Fix transparent dividerColor breaks TabBar.tabAlignment (#150350)

fixes [`TabBar.tabAlignment` property does't work when `dividerColor` is transparent](https://github.com/flutter/flutter/issues/150316)

### 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) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: DefaultTabController(
        length: 2,
        child: Scaffold(
          appBar: AppBar(
            title: const Text('TabBar'),
            bottom: const TabBar(
              dividerColor: Colors.transparent,
              tabAlignment: TabAlignment.start,
              isScrollable: true,
              tabs: <Widget>[
                Tab(text: 'TAB 1'),
                Tab(text: 'TAB 2'),
              ],
            ),
          ),
          body: const TabBarView(
            children: <Widget>[
              SizedBox.expand(),
              SizedBox.expand(),
            ],
          ),
          floatingActionButton: FloatingActionButton(
            onPressed: () {},
            child: const Icon(Icons.add),
          ),
        ),
      ),
    );
  }
}
```

</details>

### Before (`dividerColor: Colors.transparent`, `tabAlignment: TabAlignment.start`)

![Screenshot 2024-06-17 at 15 44 33](https://github.com/flutter/flutter/assets/48603081/a60e88c7-fd99-4444-b7e5-1bceacc22f4c)

### After (`dividerColor: Colors.transparent`, `tabAlignment: TabAlignment.start`)

![Screenshot 2024-06-17 at 15 44 55](https://github.com/flutter/flutter/assets/48603081/aa003d65-107f-4618-be29-095ab2660374)

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

@@ -1025,6 +1025,8 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
 
   /// The color of the divider.
   ///
+  /// If the [dividerColor] is [Colors.transparent], then the divider will not be drawn.
+  ///
   /// If null and [ThemeData.useMaterial3] is false, [TabBarTheme.dividerColor]
   /// color is used. If that is null and [ThemeData.useMaterial3] is true,
   /// [ColorScheme.outlineVariant] will be used, otherwise divider will not be drawn.
@@ -1032,6 +1034,8 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
 
   /// The height of the divider.
   ///
+  /// If the [dividerHeight] is zero or negative, then the divider will not be drawn.
+  ///
   /// 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.
@@ -1813,19 +1817,18 @@ class _TabBarState extends State<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,
+          widthFactor: dividerHeight > 0 ? null : 1.0,
           alignment: effectiveAlignment,
           child: tabBar,
         );
 
-        if (showDivider) {
+        if (dividerColor != Colors.transparent && dividerHeight > 0) {
           tabBar = CustomPaint(
             painter: _DividerPainter(
-              dividerColor: widget.dividerColor ?? tabBarTheme.dividerColor ?? _defaults.dividerColor!,
+              dividerColor: dividerColor,
               dividerHeight: widget.dividerHeight ?? tabBarTheme.dividerHeight ?? _defaults.dividerHeight!,
             ),
             child: tabBar,

packages/flutter/test/material/tabs_test.dart

@@ -6722,30 +6722,6 @@ void main() {
       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', () {
@@ -7250,4 +7226,51 @@ void main() {
     expect(find.text('Page 3'), findsOneWidget);
     expect(scrollable.controller!.position.pixels, equals(0.0));
   });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/150316.
+  testWidgets('Scrollable TabBar wuth transparent divider expands to full width', (WidgetTester tester) async {
+    Widget buildTabBar({ Color? dividerColor, TabAlignment? tabAlignment }) {
+      return boilerplate(
+        child: Center(
+          child: DefaultTabController(
+            length: 3,
+            child: TabBar(
+              dividerColor: dividerColor,
+              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 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, 800.0);
+
+    // 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, 800.0);
+
+    // 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, 800.0);
+  });
 }