TeachingTip shows an addional white rectangle popup on Linux

[Genteure]

Describe the bug

1. Add <PropertyGroup><AvaloniaVersion>11.2.0</AvaloniaVersion></PropertyGroup> to FAControlsGallery.Desktop.csproj
2. dotnet run --project samples/FAControlsGallery.Desktop/FAControlsGallery.Desktop.csproj
3. Open the page for TeachingTip and click button.

Screenshots

Desktop/Platform (please complete the following information):

• OS: Ubuntu 24.04 (GNOME, Wayland)
• FluentAvalonia Version: latest (or 2.2.0), commit 3ef356a
• Avalonia Version: 11.2.0 and up

Additional context

300, 200 seems to be coming from Avalonia.X11/X11Window.cs. This bug seems to be caused by commit 1cfa82ca in Avalonia first released in 11.2.0-beta1.

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Old incorrect bug description for archival purpose

Describe the bug

1. Change TargetFramework of FAControlsGallery.Desktop.csproj from net8.0 to net9.0
2. dotnet run --project samples/FAControlsGallery.Desktop/FAControlsGallery.Desktop.csproj
3. Open the page for TeachingTip and click button.

Screenshots

Desktop/Platform (please complete the following information):

• OS: Ubuntu 24.04 (GNOME, Wayland)
• FluentAvalonia Version: latest (or 2.2.0), commit 3ef356a
• Avalonia Version: tested on 11.1.0 and 11.2.2

Additional context
Only reproduced on dotnet 9. It works as intended on dotnet 8.

• On dotnet 8: _lightDismissIndicatorPopup.Child.Bounds = {0, 0, 0, 0}
• On dotnet 9: _lightDismissIndicatorPopup.Child.Bounds = {0, 0, 300, 200}
• On both: Width and Height are always NaN, DesiredSize is always {0, 0}

300, 200 seems to be coming from Avalonia.X11/X11Window.cs but I'm not sure. Also I still have no idea why it behaves differently on dotnet 8 and 9.

[Genteure]

I tested this in a new Avalonia 11.1.0 project without FluentAvalonia and the Bounds of the popup is the intended 0,0,0,0.

Edit: Everything below is wrong. Still investigating.

It's definitely related to PlacementTarget and AppWindow. When I change the PlacementTarget to this (TeachingTip) or null, it works correctly without explicitly setting the Panel size to 0x0.

FluentAvalonia/src/FluentAvalonia/UI/Controls/TeachingTip/TeachingTip.cs

Line 242 in 3ef356a

PlacementTarget = (Control)VisualRoot

Converting #635 to draft for now. I suspect the real root cause is in AppWindow and I prefer to fix that instead. Otherwise I feel like this kind of bug will appear again elsewhere and stab you in the back when you least expect it 😂

[Genteure]

Turns out it has nothing to do with .NET versions, not sure how did I think it's related to dotnet versions. I have updated the issue description. #635 should be a reasonable fix.