNELMGR-1791 and is a good improvement in general.