Add bottom to CupertinoNavigationBar to allow for a double-row nav bar (#155959)

Fixes [Support segmented controls in nav bars and double row nav bars](https://github.com/flutter/flutter/issues/10469)

<img width="270" height="600"  alt="Screenshot 2024-09-30 at 3 09 04 PM" src="https://github.com/user-attachments/assets/4003116f-69dd-4f8f-a185-6ca151b74d2d">

examples/api/lib/cupertino/nav_bar/cupertino_navigation_bar.1.dart

@@ -0,0 +1,72 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+
+/// Flutter code sample for [CupertinoNavigationBar] showing a
+/// [CupertinoSearchTextField] with padding at the bottom of the navigation bar.
+
+void main() => runApp(const NavBarApp());
+
+class NavBarApp extends StatelessWidget {
+  const NavBarApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const CupertinoApp(
+      theme: CupertinoThemeData(brightness: Brightness.light),
+      home: NavBarExample(),
+    );
+  }
+}
+
+class NavBarExample extends StatefulWidget {
+  const NavBarExample({super.key});
+
+  @override
+  State<NavBarExample> createState() => _NavBarExampleState();
+}
+
+class _NavBarExampleState extends State<NavBarExample> {
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: const CupertinoNavigationBar(
+        middle: Text('CupertinoNavigationBar Sample'),
+        bottom: _NavigationBarSearchField(),
+        automaticBackgroundVisibility: false,
+      ),
+      child: Column(
+        children: <Widget>[
+          Container(height: 50, color: CupertinoColors.systemRed),
+          Container(height: 50, color: CupertinoColors.systemGreen),
+          Container(height: 50, color: CupertinoColors.systemBlue),
+          Container(height: 50, color: CupertinoColors.systemYellow),
+        ],
+      ),
+    );
+  }
+}
+
+
+class _NavigationBarSearchField extends StatelessWidget implements PreferredSizeWidget {
+  const _NavigationBarSearchField();
+
+  static const double padding = 8.0;
+  static const double searchFieldHeight = 35.0;
+
+  @override
+  Widget build(BuildContext context) {
+    return const Padding(
+      padding: EdgeInsets.symmetric(horizontal: padding, vertical: padding),
+      child: SizedBox(
+        height: searchFieldHeight,
+        child: CupertinoSearchTextField()
+      ),
+    );
+  }
+
+  @override
+  Size get preferredSize => const Size.fromHeight(searchFieldHeight + padding * 2);
+}

examples/api/test/cupertino/nav_bar/cupertino_navigation_bar.1_test.dart

@@ -0,0 +1,27 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+import 'package:flutter_api_samples/cupertino/nav_bar/cupertino_navigation_bar.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('CupertinoNavigationBar with bottom widget', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.NavBarApp(),
+    );
+
+    final Finder navBarFinder = find.byType(CupertinoNavigationBar);
+    final Finder searchFieldFinder = find.byType(CupertinoSearchTextField);
+
+    expect(navBarFinder, findsOneWidget);
+    expect(searchFieldFinder, findsOneWidget);
+
+    // The bottom widget is bounded by the navigation bar.
+    expect(
+      tester.getBottomLeft(searchFieldFinder).dy,
+      lessThan(tester.getBottomLeft(navBarFinder).dy),
+    );
+  });
+}

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

@@ -289,6 +289,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
     this.padding,
     this.transitionBetweenRoutes = true,
     this.heroTag = _defaultHeroTag,
+    this.bottom,
   }) : assert(
          !transitionBetweenRoutes || identical(heroTag, _defaultHeroTag),
          'Cannot specify a heroTag override if this navigation bar does not '
@@ -465,6 +466,23 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   /// {@endtemplate}
   final Object heroTag;
 
+  /// A widget to place at the bottom of the navigation bar.
+  ///
+  /// Only widgets that implement [PreferredSizeWidget] can be used at the
+  /// bottom of a navigation bar.
+  ///
+  /// {@tool dartpad}
+  /// This example shows a [CupertinoSearchTextField] at the bottom of a
+  /// [CupertinoNavigationBar].
+  ///
+  /// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_navigation_bar.1.dart **
+  /// {@end-tool}
+  ///
+  /// See also:
+  ///
+  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
+  final PreferredSizeWidget? bottom;
+
   /// True if the navigation bar's background color has no transparency.
   @override
   bool shouldFullyObstruct(BuildContext context) {
@@ -475,7 +493,8 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
 
   @override
   Size get preferredSize {
-    return const Size.fromHeight(_kNavBarPersistentHeight);
+    final double heightForDrawer = bottom?.preferredSize.height ?? 0.0;
+    return Size.fromHeight(_kNavBarPersistentHeight + heightForDrawer);
   }
 
   @override
@@ -586,9 +605,14 @@ class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
       enableBackgroundFilterBlur: widget.enableBackgroundFilterBlur,
       child: DefaultTextStyle(
         style: CupertinoTheme.of(context).textTheme.textStyle,
-        child: _PersistentNavigationBar(
-          components: components,
-          padding: widget.padding,
+        child: Column(
+          children: <Widget>[
+            _PersistentNavigationBar(
+              components: components,
+              padding: widget.padding,
+            ),
+            if (widget.bottom != null) widget.bottom!,
+          ],
         ),
       ),
     );

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -2086,6 +2086,54 @@ void main() {
       bottomHeight,
     );
   });
+
+  testWidgets('CupertinoNavigationBar with bottom widget', (WidgetTester tester) async {
+    const double persistentHeight = 44.0;
+    const double bottomHeight = 10.0;
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoPageScaffold(
+          navigationBar: const CupertinoNavigationBar(
+            middle: Text('Middle'),
+            bottom: PreferredSize(
+              preferredSize: Size.fromHeight(bottomHeight),
+              child: Placeholder(),
+            )
+          ),
+          child: Container(),
+        ),
+      ),
+    );
+
+    final Finder navBarFinder = find.byType(CupertinoNavigationBar);
+    expect(navBarFinder, findsOneWidget);
+    final CupertinoNavigationBar navBar = tester.widget<CupertinoNavigationBar>(navBarFinder);
+
+    final Finder columnFinder = find.descendant(
+      of: navBarFinder,
+      matching: find.byType(Column),
+    );
+    expect(columnFinder, findsOneWidget);
+    final Column column = tester.widget<Column>(columnFinder);
+
+    expect(column.children.length, 2);
+    expect(
+      find.descendant(
+        of: find.byWidget(column.children.first),
+        matching: find.text('Middle'),
+      ),
+      findsOneWidget,
+    );
+    expect(
+      find.descendant(
+        of: find.byWidget(column.children.last),
+        matching: find.byType(Placeholder),
+      ),
+      findsOneWidget,
+    );
+    expect(navBar.preferredSize.height, persistentHeight + bottomHeight);
+  });
 }
 
 class _ExpectStyles extends StatelessWidget {