LANCommander/LANCommander.Launcher/Views/AlertOverlay.axaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:controls="using:LANCommander.Launcher.Controls"
             x:Class="LANCommander.Launcher.Views.AlertOverlay"
             controls:AutoFocus.IsEnabled="True">

    <!-- Full-window dim backdrop -->
    <Grid Background="#AA000000">

        <!-- Centered card -->
        <Border Background="{DynamicResource SystemControlBackgroundChromeMediumBrush}"
                Width="420"
                MaxHeight="300"
                HorizontalAlignment="Center"
                VerticalAlignment="Center"
                BoxShadow="0 8 40 8 #88000000">

            <StackPanel Spacing="0">

                <!-- Header -->
                <Border Padding="20,16,20,8">
                    <TextBlock x:Name="TitleText"
                               Text="Error"
                               FontSize="15" FontWeight="SemiBold" />
                </Border>

                <!-- Message -->
                <Border Padding="20,4,20,0">
                    <ScrollViewer MaxHeight="160"
                                  HorizontalScrollBarVisibility="Disabled"
                                  VerticalScrollBarVisibility="Auto">
                        <TextBlock x:Name="MessageText"
                                   TextWrapping="Wrap"
                                   FontSize="13"
                                   Opacity="0.85" />
                    </ScrollViewer>
                </Border>

                <!-- Footer -->
                <Border Padding="20,16,20,16" Classes="DialogFooter">
                    <Button Content="OK"
                            Classes="Primary"
                            HorizontalAlignment="Right"
                            Click="OK_Click" />
                </Border>

            </StackPanel>
        </Border>
    </Grid>
</UserControl>