How to implement an attached header view flowing on top of scrollable content in Xamarin.Forms

DoubleToTopThicknessConverter.cs

public class DoubleToTopThicknessConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        double height;

        try
        {
            height = (double)value;
        }
        catch
        {
            height = 0;
        }

        var topThickness = new Thickness
        { 
            Top = height
        };

        return topThickness;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return 0d;
    }
}

Page.xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage 
    xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    xmlns:converters="clr-namespace:Foo.Converters" 
    x:Class="Foo.Page">
    <ContentPage.Resources>
        <ResourceDictionary>
            <converters:DoubleToTopThicknessConverter x:Key="DoubleToTopThicknessConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>
    <ContentPage.Content>
        <AbsoluteLayout>
            <ScrollView
                AbsoluteLayout.LayoutFlags="All"
                AbsoluteLayout.LayoutBounds="0, 0, 1, 1"
                Padding="{Binding Source={x:Reference HeaderStackLayout}, Path=Height, Converter={StaticResource DoubleToTopThicknessConverter}}">
                <!-- ... -->
            </ScrollView>

            <StackLayout x:Name="HeaderStackLayout"
                AbsoluteLayout.LayoutFlags="PositionProportional"
                AbsoluteLayout.LayoutBounds="0, 0, AutoSize, AutoSize">
                <!-- ... -->
            </StackLayout>
        </AbsoluteLayout>
    </ContentPage.Content>
</ContentPage>