Fix Material 3 Scrollable `TabBar` (#125974)

fix https://github.com/flutter/flutter/issues/117722

### Description
1. Fix the divider doesn't stretch to take all the available width in the scrollable tab bar in M3
2. Add `dividerHeight` property.
3. Update the default tab alignment for the scrollable tab bar to match the specs (this is backward compatible for M2 with the new `tabAlignment` property).

### Bug (default tab alignment)

![Screenshot 2023-05-05 at 19 04 40](https://user-images.githubusercontent.com/48603081/236509483-1d03af21-a764-4776-acef-2126560f0d51.png)

### Fix (default tab alignment)

![Screenshot 2023-05-05 at 19 04 15](https://user-images.githubusercontent.com/48603081/236509513-2426d456-c54f-42bd-9545-a14dc6ee7e69.png)

### Code sample

<details> 
<summary>code sample</summary> 

```dart
import 'package:flutter/material.dart';

/// Flutter code sample for [TabBar].

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        //  tabBarTheme: const TabBarTheme(tabAlignment: TabAlignment.start),
          useMaterial3: true,
      ),
      home: const TabBarExample(),
    );
  }
}

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

  @override
  State<TabBarExample> createState() => _TabBarExampleState();
}

class _TabBarExampleState extends State<TabBarExample> {
  bool rtl = false;

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      initialIndex: 1,
      length: 3,
      child: Directionality(
        textDirection:  rtl ? TextDirection.rtl : TextDirection.ltr,
        child: Scaffold(
          appBar: AppBar(
            title: const Text('TabBar Sample'),
          ),
          body: const Column(
            children: <Widget>[
              Text('Scrollable-TabAlignment.start'),
              TabBar(
                isScrollable: true,
                tabAlignment: TabAlignment.start,
                tabs: <Widget>[
                  Tab(
                    icon: Icon(Icons.cloud_outlined),
                  ),
                  Tab(
                    icon: Icon(Icons.beach_access_sharp),
                  ),
                  Tab(
                    icon: Icon(Icons.brightness_5_sharp),
                  ),
                ],
              ),
              Text('Scrollable-TabAlignment.startOffset'),
              TabBar(
                isScrollable: true,
                tabAlignment: TabAlignment.startOffset,
                tabs: <Widget>[
                  Tab(
                    icon: Icon(Icons.cloud_outlined),
                  ),
                  Tab(
                    icon: Icon(Icons.beach_access_sharp),
                  ),
                  Tab(
                    icon: Icon(Icons.brightness_5_sharp),
                  ),
                ],
              ),
              Text('Scrollable-TabAlignment.center'),
              TabBar(
                isScrollable: true,
                tabAlignment: TabAlignment.center,
                tabs: <Widget>[
                  Tab(
                    icon: Icon(Icons.cloud_outlined),
                  ),
                  Tab(
                    icon: Icon(Icons.beach_access_sharp),
                  ),
                  Tab(
                    icon: Icon(Icons.brightness_5_sharp),
                  ),
                ],
              ),
              Spacer(),
              Text('Non-scrollable-TabAlignment.fill'),
              TabBar(
                tabAlignment: TabAlignment.fill,
                tabs: <Widget>[
                  Tab(
                    icon: Icon(Icons.cloud_outlined),
                  ),
                  Tab(
                    icon: Icon(Icons.beach_access_sharp),
                  ),
                  Tab(
                    icon: Icon(Icons.brightness_5_sharp),
                  ),
                ],
              ),
              Text('Non-scrollable-TabAlignment.center'),
              TabBar(
                tabAlignment: TabAlignment.center,
                tabs: <Widget>[
                  Tab(
                    icon: Icon(Icons.cloud_outlined),
                  ),
                  Tab(
                    icon: Icon(Icons.beach_access_sharp),
                  ),
                  Tab(
                    icon: Icon(Icons.brightness_5_sharp),
                  ),
                ],
              ),
              Spacer(),
            ],
          ),
          floatingActionButton: FloatingActionButton.extended(
            onPressed: () {
              setState(() {
                rtl = !rtl;
              });
            },
            label: const Text('Switch Direction'),
            icon: const Icon(Icons.swap_horiz),
          ),
        ),
      ),
    );
  }
}
``` 
	
