[Material 3] Support 24dp minimum width for Primary TabBar active indicator and allow theming primary vs. secondary tab indicators in TabBarThemeData #194716

Steps to reproduce

1. Create a Material 3 Flutter app (useMaterial3: true) with both a primary TabBar (containing a short tab label such as Tab(text: 'A') and indicatorColor: Colors.red) and a TabBar.secondary.
2. Observe that by default, selecting the short Tab(text: 'A') in the primary TabBar renders an active indicator narrower than the 24dp minimum width specified by the Material 3 Tabs specification.
3. Configure a custom indicator in ThemeData(tabBarTheme: TabBarThemeData(indicator: ...)) to customize the primary tab active indicator (e.g., with 3dp thickness, 3dp top corner radius, or custom width/insets).
4. Run the sample code below and observe:
   • The TabBar.secondary widget also inherits tabBarTheme.indicator (rendering with a 3dp rounded indicator instead of its default 2dp flat full-width underline).
   • The indicatorColor: Colors.red property on the TabBar is ignored because _TabBarState._getIndicator returns tabBarTheme.indicator! before checking widget._isPrimary or widget.indicatorColor.

Expected results

1. In a Material 3 primary TabBar (TabBar(...)), short tab labels narrower than 24dp (e.g., Tab(text: 'A')) should still render an active indicator with a minimum width of 24dp centered under the label, per the Material 3 Tabs specification
2. When a custom primary tab indicator is configured in ThemeData(tabBarTheme: TabBarThemeData(indicator: ...)), TabBar.secondary should retain its Material 3 secondary tab default (2dp flat full-width underline) instead of inheriting the primary tab indicator, and TabBar(indicatorColor: ...) overrides should still be respected.

Actual results

1. By default in Material 3, a primary TabBar with a short label (e.g., Tab(text: 'A')) renders an UnderlineTabIndicator narrower than 24dp because UnderlineTabIndicator does not support or enforce a 24dp minimum width.
2. When setting TabBarThemeData(indicator: ...) in ThemeData to customize the primary tab indicator:
   a. _TabBarState._getIndicator returns tabBarTheme.indicator! before checking widget._isPrimary or widget.indicatorColor.
   b. TabBar.secondary unexpectedly inherits the 3dp rounded primary indicator instead of its 2dp flat full-width underline.
   c. TabBar(indicatorColor: Colors.red) is ignored and continues to paint with the theme indicator's color.

Code sample

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(
        useMaterial3: true,
        // Setting a custom indicator in TabBarThemeData to style primary tabs:
        tabBarTheme: const TabBarThemeData(
          indicator: UnderlineTabIndicator(
            borderRadius: BorderRadius.only(
              topLeft: Radius.circular(3),
              topRight: Radius.circular(3),
            ),
            borderSide: BorderSide(width: 3, color: Colors.blue),
            insets: EdgeInsets.symmetric(horizontal: 2),
          ),
        ),
      ),
      home: DefaultTabController(
        length: 3,
        child: Scaffold(
          appBar: AppBar(
            title: const Text('TabBar Indicator Repro'),
            // 1. Primary TabBar with a short label ('A') and custom indicatorColor:
            // - Without tabBarTheme.indicator, 'A' has an indicator narrower than 24dp.
            // - With tabBarTheme.indicator, indicatorColor: Colors.red is ignored.
            bottom: const TabBar(
              indicatorColor: Colors.red,
              tabs: [
                Tab(text: 'A'),
                Tab(text: 'Medium'),
                Tab(text: 'Longer Tab'),
              ],
            ),
          ),
          body: const Column(
            children: [
              SizedBox(height: 24),
              Text('Secondary TabBar (should have 2dp flat full-width indicator):'),
              // 2. Secondary TabBar unexpectedly inherits the 3dp rounded indicator
              // from tabBarTheme.indicator instead of a 2dp flat underline.
              TabBar.secondary(
                tabs: [
                  Tab(text: 'Overview'),
                  Tab(text: 'Specifications'),
                  Tab(text: 'Reviews'),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Screenshots or Video

Logs

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Flutter Doctor output

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