M3 TextField not up to spec: See Flutter issue: https://github.com/flutter/flutter/issues/113329

main.dart

// MIT License
//
// Copyright (c) 2022 Mike Rydstrom
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

import 'package:flutter/material.dart';

// App breakpoints
const double phoneWidthBreakpoint = 550;
const double phoneHeightBreakpoint = 600;

// Used as M3 seed color, the M3 baseline generating seed color.
// This color is also used for the TextField spec examples.
const Color seedColor = Color(0xFF6750A4);

// Make a seed generated M3 light mode ColorScheme.
final ColorScheme myLightScheme = ColorScheme.fromSeed(
  brightness: Brightness.light,
  seedColor: seedColor,
);

// Make a seed generated M3 light mode ColorScheme.
final ColorScheme myDarkScheme = ColorScheme.fromSeed(
  brightness: Brightness.dark,
  seedColor: seedColor,
);

// A simple custom theme
ThemeData myTheme(Brightness mode) => ThemeData.from(
      colorScheme: mode == Brightness.light ? myLightScheme : myDarkScheme,
      useMaterial3: true,
    ).copyWith(
      // Use standard so desktop sizes matches mobile for this example.
      visualDensity: VisualDensity.standard,
    );

void main() {
  runApp(const TextFieldDemoApp());
}

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

  @override
  State<TextFieldDemoApp> createState() => _TextFieldDemoAppState();
}

class _TextFieldDemoAppState extends State<TextFieldDemoApp> {
  bool useMaterial3 = false;
  ThemeMode themeMode = ThemeMode.light;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      themeMode: themeMode,
      theme: myTheme(Brightness.light),
      darkTheme: myTheme(Brightness.dark),
      home: Scaffold(
        appBar: AppBar(
          title: const Text(('TextField States Issue?')),
          actions: [
            IconButton(
              icon: themeMode == ThemeMode.dark
                  ? const Icon(Icons.wb_sunny_outlined)
                  : const Icon(Icons.wb_sunny),
              onPressed: () {
                setState(() {
                  if (themeMode == ThemeMode.light) {
                    themeMode = ThemeMode.dark;
                  } else {
                    themeMode = ThemeMode.light;
                  }
                });
              },
              tooltip: "Toggle brightness",
            ),
          ],
        ),
        body: const HomePage(),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return ListView(
      padding: const EdgeInsets.symmetric(horizontal: 16),
      children: [
        const SizedBox(height: 8),
        Text(
          'Material 3 TextField Manual Test',
          style: Theme.of(context).textTheme.headlineSmall,
        ),
        const SizedBox(height: 8),
        const TextInputStatesDemo(),
        const SizedBox(height: 8),
        const ShowColorSchemeColors(),
      ],
    );
  }
}

class TextInputStatesDemo extends StatefulWidget {
  const TextInputStatesDemo({Key? key}) : super(key: key);

  @override
  State<TextInputStatesDemo> createState() => _TextInputStatesDemoState();
}

class _TextInputStatesDemoState extends State<TextInputStatesDemo> {
  late TextEditingController _textController1;
  late TextEditingController _textController2;
  bool _errorState1 = false;
  bool _errorState2 = false;

  @override
  void initState() {
    super.initState();
    _textController1 = TextEditingController();
    _textController2 = TextEditingController();
  }

  @override
  void dispose() {
    _textController1.dispose();
    _textController2.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        TextField(
          onChanged: (String text) {
            setState(() {
              if (text.contains('a') | text.isEmpty) {
                _errorState1 = false;
              } else {
                _errorState1 = true;
              }
            });
          },
          key: const Key('TextField1'),
          controller: _textController1,
          decoration: InputDecoration(
            prefixIcon: const Icon(Icons.search),
            suffixIcon: const Icon(Icons.cancel_outlined),
            // border: const UnderlineInputBorder(),
            filled: true,
            hintText: 'Write something...',
            labelText: 'Text entry',
            errorText: _errorState1
                ? "Any entry without an 'a' will trigger this error"
                : null,
          ),
        ),
        const SizedBox(height: 16),
        TextField(
          onChanged: (String text) {
            setState(() {
              if (text.contains('a') | text.isEmpty) {
                _errorState2 = false;
              } else {
                _errorState2 = true;
              }
            });
          },
          key: const Key('TextField2'),
          controller: _textController2,
          decoration: InputDecoration(
            prefixIcon: const Icon(Icons.search),
            suffixIcon: const Icon(Icons.cancel_outlined),
            border: const OutlineInputBorder(),
            hintText: 'Write something...',
            labelText: 'Another text entry',
            errorText: _errorState2
                ? "Any entry without an 'a' will trigger this error"
                : null,
          ),
        ),
      ],
    );
  }
}

