Feat: Add persistentFooterDecoration for scaffold (#167524)

Feat: Add persistentFooterDecoration for scaffold
fixes: #166478 

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

---------

Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>

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

@@ -1697,6 +1697,7 @@ class Scaffold extends StatefulWidget {
     this.floatingActionButtonAnimator,
     this.persistentFooterButtons,
     this.persistentFooterAlignment = AlignmentDirectional.centerEnd,
+    this.persistentFooterDecoration,
     this.drawer,
     this.onDrawerChanged,
     this.endDrawer,
@@ -1814,6 +1815,17 @@ class Scaffold extends StatefulWidget {
   /// Defaults to [AlignmentDirectional.centerEnd].
   final AlignmentDirectional persistentFooterAlignment;
 
+  /// Decoration for the container that holds the [persistentFooterButtons].
+  ///
+  /// By default, this container has a top border with a width of 1.0, created by
+  /// [Divider.createBorderSide].
+  ///
+  /// See also:
+  ///
+  ///  * [persistentFooterButtons], which defines the buttons to show in the footer.
+  ///  * [persistentFooterAlignment], which defines the alignment of the footer buttons.
+  final BoxDecoration? persistentFooterDecoration;
+
   /// 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])
@@ -3058,9 +3070,9 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
       _addIfNonNull(
         children,
         Container(
-          decoration: BoxDecoration(
-            border: Border(top: Divider.createBorderSide(context, width: 1.0)),
-          ),
+          decoration:
+              widget.persistentFooterDecoration ??
+              BoxDecoration(border: Border(top: Divider.createBorderSide(context, width: 1.0))),
           child: SafeArea(
             top: false,
             child: IntrinsicHeight(

packages/flutter/test/material/scaffold_test.dart

@@ -823,6 +823,33 @@ void main() {
     expect(initialPoint, finalPoint);
   });
 
+  testWidgets('Persistent bottom buttons can apply decoration', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(padding: EdgeInsets.fromLTRB(10.0, 20.0, 30.0, 40.0)),
+          child: Scaffold(
+            body: SingleChildScrollView(
+              child: Container(color: Colors.amber[500], height: 5000.0, child: const Text('body')),
+            ),
+            persistentFooterDecoration: const BoxDecoration(
+              border: Border(top: BorderSide(color: Colors.red)),
+            ),
+            persistentFooterButtons: const <Widget>[Placeholder()],
+          ),
+        ),
+      ),
+    );
+
+    final Finder persistentFooter =
+        find.ancestor(of: find.byType(OverflowBar), matching: find.byType(Container)).first;
+    final Decoration decoration = tester.widget<Container>(persistentFooter).decoration!;
+
+    expect(decoration, isA<BoxDecoration>());
+    expect((decoration as BoxDecoration).border!.top.color, Colors.red);
+  });
+
   group('back arrow', () {
     Future<void> expectBackIcon(WidgetTester tester, IconData expectedIcon) async {
       final GlobalKey rootKey = GlobalKey();