Fix `FlexibleSpaceBar` centered title position and title color (#140883)

fixes [Invisible SliverAppBar title in Material 3 light theme](https://github.com/flutter/flutter/issues/138296) 
fixes [`FlexibleSpaceBar` title is misaligned without the leading widget](https://github.com/flutter/flutter/issues/138608)

Previous attempt https://github.com/flutter/flutter/pull/138611

--- 

### Description

 - fixes the `FlexibleSpaceBar` centered title position when there is a leading widget.
 - fixes the `FlexibleSpaceBar` title color for Material 3.
 - Added documentation when using a long `FlexibleSpaceBar` title and update its test.
 - Improved documentation of default title padding.

### Code sample

### 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 const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Example(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: SafeArea(
          child: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            leading: Icon(Icons.favorite_rounded),
            flexibleSpace: FlexibleSpaceBar(
              title: ColoredBox(
                color: Color(0xffff0000),
                child: Text('SliverAppBar'),
              ),
            ),
          ),
        ],
      )),
    );
  }
}
```

</details>

###  Before

![Screenshot 2024-01-03 at 18 02 25](https://github.com/flutter/flutter/assets/48603081/92ae1062-c78f-4005-8e28-85af617acd60)

### After

![Screenshot 2024-01-03 at 18 02 16](https://github.com/flutter/flutter/assets/48603081/2ef97108-9b50-44f7-a303-018ff1b28db6)

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

@@ -99,6 +99,10 @@ class FlexibleSpaceBar extends StatefulWidget {
 
   /// Whether the title should be centered.
   ///
+  /// If the length of the title is greater than the available space, set
+  /// this property to false. This aligns the title to the start of the
+  /// flexible space bar and applies [titlePadding] to the title.
+  ///
   /// By default this property is true if the current target platform
   /// is [TargetPlatform.iOS] or [TargetPlatform.macOS], false otherwise.
   final bool? centerTitle;
@@ -120,9 +124,13 @@ class FlexibleSpaceBar extends StatefulWidget {
   /// inset from the bottom-left and it is specified along with
   /// [centerTitle] false.
   ///
-  /// By default the value of this property is
-  /// `EdgeInsetsDirectional.only(start: 72, bottom: 16)` if the title is
-  /// not centered, `EdgeInsetsDirectional.only(start: 0, bottom: 16)` otherwise.
+  /// If [centerTitle] is true, then the title is centered within the
+  /// flexible space bar with a bottom padding of 16.0 pixels.
+  ///
+  /// If [centerTitle] is false and [FlexibleSpaceBarSettings.hasLeading] is true,
+  /// then the title is aligned to the start of the flexible space bar with the
+  /// [titlePadding] applied. If [titlePadding] is null, then defaults to start
+  /// padding of 72.0 pixels and bottom padding of 16.0 pixels.
   final EdgeInsetsGeometry? titlePadding;
 
   /// Defines how much the title is scaled when the FlexibleSpaceBar is expanded
@@ -316,14 +324,15 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
 
           final double opacity = settings.toolbarOpacity;
           if (opacity > 0.0) {
-            TextStyle titleStyle = theme.primaryTextTheme.titleLarge!;
+            TextStyle titleStyle = theme.useMaterial3 ? theme.textTheme.titleLarge! : theme.primaryTextTheme.titleLarge!;
             titleStyle = titleStyle.copyWith(
               color: titleStyle.color!.withOpacity(opacity),
             );
             final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
+            final double leadingPadding = (settings.hasLeading ?? true) ? 72.0 : 0.0;
             final EdgeInsetsGeometry padding = widget.titlePadding ??
               EdgeInsetsDirectional.only(
-                start: effectiveCenterTitle && !(settings.hasLeading ?? false) ? 0.0 : 72.0,
+                start: effectiveCenterTitle ? 0.0 : leadingPadding,
                 bottom: 16.0,
               );
             final double scaleValue = Tween<double>(begin: widget.expandedTitleScale, end: 1.0).transform(t);

