Linux - Invalid Window.ClientSize on startup

Issue

On Linux Window.ClientSize is invalid on startup only. This is the root cause of WindowStartupLocation = Owner/CenterScreen is not working on Linux (Related #3454).

Repro:

XAML

<Window
    x:Class="AvaloniaApplication1.Views.MainWindow"
    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"
    xmlns:vm="clr-namespace:AvaloniaApplication1.ViewModels;assembly=AvaloniaApplication1"
    Title="AvaloniaApplication1"
    Width="450"
    Height="450"
    Icon="/Assets/avalonia-logo.ico"
    mc:Ignorable="d">
    <Grid>
      <StackPanel>
        <Button Width="200" Height="100" Name="BtnClick" Content="Refresh" />
        <TextBlock Name="Txtblock"/>
      </StackPanel>
    </Grid>
</Window>

In Code

        public MainWindow()
        {
            InitializeComponent();
#if DEBUG
            this.AttachDevTools();
#endif
            this.Opened += MainWindow_Opened;
        }

        private void MainWindow_Opened(object sender, System.EventArgs e)
        {
            var txt = this.Find<TextBlock>("Txtblock");
            var sb = new StringBuilder();
            sb.AppendLine($"ClientSize.Width:{ClientSize.Width}");
            sb.AppendLine($"ClientSize.Height:{ClientSize.Height}");
            sb.AppendLine($"this.Width:{Width}");
            sb.AppendLine($"this.Height:{Height}");
            sb.AppendLine();
            txt.Text = sb.ToString();
        }

         public void Button_Clicked(object sender, RoutedEventArgs e)
        {
            MainWindow_Opened(null, null);
        }

Opened Event - Result (Linux vs Windows):

After clicking refresh - Result (Linux only)

[kekekeks]

For now I think we should do

update the client size immediately if the window has never been onscreen before

hack to mitigate autosizing issues for most X11 systems.

[Symbai]

This issue also affects SizeToContent to not work correctly. Both makes simulating a messagebox really annoying on Linux. It always shows up at the top left corner and needs to be resized by the user.

Is there a quick workaround possible? Perhaps waiting for X11 to be finished and then resize / apply startup location?!