Why can't ScrollViewer use Offset to set the vertical scroll value for VirtualizingStackPanel

[kyuranger]

Here is my code:

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"			
			 xmlns:Local="using:Sample"			
			 xmlns:System="clr-namespace:System;assembly=System.Runtime"
			 x:DataType="Local:MainView2"		 
             x:Class="Sample.MainView2">
	<ScrollViewer BorderBrush="Gray" BorderThickness="1" Name="SV">
		<ItemsControl ItemsSource="{Binding CurrentMiddlewareOC}">
			<ItemsControl.ItemsPanel>
				<ItemsPanelTemplate>
					<VirtualizingStackPanel></VirtualizingStackPanel>
				</ItemsPanelTemplate>
			</ItemsControl.ItemsPanel>
			<ItemsControl.ItemTemplate>
				<DataTemplate x:DataType="System:Int32">
					<Border Height="500" Width="150" Background="Red" Margin="0,0,0,30">
						<ContentPresenter Content="{Binding }"></ContentPresenter>
					</Border>
				</DataTemplate>
			</ItemsControl.ItemTemplate>
		</ItemsControl>
	</ScrollViewer>
</UserControl>

Code-behind:

using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.Notifications;
using Avalonia.Input;
using Avalonia.Markup.Xaml;
using Avalonia.Media;
using Avalonia.Threading;
using System;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Drawing;
using System.Linq;
using System.Threading.Tasks;

namespace Sample;

public partial class MainView2 : UserControl
{
    public MainView2()
    {
        InitializeComponent();
        this.DataContext = this;
        SV.Offset = new Vector(0, 2500);
    }
    public ObservableCollection<int> CurrentMiddlewareOC { get; set; } = [1,2,3,4,5,6,7,8,0,10];
}

After the code runs, it always scrolls to the top but not the position I set. When I replace the VirtualizingStackPanel with StackPanel, it will scroll the position I set.

What's wrong with my code?

[msneijders]

I suspect inside MainView2 constructor the VirtualizingStackPanel is not yet fully initialized; try setting SV.Offset = new Vector(0, 2500); in MainView2.OnLoaded for example.

[kyuranger]

This doesn't work, even if I use a Task to create a delayed trigger, it doesn't work either.

[msneijders]

This is what I tested, based on default avalonia project template upgraded to avalonia 11.2.3:

VirtualizingStackPanelOffset.zip

I see the same scroll offset using StackPanel and VirtualizingStackPanel (have not checked if it actually is 2500, but looks like so).

[kyuranger]

I found the problem. I used:

this.Loaded += (sender, e) => {
SV.Offset = new Vector(0, 2500);
};

while you used

protected override void OnLoaded(RoutedEventArgs e)
{
base.OnLoaded(e);
SV.Offset = new Vector(0, 2500);
}

I always thought they were the same, but now it turns out that your method works, but mine doesn't.

[msneijders]

I would not have expected a difference in behavior. Are you really sure? I just changed the test project to Loaded += eventhandler, and I see the same behavior as using OnLoaded.