/// Draw a number of boxes showing the colors of key theme color properties
/// in the ColorScheme of the inherited ThemeData and its color properties.
class ShowColorSchemeColors extends StatelessWidget {
  const ShowColorSchemeColors({super.key, this.onBackgroundColor});

  /// The color of the background the color widget are being drawn on.
  ///
  /// Some of the theme colors may have semi transparent fill color. To compute
  /// a legible text color for the sum when it shown on a background color, we
  /// need to alpha merge it with background and we need the exact background
  /// color it is drawn on for that. If not passed in from parent, it is
  /// assumed to be drawn on card color, which usually is close enough.
  final Color? onBackgroundColor;

  // Return true if the color is light, meaning it needs dark text for contrast.
  static bool _isLight(final Color color) =>
      ThemeData.estimateBrightnessForColor(color) == Brightness.light;

  // Return true if the color is dark, meaning it needs light text for contrast.
  static bool _isDark(final Color color) =>
      ThemeData.estimateBrightnessForColor(color) == Brightness.dark;

  // On color used when a theme color property does not have a theme onColor.
  static Color _onColor(final Color color, final Color bg) =>
      _isLight(Color.alphaBlend(color, bg)) ? Colors.black : Colors.white;

  @override
  Widget build(BuildContext context) {
    final ThemeData theme = Theme.of(context);
    final ColorScheme colorScheme = theme.colorScheme;
    final bool isDark = colorScheme.brightness == Brightness.dark;
    final bool useMaterial3 = theme.useMaterial3;

    final MediaQueryData media = MediaQuery.of(context);
    final bool isPhone = media.size.width < phoneWidthBreakpoint ||
        media.size.height < phoneHeightBreakpoint;
    final double spacing = isPhone ? 3 : 6;

    // Grab the card border from the theme card shape
    ShapeBorder? border = theme.cardTheme.shape;
    // If we had one, copy in a border side to it.
    if (border is RoundedRectangleBorder) {
      border = border.copyWith(
        side: BorderSide(
          color: theme.dividerColor,
          width: 1,
        ),
      );
      // If
    } else {
      // If border was null, make one matching Card default, but with border
      // side, if it was not null, we leave it as it was.
      border ??= RoundedRectangleBorder(
        borderRadius: BorderRadius.all(Radius.circular(useMaterial3 ? 12 : 4)),
        side: BorderSide(
          color: theme.dividerColor,
          width: 1,
        ),
      );
    }

    // Get effective background color.
    final Color background =
        onBackgroundColor ?? theme.cardTheme.color ?? theme.cardColor;

    // Warning label for scaffold background when it uses to much blend.
    final String surfaceTooHigh = isDark
        ? _isLight(theme.colorScheme.surface)
            ? '\nTOO HIGH'
            : ''
        : _isDark(theme.colorScheme.surface)
            ? '\nTOO HIGH'
            : '';

    // Warning label for scaffold background when it uses to much blend.
    final String backTooHigh = isDark
        ? _isLight(theme.colorScheme.background)
            ? '\nTOO HIGH'
            : ''
        : _isDark(theme.colorScheme.background)
            ? '\nTOO HIGH'
            : '';