</details>

![Screenshot 2023-06-06 at 18 06 12](https://github.com/flutter/flutter/assets/48603081/5ee5386d-cc64-4025-a020-ed2222cb6031)

dev/tools/gen_defaults/generated/used_tokens.csv

@@ -529,6 +529,7 @@ md.comp.primary-navigation-tab.active.hover.state-layer.opacity,
 md.comp.primary-navigation-tab.active.pressed.state-layer.color,
 md.comp.primary-navigation-tab.active.pressed.state-layer.opacity,
 md.comp.primary-navigation-tab.divider.color,
+md.comp.primary-navigation-tab.divider.height,
 md.comp.primary-navigation-tab.inactive.focus.state-layer.color,
 md.comp.primary-navigation-tab.inactive.focus.state-layer.opacity,
 md.comp.primary-navigation-tab.inactive.hover.state-layer.color,
@@ -588,6 +589,7 @@ md.comp.search-view.header.supporting-text.color,
 md.comp.search-view.header.supporting-text.text-style,
 md.comp.secondary-navigation-tab.active.label-text.color,
 md.comp.secondary-navigation-tab.divider.color,
+md.comp.secondary-navigation-tab.divider.height,
 md.comp.secondary-navigation-tab.focus.state-layer.color,
 md.comp.secondary-navigation-tab.focus.state-layer.opacity,
 md.comp.secondary-navigation-tab.hover.state-layer.color,

dev/tools/gen_defaults/lib/tabs_template.dart

@@ -24,6 +24,9 @@ class _${blockName}PrimaryDefaultsM3 extends TabBarTheme {
   @override
   Color? get dividerColor => ${componentColor("md.comp.primary-navigation-tab.divider")};
 
+  @override
+  double? get dividerHeight => ${getToken('md.comp.primary-navigation-tab.divider.height')};
+
   @override
   Color? get indicatorColor => ${componentColor("md.comp.primary-navigation-tab.active-indicator")};
 
@@ -71,7 +74,7 @@ class _${blockName}PrimaryDefaultsM3 extends TabBarTheme {
   InteractiveInkFeatureFactory? get splashFactory => Theme.of(context).splashFactory;
 
   @override
-  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.start : TabAlignment.fill;
+  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.startOffset : TabAlignment.fill;
 
   static double indicatorWeight = ${getToken('md.comp.primary-navigation-tab.active-indicator.height')};
 }
@@ -88,6 +91,9 @@ class _${blockName}SecondaryDefaultsM3 extends TabBarTheme {
   @override
   Color? get dividerColor => ${componentColor("md.comp.secondary-navigation-tab.divider")};
 
+  @override
+  double? get dividerHeight => ${getToken('md.comp.secondary-navigation-tab.divider.height')};
+
   @override
   Color? get indicatorColor => ${componentColor("md.comp.primary-navigation-tab.active-indicator")};
 
@@ -135,7 +141,7 @@ class _${blockName}SecondaryDefaultsM3 extends TabBarTheme {
   InteractiveInkFeatureFactory? get splashFactory => Theme.of(context).splashFactory;
 
   @override
-  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.start : TabAlignment.fill;
+  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.startOffset : TabAlignment.fill;
 }
 ''';
 

packages/flutter/lib/src/material/tab_bar_theme.dart

@@ -32,6 +32,7 @@ class TabBarTheme with Diagnosticable {
     this.indicatorColor,
     this.indicatorSize,
     this.dividerColor,
+    this.dividerHeight,
     this.labelColor,
     this.labelPadding,
     this.labelStyle,
@@ -55,6 +56,9 @@ class TabBarTheme with Diagnosticable {
   /// Overrides the default value for [TabBar.dividerColor].
   final Color? dividerColor;
 
