Whether the popup menu is positioned over or under the popup menu button.
offset is used to change the position of the popup menu relative to the position set by this parameter.
If this property is
null, then PopupMenuThemeData.position is used. If
PopupMenuThemeData.position is also
null, then the position defaults
to PopupMenuPosition.over which makes the popup menu appear directly
over the button that was used to create it.
final PopupMenuPosition? position;