Defines the vertical direction of a ButtonBar's children if it overflows.
If the ButtonBar's children do not fit into a single row, then they are arranged in a column. The first action is at the top of the column if this property is set to VerticalDirection.down, since it "starts" at the top and "ends" at the bottom. On the other hand, the first action will be at the bottom of the column if this property is set to VerticalDirection.up, since it "starts" at the bottom and "ends" at the top.
final VerticalDirection? overflowDirection;