Empty popup behavior changed in 11.2.0

[Genteure]

Describe the bug

<Popup Name="MyPopup">
    <Panel Background="LightBlue" />
</Popup>

This Popup shows nothing in Avalonia version < 11.2.0 (for example, 11.1.5).
It shows a 300x200 rectangle in Avalonia version >= 11.2.0. It seems to always at the top left of the screen regardless of PlacementTarget.

To Reproduce

https://github.com/Genteure/avalonia-linux-empty-popup

Or if the repository is deleted in the future:

Repro steps

Run dotnet new avalonia.app (Avalonia.Templates version 11.2.1.1 but this shouldn't matter too much)

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
    <Nullable>enable</Nullable>
    <BuiltInComInteropSupport>true</BuiltInComInteropSupport>
    <ApplicationManifest>app.manifest</ApplicationManifest>
    <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
  </PropertyGroup>

  <PropertyGroup>
    <AvaloniaVersion Condition="'$(AvaloniaVersion)' == ''">11.2.0</AvaloniaVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Avalonia" Version="$(AvaloniaVersion)" />
    <PackageReference Include="Avalonia.Desktop" Version="$(AvaloniaVersion)" />
    <PackageReference Include="Avalonia.Themes.Fluent" Version="$(AvaloniaVersion)" />
    <PackageReference Include="Avalonia.Fonts.Inter" Version="$(AvaloniaVersion)" />
    <!--Condition below is needed to remove Avalonia.Diagnostics package from build output in Release configuration.-->
    <PackageReference Include="Avalonia.Diagnostics" Version="$(AvaloniaVersion)">
      <IncludeAssets Condition="'$(Configuration)' != 'Debug'">None</IncludeAssets>
      <PrivateAssets Condition="'$(Configuration)' != 'Debug'">All</PrivateAssets>
    </PackageReference>
  </ItemGroup>
</Project>

<Window ......>
    <StackPanel Margin="5">
        <TextBlock Name="RuntimeInfo" />
        <Button Click="TogglePopup" Content="Toggle Popup" />
        <TextBlock Name="PopupIsOpenStatus" />
        <Popup Name="MyPopup">
            <Panel Background="LightBlue" />
        </Popup>
    </StackPanel>
</Window>

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        var currentDotnetVersion = Environment.Version;
        var currentAvaloniaVersion = typeof(Button).Assembly.GetName().Version;
        RuntimeInfo.Text = $"Running on .NET {currentDotnetVersion} and Avalonia {currentAvaloniaVersion}";
    }

    private void TogglePopup(object sender, RoutedEventArgs e)
    {
        MyPopup.IsOpen ^= true;

        var panel = (Panel)MyPopup.Child!;
        PopupIsOpenStatus.Text = $"Popup is currently {(MyPopup.IsOpen ? "open" : "closed")}\nPopup.PlacementTarget: {MyPopup.PlacementTarget}\n" +
            $"\nHeight: {panel.Height}\nWidth: {panel.Width}" +
            $"\nDesiredSize: {panel.DesiredSize}\nBounds: {panel.Bounds}";
    }
}

dotnet run -f net8.0 -p:AvaloniaVersion=11.2.0
dotnet run -f net8.0 -p:AvaloniaVersion=11.1.5

Expected behavior

I have not tested this in other OS but the behavior should match.

Avalonia version

11.2.0

OS

Linux

Additional context

amwx/FluentAvalonia#634

300, 200 seems to be coming from Avalonia.X11/X11Window.cs.

PopupRoot got two commits in 11.2.0: 1cfa82 and b27228.

[Genteure]

I've heard this also happens on macOS but the popup window size is a lot bigger. I don't own any Apple devices so I can't test myself.