getSize method
Override this method to return the size of this object given the incoming constraints.
The size cannot reflect the sizes of the children. If this layout has a fixed width or height the returned size can reflect that; the size will be constrained to the given constraints.
By default, attempts to size the box to the biggest size possible given the constraints.
Implementation
@override
Size getSize(BoxConstraints constraints) {
final height = _initItemsPosition(constraints);
return Size(constraints.maxWidth, height);
}