Viewport in Efl.Ui.CollectionView is a pre step for faster scrolling. The idea is that 2/3 of the needed element in the view are in one viewport. They are stored in 3 viewport so that the previous and the next viewport can be available at all time. This is simply stored in an array and is more efficient to access than an rbtree. The rbtree should be kept for caching things that might be useful for the next few loop iteration.