Fix `FlexibleSpaceBar.title` doesn't respect the leading widget (#132573)

fixes [Long `FlexibleSpaceBar.title` doesn't respect the leading widget 
](https://github.com/flutter/flutter/issues/132030)

### Description

- This adds `FlexibleSpaceBarSettings.hasLeading` for the `FlexibleSpaceBar`'s title to respect the leading widget.
- Use the new `FlexibleSpaceBarSettings.hasLeading` property in the  `SliverAppBar` for its `FlexibleSpaceBar`.

### 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,
      theme: ThemeData(
        useMaterial3: true,
        brightness: Brightness.dark,
      ),
      home: const Example(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          const Text('TargetPlatform.Android'),
          Theme(
            data: Theme.of(context).copyWith(
              platform: TargetPlatform.android,
            ),
            child: Container(
              height: 250,
              padding: const EdgeInsets.all(8),
              decoration: BoxDecoration(
                border: Border.all(
                  color: Colors.amber,
                  width: 4,
                ),
              ),
              child: const AppBarLeading(
                showLeading: true,
                showTitle: false,
              ),
            ),
          ),
          const Text('TargetPlatform.iOS'),
          Theme(
            data: Theme.of(context).copyWith(
              platform: TargetPlatform.iOS,
            ),
            child: Container(
              height: 250,
              padding: const EdgeInsets.all(8),
              decoration: BoxDecoration(
                border: Border.all(
                  color: Colors.amber,
                  width: 2,
                ),
              ),
              child: const AppBarLeading(
                showLeading: true,
                showTitle: false,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

class AppBarLeading extends StatelessWidget {
  const AppBarLeading({
    super.key,
    required this.showLeading,
    required this.showTitle,
  });

  final bool showLeading;
  final bool showTitle;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      drawer: const Drawer(),
      body: CustomScrollView(
        slivers: [
          SliverAppBar(
            automaticallyImplyLeading: showLeading,
            iconTheme: const IconThemeData(
              color: Colors.amber,
            ),
            title: showTitle ? const Text('AppBar') : null,
            flexibleSpace: FlexibleSpaceBar(
              title: Text('Title ' * 15),
              // centerTitle: true,
            ),
            toolbarHeight: showTitle ? 170 : 100,
          ),
        ],
      ),
    );
  }
}
``` 
	
</details>

### Before 

![Screenshot 2023-08-15 at 18 11 34](https://github.com/flutter/flutter/assets/48603081/4b798998-8549-43aa-b564-933ea14f494c)

### After

![Screenshot 2023-08-15 at 18 11 45](https://github.com/flutter/flutter/assets/48603081/b085a33a-db7d-40d4-8a12-ee37197b5bd4)

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

@@ -1261,6 +1261,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
       currentExtent: math.max(minExtent, maxExtent - shrinkOffset),
       toolbarOpacity: toolbarOpacity,
       isScrolledUnder: isScrolledUnder,
+      hasLeading: leading != null || automaticallyImplyLeading,
       child: AppBar(
         clipBehavior: clipBehavior,
         leading: leading,

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

@@ -156,6 +156,7 @@ class FlexibleSpaceBar extends StatefulWidget {
     double? minExtent,
     double? maxExtent,
     bool? isScrolledUnder,
+    bool? hasLeading,
     required double currentExtent,
     required Widget child,
   }) {
@@ -164,6 +165,7 @@ class FlexibleSpaceBar extends StatefulWidget {
       minExtent: minExtent ?? currentExtent,
       maxExtent: maxExtent ?? currentExtent,
       isScrolledUnder: isScrolledUnder,
+      hasLeading: hasLeading,
       currentExtent: currentExtent,
       child: child,
     );
@@ -321,7 +323,7 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
             final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
             final EdgeInsetsGeometry padding = widget.titlePadding ??
               EdgeInsetsDirectional.only(
-                start: effectiveCenterTitle ? 0.0 : 72.0,
+                start: effectiveCenterTitle && !(settings.hasLeading ?? false) ? 0.0 : 72.0,
                 bottom: 16.0,
               );
             final double scaleValue = Tween<double>(begin: widget.expandedTitleScale, end: 1.0).transform(t);
@@ -380,6 +382,7 @@ class FlexibleSpaceBarSettings extends InheritedWidget {
     required this.currentExtent,
     required super.child,
     this.isScrolledUnder,
+    this.hasLeading,
   }) : assert(minExtent >= 0),
        assert(maxExtent >= 0),
        assert(currentExtent >= 0),
@@ -413,13 +416,24 @@ class FlexibleSpaceBarSettings extends InheritedWidget {
   /// overlaps the primary scrollable's contents.
   final bool? isScrolledUnder;
 
+  /// True if the FlexibleSpaceBar has a leading widget.
+  ///
+  /// This value is used by the [FlexibleSpaceBar] to determine
+  /// if there should be a gap between the leading widget and
+  /// the title.
+  ///
+  /// Null if the caller hasn't determined if the FlexibleSpaceBar
+  /// has a leading widget.
+  final bool? hasLeading;
+
   @override
   bool updateShouldNotify(FlexibleSpaceBarSettings oldWidget) {
     return toolbarOpacity != oldWidget.toolbarOpacity
         || minExtent != oldWidget.minExtent
         || maxExtent != oldWidget.maxExtent
         || currentExtent != oldWidget.currentExtent
-        || isScrolledUnder != oldWidget.isScrolledUnder;
+        || isScrolledUnder != oldWidget.isScrolledUnder
+        || hasLeading != oldWidget.hasLeading;
   }
 }
 

packages/flutter/test/material/flexible_space_bar_test.dart

@@ -821,6 +821,94 @@ void main() {
     expect(RenderRebuildTracker.count, greaterThan(1));
     expect(tester.layers.whereType<OpacityLayer>(), isEmpty);
   });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/132030.
+  testWidgetsWithLeakTracking('FlexibleSpaceBarSettings.hasLeading provides a gap between leading and title', (WidgetTester tester) async {
+    final FlexibleSpaceBarSettings customSettings = FlexibleSpaceBar.createSettings(
+      currentExtent: 200.0,
+      hasLeading: true,
+      child: AppBar(
+        leading: const Icon(Icons.menu),
+        flexibleSpace: FlexibleSpaceBar(
+          title: Text('title ' * 10),
+          centerTitle: true,
+        ),
+      ),
+    ) as FlexibleSpaceBarSettings;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: CustomScrollView(
+            slivers: <Widget>[
+              SliverPersistentHeader(
+                floating: true,
+                pinned: true,
+                delegate: TestDelegate(settings: customSettings),
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  height: 1200.0,
+                  color: Colors.orange[400],
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+    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.
+  testWidgetsWithLeakTracking('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);
+  });
 }
 
 class TestDelegate extends SliverPersistentHeaderDelegate {