packages/flutter/test/material/flexible_space_bar_test.dart

@@ -719,7 +719,6 @@ void main() {
 
     await tester.pumpWidget(buildFrame(TargetPlatform.macOS, false));
     expect(getTitleBottomLeft(), const Offset(72.0, 16.0));
-
   });
 
   testWidgets('FlexibleSpaceBar test titlePadding override', (WidgetTester tester) async {
@@ -830,7 +829,9 @@ void main() {
         leading: const Icon(Icons.menu),
         flexibleSpace: FlexibleSpaceBar(
           title: Text('title ' * 10),
-          centerTitle: true,
+          // Set centerTitle to false to create a gap between the leading widget
+          // and the long title.
+          centerTitle: false,
         ),
       ),
     ) as FlexibleSpaceBarSettings;
@@ -861,54 +862,6 @@ void main() {
     expect(tester.getTopLeft(find.byType(Text)).dx, closeTo(72.0, 0.01));
   });
 
-  // This is a regression test for https://github.com/flutter/flutter/issues/132030.
-  testWidgets('Long centered FlexibleSpaceBar.title respects leading widget', (WidgetTester tester) async {
-    // Test start position of a long title when the leading widget is
-    // shown by default and the long title is centered.
-    await tester.pumpWidget(
-      MaterialApp(
-        home: Scaffold(
-          drawer: const Drawer(),
-          body: CustomScrollView(
-            slivers: <Widget>[
-              SliverAppBar(
-                flexibleSpace: FlexibleSpaceBar(
-                  title: Text('Title ' * 10),
-                  centerTitle: true,
-                ),
-              ),
-            ],
-          ),
-        ),
-      ),
-    );
-
-    expect(tester.getTopLeft(find.byType(Text)).dx, 72.0);
-
-    // Test start position of a long title when the leading widget is provided
-    // and the long title is centered.
-    await tester.pumpWidget(
-      MaterialApp(
-        home: Scaffold(
-          body: CustomScrollView(
-            slivers: <Widget>[
-              SliverAppBar(
-                leading: const Icon(Icons.menu),
-                flexibleSpace: FlexibleSpaceBar(
-                  title: Text('Title ' * 10),
-                  centerTitle: true,
-                ),
-              ),
-            ],
-          ),
-        ),
-      ),
-    );
-
-    await tester.pumpAndSettle();
-    expect(tester.getTopLeft(find.byType(Text)).dx, 72.0);
-  });
-
   // This is a regression test for https://github.com/flutter/flutter/issues/135698.
   testWidgets('_FlexibleSpaceHeaderOpacity with near zero opacity avoids compositing', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -948,6 +901,55 @@ void main() {
 
     expect(tester.takeException(), isNull);
   }, variant: TargetPlatformVariant.mobile());
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/138608.
+  testWidgets('FlexibleSpaceBar centers title with a leading widget', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Material(
+          child: CustomScrollView(
+            slivers: <Widget>[
+              SliverAppBar(
+                leading: Icon(Icons.menu),
+                flexibleSpace: FlexibleSpaceBar(
+                  centerTitle: true,
+                  title: Text('X'),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final Offset appBarCenter = tester.getCenter(find.byType(AppBar));
+    final Offset titleCenter = tester.getCenter(find.text('X'));
+    expect(appBarCenter.dx, titleCenter.dx);
+  });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/138296.
+  testWidgets('Material3 - Default title color', (WidgetTester tester) async {
+    final ThemeData theme = ThemeData();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: theme, // Provide the expected theme data.
+        home: const Material(
+          child: CustomScrollView(
+            slivers: <Widget>[
+              SliverAppBar(
+                flexibleSpace: FlexibleSpaceBar(
+                  title: Text('Title'),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final DefaultTextStyle textStyle = DefaultTextStyle.of(tester.element(find.text('Title')));
+    expect(textStyle.style.color, theme.textTheme.titleLarge!.color);
+  });
 }
 
 class TestDelegate extends SliverPersistentHeaderDelegate {