DropdownButtonFormField should allow definition of underline

[gildurao]

Use case

As a dev, I want to be able to customize my DropdownButtonFormFields to the fullest possibilities. Therefore, I should be able to inject a custom underline Widget

Proposal

Add an underline argument to DropdownButtonFormField constructor:

DropdownButtonFormField({
    Key? key,
    required List<DropdownMenuItem<T>>? items,
    DropdownButtonBuilder? selectedItemBuilder,
    T? value,
    Widget? hint,
    Widget? disabledHint,
    required this.onChanged,
    VoidCallback? onTap,
    int elevation = 8,
    TextStyle? style,
    Widget? icon,
    Color? iconDisabledColor,
    Color? iconEnabledColor,
    double iconSize = 24.0,
    bool isDense = true,
    bool isExpanded = false,
    double? itemHeight,
    Color? focusColor,
    FocusNode? focusNode,
    bool autofocus = false,
    Color? dropdownColor,
    InputDecoration? decoration,
    FormFieldSetter<T>? onSaved,
    FormFieldValidator<T>? validator,
    AutovalidateMode? autovalidateMode,
    double? menuMaxHeight,
    bool? enableFeedback,
    AlignmentGeometry alignment = AlignmentDirectional.centerStart,
    BorderRadius? borderRadius,
    Widget? underline, // <------------------------------------------------
    // When adding new arguments, consider adding similar arguments to
    // DropdownButton.
  })

DropdownButtonHideUnderline does not seem to be working, could it be a bug?

[danagbemava-nc]

Related #80416

Hi @gildurao, kindly file a new issue and provide the relevant information required to reproduce the issue you're facing with DropdownButtonHideUnderline so that we can investigate it.

Thank you