Scrolls the list within an enclosing viewport to make the specified
 cell completely visible. This calls  scrollRectToVisible with
 the bounds of the specified cell. For this method to work, the
  JList must be within a JViewport.
 
If the given index is outside the list's range of cells, this method results in nothing.
index | the index of the cell to make visible | 
JComponent.scrollRectToVisible, getVisibleRect
Diagram: JList, JComboBox