+  /// Overrides the default value for [TabBar.dividerHeight].
+  final double? dividerHeight;
+
   /// Overrides the default value for [TabBar.labelColor].
   ///
   /// If [labelColor] is a [MaterialStateColor], then the effective color will
@@ -101,6 +105,7 @@ class TabBarTheme with Diagnosticable {
     Color? indicatorColor,
     TabBarIndicatorSize? indicatorSize,
     Color? dividerColor,
+    double? dividerHeight,
     Color? labelColor,
     EdgeInsetsGeometry? labelPadding,
     TextStyle? labelStyle,
@@ -116,6 +121,7 @@ class TabBarTheme with Diagnosticable {
       indicatorColor: indicatorColor ?? this.indicatorColor,
       indicatorSize: indicatorSize ?? this.indicatorSize,
       dividerColor: dividerColor ?? this.dividerColor,
+      dividerHeight: dividerHeight ?? this.dividerHeight,
       labelColor: labelColor ?? this.labelColor,
       labelPadding: labelPadding ?? this.labelPadding,
       labelStyle: labelStyle ?? this.labelStyle,
@@ -147,6 +153,7 @@ class TabBarTheme with Diagnosticable {
       indicatorColor: Color.lerp(a.indicatorColor, b.indicatorColor, t),
       indicatorSize: t < 0.5 ? a.indicatorSize : b.indicatorSize,
       dividerColor: Color.lerp(a.dividerColor, b.dividerColor, t),
+      dividerHeight: t < 0.5 ? a.dividerHeight : b.dividerHeight,
       labelColor: Color.lerp(a.labelColor, b.labelColor, t),
       labelPadding: EdgeInsetsGeometry.lerp(a.labelPadding, b.labelPadding, t),
       labelStyle: TextStyle.lerp(a.labelStyle, b.labelStyle, t),
@@ -165,6 +172,7 @@ class TabBarTheme with Diagnosticable {
     indicatorColor,
     indicatorSize,
     dividerColor,
+    dividerHeight,
     labelColor,
     labelPadding,
     labelStyle,
@@ -189,6 +197,7 @@ class TabBarTheme with Diagnosticable {
         && other.indicatorColor == indicatorColor
         && other.indicatorSize == indicatorSize
         && other.dividerColor == dividerColor
+        && other.dividerHeight == dividerHeight
         && other.labelColor == labelColor
         && other.labelPadding == labelPadding
         && other.labelStyle == labelStyle

packages/flutter/lib/src/material/tabs.dart

@@ -397,6 +397,8 @@ class _IndicatorPainter extends CustomPainter {
     required this.indicatorPadding,
     required this.labelPaddings,
     this.dividerColor,
+    this.dividerHeight,
+    required this.width,
   }) : super(repaint: controller.animation) {
     if (old != null) {
       saveTabOffsets(old._currentTabOffsets, old._currentTextDirection);
@@ -408,8 +410,10 @@ class _IndicatorPainter extends CustomPainter {
   final TabBarIndicatorSize? indicatorSize;
   final EdgeInsetsGeometry indicatorPadding;
   final List<GlobalKey> tabKeys;
-  final Color? dividerColor;
   final List<EdgeInsetsGeometry> labelPaddings;
+  final Color? dividerColor;
+  final double? dividerHeight;
+  final double width;
 
   // _currentTabOffsets and _currentTextDirection are set each time TabBar
   // layout is completed. These values can be null when TabBar contains no
@@ -502,8 +506,10 @@ class _IndicatorPainter extends CustomPainter {
       textDirection: _currentTextDirection,
     );
     if (dividerColor != null) {
-      final Paint dividerPaint = Paint()..color = dividerColor!..strokeWidth = 1;
-      canvas.drawLine(Offset(0, size.height), Offset(size.width, size.height), dividerPaint);
+      final Paint dividerPaint = Paint()..color = dividerColor!..strokeWidth = dividerHeight!;
+      final Offset dividerP1 = Offset(-width, size.height - (dividerPaint.strokeWidth / 2));
+      final Offset dividerP2 = Offset(width, size.height - (dividerPaint.strokeWidth / 2));
+      canvas.drawLine(dividerP1, dividerP2, dividerPaint);
     }
     _painter!.paint(canvas, _currentRect!.topLeft, configuration);
   }
@@ -718,6 +724,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.indicator,
     this.indicatorSize,
     this.dividerColor,
+    this.dividerHeight,
     this.labelColor,
     this.labelStyle,
     this.labelPadding,
@@ -768,6 +775,7 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.indicator,
     this.indicatorSize,
     this.dividerColor,
+    this.dividerHeight,
     this.labelColor,
     this.labelStyle,
     this.labelPadding,
@@ -895,6 +903,13 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
   /// [ColorScheme.surfaceVariant] will be used, otherwise divider will not be drawn.
   final Color? dividerColor;
 
