ScrollView.Content is not re-layouted on Android

[rodja]

Description

When changing Views inside of a ScrollView they are not updated properly on Android.

Steps to Reproduce

This example is a good way to reproduce the issue. Swipe in the menu to switch to the broken pages:

public class App : Application {
    public App() {
        MainPage = new MasterDetailPage {
            Master = new ContentPage {
                Title = "Menu",
                Content = new StackLayout {
                    Children = {
                        new Button { Text = "Broken ScrollView", Command = new Command(obj => Open(new ScrollPage())) },
                        new Button { Text = "Fixed Label on ScrollView", Command = new Command(obj => Open(new FixedLabelOnScrollPage())) },
                        new Button { Text = "Fully Fixed ScrollView", Command = new Command(obj => Open(new FixedScrollPage())) },
                    },
                },
            },
            Detail = new FixedScrollPage(),
        };
    }

    void Open(ContentPage page) {
        (MainPage as MasterDetailPage).Detail = page;
        (MainPage as MasterDetailPage).IsPresented = false;
    }
}

public class FixedScrollPage : ScrollPage {
    protected override async void OnAppearing() {
        await Task.Delay(1);
        var content = ScrollContent;
        ScrollContent = null;
        ScrollContent = content;
        base.OnAppearing();
    }
}

public class FixedLabelOnScrollPage : ScrollPage {
    public FixedLabelOnScrollPage() {
        Label.HorizontalOptions = LayoutOptions.Fill;
    }

}

public class ScrollPage : ContentPage {
    Timer timer = new Timer(1000);
    protected Label Label;

    public ScrollPage() {
        Content = new ScrollView {
            Content = new StackLayout {
                Children = {
                    (Label = new Label {
                        Text = "this text should disappear after 1 sec",
                        BackgroundColor = Color.LightBlue,
                        HorizontalOptions = LayoutOptions.StartAndExpand,
                    })
                }
            }
        };
    }

    protected StackLayout ScrollContent {
        get => (Content as ScrollView).Content as StackLayout;
        set => (Content as ScrollView).Content = value;
    }

    protected override void OnAppearing() {
        base.OnAppearing();
        timer.Elapsed += (s, e) => Device.BeginInvokeOnMainThread(OnTimerElapsed);

        timer.Start();
    }

    void OnTimerElapsed() {
        Label.Text = $"{ DateTime.Now.ToString() }: expecting {ScrollContent?.Children.Count} dates to show up.";
        ScrollContent.Children.Add(new Label { Text = DateTime.Now.ToString() });
    }
}

Expected Behavior

When changing Label.Text or adding children to an StackLayout inside of an ScrollView we should see it.

Actual Behavior

The new content is not visible.

Basic Information

• Version with issue: Xamrin.Forms 3.0.0
• Last known good version: ?
• IDE: Visual Studio 7.3.3
• Platform Target Frameworks:
  • Android: 8.0
• Android Support Library Version: 25.4
• Affected Devices: all

[hartez]

This may be a duplicate of #1760.