Fix Cupertino sheet covered top gap transition - #187058

  testWidgets('covered sheet does not reveal the root route through its top gap', (
    WidgetTester tester,
  ) async {
    final GlobalKey scaffoldKey = GlobalKey();

    await tester.pumpWidget(
      CupertinoApp(
        home: CupertinoPageScaffold(
          key: scaffoldKey,
          child: Column(
            children: <Widget>[
              const Text('Page 1'),
              CupertinoButton(
                onPressed: () {
                  Navigator.push<void>(
                    scaffoldKey.currentContext!,
                    CupertinoSheetRoute<void>(
                      builder: (BuildContext context) {
                        return CupertinoPageScaffold(
                          child: Column(
                            children: <Widget>[
                              const Text('Page 2'),
                              CupertinoButton(
                                onPressed: () {
                                  Navigator.push<void>(
                                    context,
                                    CupertinoSheetRoute<void>(
                                      builder: (BuildContext context) {
                                        return const CupertinoPageScaffold(child: Text('Page 3'));
                                      },
                                    ),
                                  );
                                },
                                child: const Text('Push Page 3'),
                              ),
                            ],
                          ),
                        );
                      },
                    ),
                  );
                },
                child: const Text('Push Page 2'),
              ),
            ],
          ),
        ),
      ),
    );

    await tester.tap(find.text('Push Page 2'));
    await tester.pumpAndSettle();

    await tester.tap(find.text('Push Page 3'));
    await tester.pumpAndSettle();

    final double page1Top = tester
        .getTopLeft(
          find.ancestor(of: find.text('Page 1'), matching: find.byType(CupertinoPageScaffold)),
        )
        .dy;
    final double page2Top = tester
        .getTopLeft(
          find.ancestor(of: find.text('Page 2'), matching: find.byType(CupertinoPageScaffold)),
        )
        .dy;
    final double page3Top = tester
        .getTopLeft(
          find.ancestor(of: find.text('Page 3'), matching: find.byType(CupertinoPageScaffold)),
        )
        .dy;

    // Sheet 2 moves up above Page 1 so that the root route (Page 1) is completely hidden behind Sheet 2.
    expect(page2Top, lessThanOrEqualTo(page1Top));
    // Sheet 3 is stacked on top of Sheet 2, with Sheet 2 peeking out above Sheet 3.
    expect(page2Top, lessThanOrEqualTo(page3Top));
  });