Adding an element to a long existing list may cause the first visible element to jump

An implicit requirement of controls using VirtualFlow is that when adding an element to the bottom of a list, the first visible element in the UI should not change its position (avoiding pseudo-random jumps).

In case the considered list contains cells with different heights, a jump may still happen as the cellsize cache is invalid when the count changes. Since we have no idea about how different the new elements are from the previous ones, we need to make sure that the element that is rendered at the top of the VirtualFlow before the cell count changes stays in the exact same position after the change is processed.