Remove unwanted gap between navigation bar and safe area's child (#29943)

Remove the additional top padding from CupertinoPageScaffold's MediaQuery when the navigation bar is opaque (as the padding was already consumed by the navigation bar).

Related Issue: #29136

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

@@ -34,7 +34,8 @@ class CupertinoPageScaffold extends StatelessWidget {
   /// If translucent, the main content may slide behind it.
   /// Otherwise, the main content's top margin will be offset by its height.
   ///
-  /// The scaffold assumes the navigation bar will consume the [MediaQuery] top padding.
+  /// The scaffold assumes the navigation bar will account for the [MediaQuery] top padding,
+  /// also consume it if the navigation bar is opaque.
   // TODO(xster): document its page transition animation when ready
   final ObstructingPreferredSizeWidget navigationBar;
 
@@ -92,7 +93,10 @@ class CupertinoPageScaffold extends StatelessWidget {
       // obstructed area.
       if (fullObstruction) {
         paddedContent = MediaQuery(
-          data: existingMediaQuery.copyWith(
+          data: existingMediaQuery
+          // If the navigation bar is opaque, the top media query padding is fully consumed by the navigation bar.
+          .removePadding(removeTop: true)
+          .copyWith(
             viewInsets: newViewInsets,
           ),
           child: Padding(

packages/flutter/test/cupertino/scaffold_test.dart

@@ -25,6 +25,34 @@ void main() {
     expect(tester.getTopLeft(find.byType(Center)), const Offset(0.0, 0.0));
   });
 
+testWidgets('Opaque bar pushes contents down', (WidgetTester tester) async {
+    BuildContext childContext;
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: MediaQuery(
+        data: const MediaQueryData(viewInsets: EdgeInsets.only(top: 20)),
+        child: CupertinoPageScaffold(
+          navigationBar: const CupertinoNavigationBar(
+            middle: Text('Opaque'),
+            backgroundColor: Color(0xFFF8F8F8),
+          ),
+          child: Builder(
+            builder: (BuildContext context) {
+              childContext = context;
+              return Container();
+            },
+          ),
+        ),
+      ),
+    ));
+
+    expect(MediaQuery.of(childContext).padding.top, 0);
+    // The top of the [Container] is 44 px from the top of the screen because
+    // it's pushed down by the opaque navigation bar whose height is 44 px,
+    // and the 20 px [MediaQuery] top padding is fully absorbed by the navigation bar.
+    expect(tester.getRect(find.byType(Container)), Rect.fromLTRB(0, 44, 800, 600));
+  });
+
   testWidgets('Contents padding from viewInsets', (WidgetTester tester) async {
     await tester.pumpWidget(Directionality(
       textDirection: TextDirection.ltr,