Optional size constraints for the search view.
By default, the search view has the same width as the anchor and is 2/3 the height of the screen. If the width and height of the view are within the viewConstraints, the view will show its default size. Otherwise, the size of the view will be constrained by this property.
If null, the value of SearchViewThemeData.constraints will be used. If this is also null, then the constraints defaults to:
const BoxConstraints(minWidth: 360.0, minHeight: 240.0)
final BoxConstraints? viewConstraints;