Add alignment parameter for persistentFooterButtons (#101297)

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

@@ -1463,6 +1463,7 @@ class Scaffold extends StatefulWidget {
     this.floatingActionButtonLocation,
     this.floatingActionButtonAnimator,
     this.persistentFooterButtons,
+    this.persistentFooterAlignment = AlignmentDirectional.centerEnd,
     this.drawer,
     this.onDrawerChanged,
     this.endDrawer,
@@ -1569,6 +1570,11 @@ class Scaffold extends StatefulWidget {
   /// [bottomNavigationBar] but below the [body].
   final List<Widget>? persistentFooterButtons;
 
+  /// The alignment of the [persistentFooterButtons] inside the [OverflowBar].
+  ///
+  /// Defaults to [AlignmentDirectional.centerEnd].
+  final AlignmentDirectional persistentFooterAlignment;
+
   /// A panel displayed to the side of the [body], often hidden on mobile
   /// devices. Swipes in from either left-to-right ([TextDirection.ltr]) or
   /// right-to-left ([TextDirection.rtl])
@@ -2725,7 +2731,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
             top: false,
             child: IntrinsicHeight(
               child: Container(
-                alignment: AlignmentDirectional.centerEnd,
+                alignment: widget.persistentFooterAlignment,
                 padding: const EdgeInsets.all(8),
                 child: OverflowBar(
                   spacing: 8,

packages/flutter/test/material/scaffold_test.dart

@@ -533,6 +533,41 @@ void main() {
     expect(didPressButton, isTrue);
   });
 
+  testWidgets('Persistent bottom buttons alignment', (WidgetTester tester) async {
+    Widget buildApp(AlignmentDirectional persistentAligment) {
+      return MaterialApp(
+          home: Scaffold(
+            body: SingleChildScrollView(
+              child: Container(
+                color: Colors.amber[500],
+                height: 5000.0,
+                child: const Text('body'),
+              ),
+            ),
+            persistentFooterAlignment: persistentAligment,
+            persistentFooterButtons: <Widget>[
+              TextButton(
+                onPressed: () { },
+                child: const Text('X'),
+              ),
+            ],
+          ),
+      );
+    }
+
+    await tester.pumpWidget(buildApp(AlignmentDirectional.centerEnd));
+    Finder footerButton = find.byType(TextButton);
+    expect(tester.getTopRight(footerButton).dx, 800.0 - 8.0);
+
+    await tester.pumpWidget(buildApp(AlignmentDirectional.center));
+    footerButton = find.byType(TextButton);
+    expect(tester.getCenter(footerButton).dx, 800.0 / 2);
+
+    await tester.pumpWidget(buildApp(AlignmentDirectional.centerStart));
+    footerButton = find.byType(TextButton);
+    expect(tester.getTopLeft(footerButton).dx, 8.0);
+  });
+
   testWidgets('Persistent bottom buttons apply media padding', (WidgetTester tester) async {
     await tester.pumpWidget(
       Directionality(