Defines minimum and maximum sizes for the InputDecorator.
Typically the decorator will fill the horizontal space it is given. For larger screens, it may be useful to have the maximum width clamped to a given value so it doesn't fill the whole screen. This property allows you to control how big the decorator will be in its available space.
If null, then the ambient ThemeData.inputDecorationTheme's InputDecorationTheme.constraints will be used. If that is null then the decorator will fill the available width with a default height based on text size.
final BoxConstraints? constraints;