LanMountainDesktop/LanMountainDesktop.Launcher/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:sty="using:FluentAvalonia.Styling"
             x:Class="LanMountainDesktop.Launcher.App"
             RequestedThemeVariant="Default">
  <Application.Styles>
    <sty:FluentAvaloniaTheme />
    <Style Selector="Window">
      <Setter Property="Topmost" Value="True" />
    </Style>
  </Application.Styles>
</Application>