Problem description: WPF will initialize pen thread when the window initializing. But I find the window may be locked in PenThreadWorker constructor. The UnsafeNativeMethods.CreateResetEvent will lock the thread.

Actual behavior: I use multithreading to create some windows by this code and maybe the thread be locked in PenThreadWorker constructor. The Demo code

    public partial class App : Application
    {
        /// <inheritdoc />
        protected override void OnStartup(StartupEventArgs e)
        {
            var thread = new Thread(() =>
            {
                var mainWindow = new MainWindow();
                mainWindow.Show();
                Dispatcher.Run();
            });
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
            base.OnStartup(e);
        }
    }

The step

1. Run the code
2. Use the VisualStudio to suspend the application Maybe you can find the application only create a Stylus Input thread and show only a main window and the other main window and the thread be locked in PenThreadWorker constructor and the window can not show Why I need multithreading to show the windows? I need the splash window to show the welcome page and then I should show the main window in other thread to do the bussiness code and I should close the splash window when the main window showed. But I found some users could not show the main window and the main window be lock in PenThreadWorker constructor. I use dnspy to suspend the application and find the code run in PenThreadWorker constructor and the UnsafeNativeMethods.CreateResetEvent will lock the thread.

    PenThreadWorker..ctor()  
    PenThread..ctor()  
    PenThreadPool.GetPenThreadForPenContextHelper(PenContext penContext)  
    PenThreadPool.GetPenThreadForPenContext(PenContext penContext)  
    WispTabletDeviceCollection.UpdateTabletsImpl()  
    WispTabletDeviceCollection.UpdateTablets()  
    WispTabletDeviceCollection..ctor()  
    WispLogic.get_WispTabletDevices()  
    WispLogic.RegisterHwndForInput(InputManager inputManager, PresentationSource inputSource)  
    HwndStylusInputProvider..ctor(HwndSource source)  
    HwndSource.Initialize(HwndSourceParameters parameters)  
    HwndSource..ctor(HwndSourceParameters parameters)  
    PresentationFramework.dll!System.Windows.Window.CreateSourceWindow(bool duringShow)  
    PresentationFramework.dll!System.Windows.Window.CreateSourceWindowDuringShow()  
    PresentationFramework.dll!System.Windows.Window.ShowHelper(object booleanBox)  
   

   Expected behavior: The UnsafeNativeMethods.CreateResetEvent do not lock the thread

Minimal repro:

Create a empty WPF application and then change the app.xaml.cs code

    public partial class App : Application
    {
        /// <inheritdoc />
        protected override void OnStartup(StartupEventArgs e)
        {
            var thread = new Thread(() =>
            {
                var mainWindow = new MainWindow();
                mainWindow.Show();
                Dispatcher.Run();
            });
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
            base.OnStartup(e);
        }
    }

Run the code and maybe you can find one of the main window can not show and it stop in UnsafeNativeMethods.CreateResetEvent and I wait for a hour but it can not run.