Virtgroup class generates special kinds of group objects whose children can be a lot larger than the actual group. The group acts as a (small) window through which a rectangle area of its contents is visible.
During layout, MUI tries to place the children of a virtual group in the visible part. If this is impossible, space is extended as long as all children fit.
Virtual groups themselves don't offer any scrollbars to allow user interaction. These things are handled by scrollgroup class. Usually, you don't want to use a virtual group without a scrollgroup. See Scrollgroup class for details.