Close button flickers on hover and sometimes cannot be clicked of Window

[Koichi-Kobayashi]

Describe the bug

2026-01-15_15h35_06.mp4

The issue can now be consistently reproduced.

When hovering the mouse cursor near the close button (top-right corner) of the window, the button flickers and sometimes becomes unclickable, as shown in the attached video.

To Reproduce

1. Create a new project
2. Select WPF UI – Fluent Navigation template
3. Place the attached XAML and C# files under:
   Views / Windows

TestWindow.zip

4. Add the following method to SettingsViewModel.cs:

[RelayCommand]
private void OnTestWindow()
{
    var testWindow = new Views.Windows.TestWindow()
    {
        Owner = App.Current.MainWindow
    };
    testWindow.ShowDialog();
}

5. Run the application
6. Click the Settings button
7. Click the Test Window button
8. When the window appears, move the mouse cursor near the close button

Expected behavior

• The close button should remain stable on hover
• The button should always be clickable

Screenshots

No response

OS version

Windows 11 Pro

.NET version

.NET 10

WPF-UI NuGet version

4.1.0 - 4.2.0

Additional context

If it helps, I can upload the full project files for investigation.
Please let me know if that would be useful.

UiDesktopApp3.zip

[Koichi-Kobayashi]

I updated the bug report since it reproduced in a new project as well.

[luca-domenichini]

Also happens here as well