Fix scrollable `TabBar` expands to full width when the divider is removed (#140963)

fixes [TabBar Expands to full width of the screen isScrollable: true after upgrading to flutter 3.16.4](https://github.com/flutter/flutter/issues/140338)

---

## Description

Fixes the scrollable `TabBar` width when the divider is removed. (when the divider height is set to `0` or divider color is set to `Colors.transparent`)

### Code sample

<details>
<summary>expand to view the code sample</summary> 

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

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

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

  @override
  Widget build(BuildContext context) {
    const int tabsCount = 2;

    return MaterialApp(
      home: DefaultTabController(
        initialIndex: 1,
        length: tabsCount,
        child: Scaffold(
          appBar: AppBar(
            title: const Text('TabBar Sample'),
            bottom: PreferredSize(
              preferredSize: const Size.fromHeight(48.0),
              child: ColoredBox(
                color: Theme.of(context).colorScheme.secondaryContainer,
                child:  TabBar(
                  // dividerColor: Theme.of(context).colorScheme.onSurface,
                  dividerColor: Colors.transparent, // remove divider
                  // dividerHeight: 0, // remove divider
                  isScrollable: true,
                  tabAlignment: TabAlignment.center,
                  tabs: <Widget>[
                    for (int i = 0; i < tabsCount; i++)
                      Tab(
                        text: 'Tab $i',
                      ),
                  ],
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}
```

</details>

### Before
![Simulator Screenshot - iPhone 15 Pro - 2024-01-04 at 15 16 15](https://github.com/flutter/flutter/assets/48603081/b776e7e6-e5f0-49df-8a79-55032eaad631)

### After
![Simulator Screenshot - iPhone 15 Pro - 2024-01-04 at 15 16 23](https://github.com/flutter/flutter/assets/48603081/9ad13793-43a9-4ae8-977e-7cf84cb59bb1)

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

@@ -1693,17 +1693,26 @@ class _TabBarState extends State<TabBar> {
           TabAlignment.start || TabAlignment.startOffset || TabAlignment.fill => AlignmentDirectional.centerStart,
         };
 
-        tabBar = CustomPaint(
-          painter: _DividerPainter(
-            dividerColor: widget.dividerColor ?? tabBarTheme.dividerColor ?? _defaults.dividerColor!,
-            dividerHeight: widget.dividerHeight ?? tabBarTheme.dividerHeight ?? _defaults.dividerHeight!,
-          ),
-          child: Align(
-            heightFactor: 1.0,
-            alignment: effectiveAlignment,
-            child: tabBar,
-          ),
+        final Color dividerColor = widget.dividerColor ?? tabBarTheme.dividerColor ?? _defaults.dividerColor!;
+        final double dividerHeight = widget.dividerHeight ?? tabBarTheme.dividerHeight ?? _defaults.dividerHeight!;
+        final bool showDivider = dividerColor != Colors.transparent && dividerHeight > 0;
+
+        tabBar = Align(
+          heightFactor: 1.0,
+          widthFactor: showDivider ? null : 1.0,
+          alignment: effectiveAlignment,
+          child: tabBar,
         );
+
+        if (showDivider) {
+          tabBar = CustomPaint(
+            painter: _DividerPainter(
+              dividerColor: widget.dividerColor ?? tabBarTheme.dividerColor ?? _defaults.dividerColor!,
+              dividerHeight: widget.dividerHeight ?? tabBarTheme.dividerHeight ?? _defaults.dividerHeight!,
+            ),
+            child: tabBar,
+          );
+        }
       }
     } else if (widget.padding != null) {
       tabBar = Padding(

packages/flutter/test/material/tabs_test.dart

@@ -6636,6 +6636,97 @@ void main() {
     expect(unselectedTextStyle.fontStyle, unselectedLabelStyle.fontStyle);
   });
 
+  // This is a regression test for https://github.com/flutter/flutter/issues/140338.
+  testWidgets('Material3 - Scrollable TabBar without a divider does not expand to full width', (WidgetTester tester) async {
+    Widget buildTabBar({
+      Color? dividerColor,
+      double? dividerHeight,
+      TabAlignment? tabAlignment,
+     }) {
+      return boilerplate(
+        child: Center(
+          child: DefaultTabController(
+            length: 3,
+            child: TabBar(
+              dividerColor: dividerColor,
+              dividerHeight: dividerHeight,
+              tabAlignment: tabAlignment,
+              isScrollable: true,
+              tabs: const <Widget>[
+                Tab(text: 'Tab 1'),
+                Tab(text: 'Tab 2'),
+                Tab(text: 'Tab 3'),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Test default tab bar width when there is a divider and tabAlignment
+    // is set to startOffset.
+    await tester.pumpWidget(buildTabBar(tabAlignment: TabAlignment.start));
+    expect(tester.getSize(find.byType(TabBar)).width, 800.0);
+
+    // Test default tab bar width when there is a divider and tabAlignment
+    // is set to start.
+    await tester.pumpWidget(buildTabBar(tabAlignment: TabAlignment.startOffset));
+    expect(tester.getSize(find.byType(TabBar)).width, 800.0);
+
+    // Test default tab bar width when there is a divider and tabAlignment
+    // tabAlignment is set to center.
+    await tester.pumpWidget(buildTabBar(tabAlignment: TabAlignment.center));
+    expect(tester.getSize(find.byType(TabBar)).width, 800.0);
+
+    // Test default tab bar width when the divider height is set to 0.0
+    // and tabAlignment is set to startOffset.
+    await tester.pumpWidget(buildTabBar(
+      dividerHeight: 0.0,
+      tabAlignment: TabAlignment.startOffset,
+    ));
+    expect(tester.getSize(find.byType(TabBar)).width, 359.5);
+
+    // Test default tab bar width when the divider height is set to 0.0
+    // and tabAlignment is set to start.
+    await tester.pumpWidget(buildTabBar(
+      dividerHeight: 0.0,
+      tabAlignment: TabAlignment.start,
+    ));
+    expect(tester.getSize(find.byType(TabBar)).width, 307.5);
+
+    // Test default tab bar width when the divider height is set to 0.0
+    // and tabAlignment is set to center.
+    await tester.pumpWidget(buildTabBar(
+      dividerHeight: 0.0,
+      tabAlignment: TabAlignment.center,
+    ));
+    expect(tester.getSize(find.byType(TabBar)).width, 307.5);
+
+    // Test default tab bar width when the divider color is set to transparent
+    // and tabAlignment is set to startOffset.
+    await tester.pumpWidget(buildTabBar(
+      dividerColor: Colors.transparent,
+      tabAlignment: TabAlignment.startOffset,
+    ));
+    expect(tester.getSize(find.byType(TabBar)).width, 359.5);
+
+    // Test default tab bar width when the divider color is set to transparent
+    // and tabAlignment is set to start.
+    await tester.pumpWidget(buildTabBar(
+      dividerColor: Colors.transparent,
+      tabAlignment: TabAlignment.start,
+    ));
+    expect(tester.getSize(find.byType(TabBar)).width, 307.5);
+
+    // Test default tab bar width when the divider color is set to transparent
+    // and tabAlignment is set to center.
+    await tester.pumpWidget(buildTabBar(
+      dividerColor: Colors.transparent,
+      tabAlignment: TabAlignment.center,
+    ));
+    expect(tester.getSize(find.byType(TabBar)).width, 307.5);
+  });
+
   group('Material 2', () {
     // These tests are only relevant for Material 2. Once Material 2
     // support is deprecated and the APIs are removed, these tests