Scrollview in a stacklayout eats up views above it when scrolled upwards (in Charles Petzold's book)

Hi, I am following Charles Petzold's new Xamarin Book published by MSFT and got stuck in the paragraph about scrollviews inside stack layout.
I haven't found a way to have this example (page 74) working correctly. Anyone can help?

It has an inner stacklayout inside a scrollview inside an outer stacklayout. The outer stacklayout has a label above the scroll view.

Mr Petzel writes:
_Notice that the ScrollView has its VerticalOptions property set to LayoutOptions.-
FillAndExpand. Without that, this program won’t work. With it, the text is scrollable while the title
stays in place.
__

It is not what I see... if I scroll the view upwards, the label is semi-covered by the label (see first Capture)

If I set the label's background to WHITE, it is even worse: the label disappears completely (see second screenshot)

How to tell the scrollview: do not expand ABOVE the label in your stacklayout?

This is the code:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;


namespace BlackCat
{
    class BlackCatPage : ContentPage
    {
        public BlackCatPage()
        {
            StackLayout mainStack = new StackLayout();
            StackLayout textStack = new StackLayout
            {
                Padding = new Thickness(5),
                Spacing = 10

            };
            // Get access to the text resource.
            Assembly assembly = this.GetType().GetTypeInfo().Assembly;
            string resource = "BlackCat.Texts.TheBlackCat.txt";
            using (Stream stream = assembly.GetManifestResourceStream(resource))
            {
                using (StreamReader reader = new StreamReader(stream))
                {
                    bool gotTitle = false;
                    string line;
                    // Read in a line (which is actually a paragraph).
                    while (null != (line = reader.ReadLine()))
                    {
                        Label label = new Label
                        {
                            Text = line,
                            // Black text for ebooks!
                            TextColor = Color.Black
                        };
                        if (!gotTitle)
                        {
                            // Add first label (the title to mainStack.
                            label.HorizontalOptions = LayoutOptions.Center;
                            label.Font = Font.SystemFontOfSize(NamedSize.Medium,
                            FontAttributes.Bold);
                            mainStack.Children.Add(label);
                            gotTitle = true;
                        }
                        else
                        {
                            // Add subsequent labels to textStack.
                            textStack.Children.Add(label);
                        }
                    }
                }
            }
            // Put the textStack in a ScrollView with FillAndExpand.
            ScrollView scrollView = new ScrollView
            {
                Content = textStack,
                VerticalOptions = LayoutOptions.FillAndExpand,
                Padding = new Thickness(5, 0)
            };
            // Add the ScrollView as a second child of mainStack.
            mainStack.Children.Add(scrollView);
            // Set page content to mainStack.

            this.Content = mainStack;
            // White background for ebooks!
            this.BackgroundColor = Color.White;
            // Add some iOS padding for the page
            this.Padding = new Thickness(0, Device.OnPlatform(20, 0, 0), 0, 0);
        }
    }
}