MahApps sample code for issue #1690

App.xaml

<Application x:Class="MahAppsTest.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Colors.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

InfoWindow.xaml

<Window x:Class="MahAppsTest.InfoWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="InfoWindow" Height="300" Width="300">
    <TextBox>asd</TextBox>
</Window>

MainWindow.xaml

<Window x:Class="MahAppsTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Button Click="OpenInfoWindow_OnClick">Open window</Button>
</Window>

MainWindow.xaml.cs

using System.Threading;
using System.Windows;

namespace MahAppsTest
{
    public partial class MainWindow
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    
        private void OpenInfoWindow_OnClick(object sender, RoutedEventArgs e)
        {
            var t = new Thread(() =>
            {
                var window = new InfoWindow();
                window.Show();
                System.Windows.Threading.Dispatcher.Run();
            }) { IsBackground = true };
            t.SetApartmentState(ApartmentState.STA);
            t.Start();
        }
    }
}

I guess your thread should look like this:

var t = new Thread(() =>
    {
    var window = new InfoWindow();
    window.Show();
    System.Windows.Threading.Dispatcher.Run();
}) { IsBackground = true };
t.SetApartmentState(ApartmentState.STA);
t.Start();

See Running WPF Application with Multiple UI Threads

Thanks, I tried that, but still, using MahApps.Metro's ResourceDictionarys it fails. Anyway, I will update the gist.