M3 Checkbox error sides

main.dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.light(useMaterial3: true),
      home: const Scaffold(
        body: Center(
          child: MyStatefulWidget(),
        ),
      ),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({super.key});

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  bool isChecked = false;

  @override
  Widget build(BuildContext context) {
    final side = MaterialStateBorderSide.resolveWith((states) {
      // EXPECTATION: border side will be red in all error states.
      if (states.contains(MaterialState.error)) {
        return const BorderSide(color: Colors.red, width: 4);
      }
      // REALITY: border side override only takes effect for non-error state (like hovered, below).
      if (states.contains(MaterialState.hovered)) {
        return const BorderSide(color: Colors.red, width: 4);
      }
      return const BorderSide(color: Colors.green);
    });

    // Error states works correctly for fill color.
    final fillColor = MaterialStateProperty.resolveWith<Color?>((states) {
      if (states.contains(MaterialState.error)) {
        return Colors.green;
      }
    });

    return Checkbox(
      checkColor: Colors.white,
      isError: true,
      fillColor: fillColor,
      side: side,
      value: isChecked,
      onChanged: (bool? value) {
        setState(() {
          isChecked = value!;
        });
      },
    );
  }
}