+  /// The height of the divider.
+  ///
+  /// If null and [ThemeData.useMaterial3] is true, [TabBarTheme.dividerHeight] is used.
+  /// If that is also null and [ThemeData.useMaterial3] is true, 1.0 will be used.
+  /// Otherwise divider will not be drawn.
+  final double? dividerHeight;
+
   /// The color of selected tab labels.
   ///
   /// If null, then [TabBarTheme.labelColor] is used. If that is also null and
@@ -1269,8 +1284,10 @@ class _TabBarState extends State<TabBar> {
       indicatorPadding: widget.indicatorPadding,
       tabKeys: _tabKeys,
       old: _indicatorPainter,
-      dividerColor: theme.useMaterial3 ? widget.dividerColor ?? tabBarTheme.dividerColor ?? _defaults.dividerColor : null,
       labelPaddings: _labelPaddings,
+      dividerColor: theme.useMaterial3 ? widget.dividerColor ?? tabBarTheme.dividerColor ?? _defaults.dividerColor : null,
+      dividerHeight: theme.useMaterial3 ? widget.dividerHeight ?? tabBarTheme.dividerHeight ?? _defaults.dividerHeight : null,
+      width: MediaQuery.sizeOf(context).width,
     );
   }
 
@@ -1475,6 +1492,7 @@ class _TabBarState extends State<TabBar> {
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterialLocalizations(context));
     assert(_debugScheduleCheckHasValidTabsCount());
+    final ThemeData theme = Theme.of(context);
     final TabBarTheme tabBarTheme = TabBarTheme.of(context);
     final TabAlignment effectiveTabAlignment = widget.tabAlignment ?? tabBarTheme.tabAlignment ?? _defaults.tabAlignment!;
     assert(_debugTabAlignmentIsValid(effectiveTabAlignment));
@@ -1627,6 +1645,17 @@ class _TabBarState extends State<TabBar> {
           child: tabBar,
         ),
       );
+      if (theme.useMaterial3) {
+        final AlignmentGeometry effectiveAlignment = switch (effectiveTabAlignment) {
+          TabAlignment.center => Alignment.center,
+          TabAlignment.start || TabAlignment.startOffset || TabAlignment.fill => AlignmentDirectional.centerStart,
+        };
+        tabBar = Align(
+          heightFactor: 1.0,
+          alignment: effectiveAlignment,
+          child: tabBar,
+        );
+      }
     } else if (widget.padding != null) {
       tabBar = Padding(
         padding: widget.padding!,
@@ -2177,6 +2206,9 @@ class _TabsPrimaryDefaultsM3 extends TabBarTheme {
   @override
   Color? get dividerColor => _colors.surfaceVariant;
 
+  @override
+  double? get dividerHeight => 1.0;
+
   @override
   Color? get indicatorColor => _colors.primary;
 
@@ -2224,7 +2256,7 @@ class _TabsPrimaryDefaultsM3 extends TabBarTheme {
   InteractiveInkFeatureFactory? get splashFactory => Theme.of(context).splashFactory;
 
   @override
-  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.start : TabAlignment.fill;
+  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.startOffset : TabAlignment.fill;
 
   static double indicatorWeight = 3.0;
 }
@@ -2241,6 +2273,9 @@ class _TabsSecondaryDefaultsM3 extends TabBarTheme {
   @override
   Color? get dividerColor => _colors.surfaceVariant;
 
+  @override
+  double? get dividerHeight => 1.0;
+
   @override
   Color? get indicatorColor => _colors.primary;
 
@@ -2288,7 +2323,7 @@ class _TabsSecondaryDefaultsM3 extends TabBarTheme {
   InteractiveInkFeatureFactory? get splashFactory => Theme.of(context).splashFactory;
 
   @override
-  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.start : TabAlignment.fill;
+  TabAlignment? get tabAlignment => isScrollable ? TabAlignment.startOffset : TabAlignment.fill;
 }
 
 // END GENERATED TOKEN PROPERTIES - Tabs