Add stretch property to CupertinoSliverNavigationBar (#71707)

* Add stretch property to CupertinoSliverNavigationBar

* Added tests

* Fix trailling spaces

* xster review: fix tests

* xster review: fix tests

* trim extra spaces

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -551,6 +551,11 @@ class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
 /// from the operating system can be retrieved in many ways, such as querying
 /// [MediaQuery.textScaleFactorOf] against [CupertinoApp]'s [BuildContext].
 ///
+/// The [stretch] parameter determines whether the nav bar should stretch to
+/// fill the over-scroll area. The nav bar can still expand and contract as the
+/// user scrolls, but it will also stretch when the user over-scrolls if the
+/// [stretch] value is `true`. Defaults to `true`.
+///
 /// See also:
 ///
 ///  * [CupertinoNavigationBar], an iOS navigation bar for use on non-scrolling
@@ -575,6 +580,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
     this.actionsForegroundColor,
     this.transitionBetweenRoutes = true,
     this.heroTag = _defaultHeroTag,
+    this.stretch = true,
   }) : assert(automaticallyImplyLeading != null),
        assert(automaticallyImplyTitle != null),
        assert(
@@ -672,6 +678,13 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   /// True if the navigation bar's background color has no transparency.
   bool get opaque => backgroundColor?.alpha == 0xFF;
 
+  /// Whether the nav bar should stretch to fill the over-scroll area.
+  ///
+  /// The nav bar can still expand and contract as the user scrolls, but it will
+  /// also stretch when the user over-scrolls if the [stretch] value is `true`.
+  /// Defaults to `true`.
+  final bool stretch;
+
   @override
   _CupertinoSliverNavigationBarState createState() => _CupertinoSliverNavigationBarState();
 }
@@ -729,6 +742,7 @@ class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigation
             heroTag: widget.heroTag,
             persistentHeight: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
             alwaysShowMiddle: widget.middle != null,
+            stretchConfiguration: widget.stretch ? OverScrollHeaderStretchConfiguration() : null,
           ),
         ),
       ),
@@ -751,6 +765,7 @@ class _LargeTitleNavigationBarSliverDelegate
     required this.heroTag,
     required this.persistentHeight,
     required this.alwaysShowMiddle,
+    required this.stretchConfiguration,
   }) : assert(persistentHeight != null),
        assert(alwaysShowMiddle != null),
        assert(transitionBetweenRoutes != null);
@@ -774,6 +789,9 @@ class _LargeTitleNavigationBarSliverDelegate
   @override
   double get maxExtent => persistentHeight + _kNavBarLargeTitleHeightExtension;
 
+  @override
+  OverScrollHeaderStretchConfiguration? stretchConfiguration;
+
   @override
   Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
     final bool showLargeTitle = shrinkOffset < maxExtent - minExtent - _kNavBarShowLargeTitleThreshold;

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -1185,6 +1185,113 @@ void main() {
     expect(barItems2.length, greaterThan(0));
     expect(barItems2.any((RichText t) => t.textScaleFactor != 1), isFalse);
   });
+
+  testWidgets(
+    'CupertinoSliverNavigationBar stretches upon over-scroll and bounces back once over-scroll ends',
+    (WidgetTester tester) async {
+    const Text trailingText = Text('Bar Button');
+    const Text titleText = Text('Large Title');
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoPageScaffold(
+          child: CustomScrollView(
+            slivers: <Widget>[
+              const CupertinoSliverNavigationBar(
+                trailing: trailingText,
+                largeTitle: titleText,
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  height: 1200.0,
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final Finder trailingTextFinder = find.byWidget(trailingText).first;
+    final Finder titleTextFinder = find.byWidget(titleText).first;
+
+    final Offset initialTrailingTextToLargeTitleOffset = tester.getTopLeft(trailingTextFinder) - tester.getTopLeft(titleTextFinder);
+
+    // Drag for overscroll
+    await tester.drag(find.byType(Scrollable), const Offset(0.0, 150.0));
+    await tester.pump();
+
+    final Offset stretchedTrailingTextToLargeTitleOffset = tester.getTopLeft(trailingTextFinder) - tester.getTopLeft(titleTextFinder);
+
+    expect(
+      stretchedTrailingTextToLargeTitleOffset.dy.abs(),
+      greaterThan(initialTrailingTextToLargeTitleOffset.dy.abs())
+    );
+
+    // Ensure overscroll retracts to original size after releasing gesture
+    await tester.pumpAndSettle();
+
+    final Offset finalTrailingTextToLargeTitleOffset = tester.getTopLeft(trailingTextFinder) - tester.getTopLeft(titleTextFinder);
+
+    expect(
+      finalTrailingTextToLargeTitleOffset.dy.abs(),
+      initialTrailingTextToLargeTitleOffset.dy.abs(),
+    );
+  });
+
+  testWidgets(
+    'CupertinoSliverNavigationBar does not stretch upon over-scroll if stretch parameter is false',
+    (WidgetTester tester) async {
+    const Text trailingText = Text('Bar Button');
+    const Text titleText = Text('Large Title');
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoPageScaffold(
+          child: CustomScrollView(
+            slivers: <Widget>[
+              const CupertinoSliverNavigationBar(
+                trailing: trailingText,
+                largeTitle: titleText,
+                stretch: false,
+              ),
+              SliverToBoxAdapter(
+                child: Container(
+                  height: 1200.0,
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final Finder trailingTextFinder = find.byWidget(trailingText).first;
+    final Finder titleTextFinder = find.byWidget(titleText).first;
+
+    final Offset initialTrailingTextToLargeTitleOffset = tester.getTopLeft(trailingTextFinder) - tester.getTopLeft(titleTextFinder);
+
+    // Drag for overscroll
+    await tester.drag(find.byType(Scrollable), const Offset(0.0, 150.0));
+    await tester.pump();
+
+    final Offset stretchedTrailingTextToLargeTitleOffset = tester.getTopLeft(trailingTextFinder) - tester.getTopLeft(titleTextFinder);
+
+    expect(
+      stretchedTrailingTextToLargeTitleOffset.dy.abs(),
+      initialTrailingTextToLargeTitleOffset.dy.abs(),
+    );
+
+    // Ensure overscroll is zero after releasing gesture
+    await tester.pumpAndSettle();
+
+    final Offset finalTrailingTextToLargeTitleOffset = tester.getTopLeft(trailingTextFinder) - tester.getTopLeft(titleTextFinder);
+
+    expect(
+      finalTrailingTextToLargeTitleOffset.dy.abs(),
+      initialTrailingTextToLargeTitleOffset.dy.abs(),
+    );
+  });
 }
 
 class _ExpectStyles extends StatelessWidget {