Test Xamarin.Forms app with Picker input issues

MyPage.cs

using System;
using System.Threading.Tasks;
using Xamarin.Forms;

public class Scene : object
{

    protected RelativeLayout _slidein_layout = null;
    protected RelativeLayout _static_layout = null;
    protected StackLayout _next_button = null;
    protected Grid _grid = null;

    public Scene(Grid parent_grid)
    {
        this._grid = parent_grid;
    }

    public void CleanupPreviousScene()
    {
        if ((_grid != null) && (_slidein_layout != null))
        {
            _grid.Children.Remove(_slidein_layout);
        }
        if ((_grid != null) && (_static_layout != null))
        {
            _grid.Children.Remove(_static_layout);
        }
    }

    public void SetupScene(String mode)
    {

        StackLayout stack = new StackLayout
        {
            // inner black box that directly contains text and widgets
            Margin = 15,
            Orientation = StackOrientation.Vertical,
            HorizontalOptions = LayoutOptions.Center

        };

        switch (mode)
        {
            case "welcome":
                Label welcome_label = new Label
                {
                    TextColor = Color.White,
                    Text = "Welcome to app!",
                    FontSize = 18
                };
                stack.Children.Add(welcome_label);
                break;

            case "picker":
                Label picker_label = new Label
                {
                    TextColor = Color.White,
                    Text = "Choose something",
                };
                stack.Children.Add(picker_label);
                Picker picker = new Picker
                {
                    BackgroundColor = Color.FromHex("111111"),
                    TextColor = Color.White,
                    WidthRequest = 400
                };

                picker.Items.Add("Item 1");
                picker.Items.Add("Item 2");

                stack.Children.Add(picker);
                break;
        }

        StackLayout stack_outer = new StackLayout
        {
            /* outer black box that contains the "Welcome to app!" text */
            Opacity = 0.95,
            HeightRequest = 100,
            BackgroundColor = Color.Black,
            Children = { stack }
        };

        _slidein_layout = new RelativeLayout
        {
            VerticalOptions = LayoutOptions.FillAndExpand,
            HorizontalOptions = LayoutOptions.FillAndExpand,
            Opacity = 1.0
        };

        _slidein_layout.Children.Add(stack_outer,
                                   /* x constraint */
                                   Constraint.RelativeToParent((parent) =>
                                   {
                                       // this means it's initially off-screen
                                       return parent.Width;
                                   }),
                                   /* y constraint */
                                   Constraint.RelativeToParent((parent) =>
                                   {
                                       return parent.Height * 0.15;
                                   }),
                                   /* width constraint */
                                   Constraint.RelativeToParent((parent) =>
                                   {
                                       return parent.Width;
                                   }),
                                   /* height constraint */
                                   Constraint.RelativeToParent((parent) =>
                                   {
                                       return 100;
                                   })
                                  );

        _static_layout = new RelativeLayout
        {
            VerticalOptions = LayoutOptions.FillAndExpand,
            HorizontalOptions = LayoutOptions.FillAndExpand,

        };

        CreateNextButton();
        _grid.Children.Add(_static_layout, 0, 0);
        _grid.Children.Add(_slidein_layout, 0, 0);

    }

    public void CreateNextButton()
    {

        Label next_label = new Label
        {
            Margin = 15,
            TextColor = Color.White,
            Text = "Next >",
            FontSize = 18,
            InputTransparent = true
        };

        StackLayout next_button_inner = new StackLayout
        {
            Opacity = 100.0,
            HorizontalOptions = LayoutOptions.Center,
            Orientation = StackOrientation.Vertical,
            Margin = 15,
            Children = { new Label { TextColor = Color.White, Text = "Next >", FontSize = 18 } },
        };

        _next_button = new StackLayout
        {
            Opacity = 100.0,
            HeightRequest = 100,
            BackgroundColor = Color.Green,
            Children = { next_button_inner }
        };

        var tgr = new TapGestureRecognizer();
        tgr.Tapped += async (object sender, EventArgs e) =>
        {
            await _next_button.FadeTo(0.0, 250, Easing.CubicOut);
            await ExitTransition();
        };
        _next_button.GestureRecognizers.Add(tgr);

        _static_layout.Children.Add(_next_button,
                    /* x constraint */
                    Constraint.RelativeToParent((parent) =>
                    {
                        return parent.Width * 0.7;
                    }),
                    /* y constraint */
                    Constraint.RelativeToParent((parent) =>
                    {
                        return parent.Height * 0.8;
                    }),
                    /* width constraint */
                    Constraint.RelativeToParent((parent) =>
                    {
                        return parent.Width * 0.3;
                    }),
                    /* height constraint */
                    Constraint.RelativeToParent((parent) =>
                    {
                        return 100;
                    })
                   );

    }

    public async Task ExitTransition()
    {
        await _next_button.ScaleTo(0.9, 250, Easing.CubicOut);
        await _next_button.FadeTo(0.0, 250, Easing.CubicOut);
        await _slidein_layout.TranslateTo(-2 * Application.Current.MainPage.Width, 0, 500, Easing.CubicIn);
        _slidein_layout.IsVisible = false;
        CleanupPreviousScene();
        SetupScene("picker");
        EnterTransition();
    }

    public void EnterTransition()
    {
        Device.BeginInvokeOnMainThread(async () =>
        {
            await _slidein_layout.TranslateTo(-Application.Current.MainPage.Width, 0, 500, Easing.CubicIn);
            await _next_button.ScaleTo(1.0, 250, Easing.CubicOut);
            await _next_button.FadeTo(1.0, 250, Easing.CubicOut);
        });
    }

}

namespace testapp
{
    public class MyPage : ContentPage
    {

        protected Scene _scene = null;
        protected Grid _grid = null;

        public MyPage()
        {
            NavigationPage.SetHasNavigationBar(this, false);

            this._grid = new Grid
            {
                VerticalOptions = LayoutOptions.FillAndExpand,
                HorizontalOptions = LayoutOptions.FillAndExpand,
                RowDefinitions =
                {
                    new RowDefinition { Height = new GridLength(1, GridUnitType.Star) },
                    new RowDefinition { Height = new GridLength(30, GridUnitType.Absolute) }
                },
                ColumnDefinitions =
                {
                    new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) }
                },
                RowSpacing = 0,
                ColumnSpacing = 0
            };

        }
        protected override void OnAppearing()
        {
            this.Content = this._grid;
            this._scene = new Scene(_grid);
            _scene.SetupScene("welcome");
            _scene.EnterTransition();
        }
    }


}