VirtualFlow jump when cellcount changes

When a Control using a VirtualFlow contains variable sized cells, and a cell is added, the position of the visible cells is sometimes wrong.
This happens when the top-most cell is larger than average, and positioned near its bottom.
When a cell is added, the cache with the cell sizes is cleared, but the current index and the offset of the first cell is preserved. In case the offset is larger than the average cell size, this will lead to a shift to the next cell to become visible.