    // Wrap this widget branch in a custom theme where card has a border outline
    // if it did not have one, but retains in ambient themed border radius.
    return Theme(
      data: Theme.of(context).copyWith(
        cardTheme: CardTheme.of(context).copyWith(
          elevation: 0,
          shape: border,
        ),
      ),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 8),
            child: Text(
              'ColorScheme Colors (M3 Baseline colors)',
              style: theme.textTheme.titleMedium,
            ),
          ),
          Wrap(
            alignment: WrapAlignment.start,
            crossAxisAlignment: WrapCrossAlignment.center,
            spacing: spacing,
            runSpacing: spacing,
            children: <Widget>[
              ColorCard(
                label: 'Primary',
                color: colorScheme.primary,
                textColor: colorScheme.onPrimary,
              ),
              ColorCard(
                label: 'on\nPrimary',
                color: colorScheme.onPrimary,
                textColor: colorScheme.primary,
              ),
              ColorCard(
                label: 'Primary\nContainer',
                color: colorScheme.primaryContainer,
                textColor: colorScheme.onPrimaryContainer,
              ),
              ColorCard(
                label: 'onPrimary\nContainer',
                color: colorScheme.onPrimaryContainer,
                textColor: colorScheme.primaryContainer,
              ),
              ColorCard(
                label: 'Secondary',
                color: colorScheme.secondary,
                textColor: colorScheme.onSecondary,
              ),
              ColorCard(
                label: 'on\nSecondary',
                color: colorScheme.onSecondary,
                textColor: colorScheme.secondary,
              ),
              ColorCard(
                label: 'Secondary\nContainer',
                color: colorScheme.secondaryContainer,
                textColor: colorScheme.onSecondaryContainer,
              ),
              ColorCard(
                label: 'on\nSecondary\nContainer',
                color: colorScheme.onSecondaryContainer,
                textColor: colorScheme.secondaryContainer,
              ),
              ColorCard(
                label: 'Tertiary',
                color: colorScheme.tertiary,
                textColor: colorScheme.onTertiary,
              ),
              ColorCard(
                label: 'on\nTertiary',
                color: colorScheme.onTertiary,
                textColor: colorScheme.tertiary,
              ),
              ColorCard(
                label: 'Tertiary\nContainer',
                color: colorScheme.tertiaryContainer,
                textColor: colorScheme.onTertiaryContainer,
              ),
              ColorCard(
                label: 'on\nTertiary\nContainer',
                color: colorScheme.onTertiaryContainer,
                textColor: colorScheme.tertiaryContainer,
              ),
              ColorCard(
                label: 'Error',
                color: colorScheme.error,
                textColor: colorScheme.onError,
              ),
              ColorCard(
                label: 'on\nError',
                color: colorScheme.onError,
                textColor: colorScheme.error,
              ),
              ColorCard(
                label: 'Error\nContainer',
                color: colorScheme.errorContainer,
                textColor: colorScheme.onErrorContainer,
              ),
              ColorCard(
                label: 'onError\nContainer',
                color: colorScheme.onErrorContainer,
                textColor: colorScheme.errorContainer,
              ),
              ColorCard(
                label: 'Background$backTooHigh',
                color: colorScheme.background,
                textColor: colorScheme.onBackground,
              ),
              ColorCard(
                label: 'on\nBackground',
                color: colorScheme.onBackground,
                textColor: colorScheme.background,
              ),
              ColorCard(
                label: 'Surface$surfaceTooHigh',
                color: colorScheme.surface,
                textColor: colorScheme.onSurface,
              ),
              ColorCard(
                label: 'on\nSurface',
                color: colorScheme.onSurface,
                textColor: colorScheme.surface,
              ),
              ColorCard(
                label: 'Surface\nVariant',
                color: colorScheme.surfaceVariant,
                textColor: colorScheme.onSurfaceVariant,
              ),
              ColorCard(
                label: 'onSurface\nVariant',
                color: colorScheme.onSurfaceVariant,
                textColor: colorScheme.surfaceVariant,
              ),
              ColorCard(
                label: 'Outline',
                color: colorScheme.outline,
                textColor: colorScheme.background,
              ),
              ColorCard(
                label: 'Outline\nVariant',
                color: colorScheme.outlineVariant,
                textColor: colorScheme.onBackground,
              ),
              ColorCard(
                label: 'Shadow',
                color: colorScheme.shadow,
                textColor: _onColor(colorScheme.shadow, background),
              ),
              ColorCard(
                label: 'Scrim',
                color: colorScheme.scrim,
                textColor: _onColor(colorScheme.scrim, background),
              ),
              ColorCard(
                label: 'Inverse\nSurface',
                color: colorScheme.inverseSurface,
                textColor: colorScheme.onInverseSurface,
              ),
              ColorCard(
                label: 'onInverse\nSurface',
                color: colorScheme.onInverseSurface,
                textColor: colorScheme.inverseSurface,
              ),
              ColorCard(
                label: 'Inverse\nPrimary',
                color: colorScheme.inversePrimary,
                textColor: colorScheme.inverseSurface,
              ),
              ColorCard(
                label: 'Surface\nTint',
                color: colorScheme.surfaceTint,
                textColor: colorScheme.onPrimary,
              ),
            ],
          ),
        ],
      ),
    );
  }
}

/// A [SizedBox] with a [Card] and string text in it. Used in this demo to
/// display theme color boxes.
///
/// Can specify label text color and background color.
class ColorCard extends StatelessWidget {
  const ColorCard({
    super.key,
    required this.label,
    required this.color,
    required this.textColor,
    this.size,
  });

  final String label;
  final Color color;
  final Color textColor;
  final Size? size;

  @override
  Widget build(BuildContext context) {
    const double fontSize = 11;
    const Size effectiveSize = Size(86, 58);

    return SizedBox(
      width: effectiveSize.width,
      height: effectiveSize.height,
      child: Card(
        margin: EdgeInsets.zero,
        clipBehavior: Clip.antiAlias,
        color: color,
        child: Center(
          child: Text(
            label,
            style: TextStyle(color: textColor, fontSize: fontSize),
            textAlign: TextAlign.center,
          ),
        ),
      ),
    );
  }
}

Source code for Flutter issue: flutter/flutter#113329