wpf HwndHost MouseMove event before previous element MouseLeave event

Question

We have a Wpf application with an Hwndhost element we use for rendering 3d content via DirectX. We expose a "Panel" interface to clients which contains a MouseEnter and MouseLeave event. For the Hwndhost "panel" we simulate a MouseEnter event when we first receive a WM_MOUSEMOVE event to the HwndHost.WndProc since win32 does not have a WM_MOUSEENTER event. Also, for the HwndHost we request a WM_MOUSELEAVE event via TrackMouseEvent() which simulates the MouseLeave event on our "panel" interface.

The problem is that when the user moves the mouse very rapidly to, or from the HwndHost panel we receive the enter event from the panel the mouse is moving to before we receive the leave event for the panel the mouse is moving from. This does not happen when both the source and destination panels are both wpf elements, only when the hwndhost element is involved. Also, if there are two of these HwndHost elements next to each other the same behavior occurs, so that it looks like win32 is not sending messages in the order they occur.

Does anyone know of a way around this? It would be nice if there was a way to get Win32 to send events in the order they occur.

Here is a sample application which dimonstrates the problem with print messages to the debugger output window:

// App.xaml.cs
using System;
using System.Windows;
using System.Runtime.InteropServices;

namespace WpfHostingWin32Control
{
    public partial class App : Application
    {
        [DllImport("comctl32.dll", EntryPoint = "InitCommonControls", CharSet =         CharSet.Auto)]
        public static extern void InitCommonControls();
        private void ApplicationStartup(object sender, StartupEventArgs args)
        {
            InitCommonControls();
            HostWindow host = new HostWindow();
            host.InitializeComponent();
        }
    }
}

// App.xaml
<Application x:Class="WpfHostingWin32Control.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    StartupUri="MainWindow.xaml"
    Startup="ApplicationStartup">
</Application>

// HostWindow.xaml.cs
using System;
using System.Windows;
using System.Windows.Input;  

namespace WpfHostingWin32Control
{
    public partial class HostWindow : Window
    {
        WpfHostingWin32Control.ControlHost ctrlHost;

        public HostWindow()
        {
            InitializeComponent();
            txtbox1.MouseEnter += MouseEnterHandler;
            txtbox1.MouseLeave += MouseLeaveHandler;
        }

        void MouseEnterHandler(object sender, MouseEventArgs e)
        {
            System.Diagnostics.Debug.WriteLine("Wpf MouseEnter");
        }

        void MouseLeaveHandler(object sender, MouseEventArgs e)
        {
            System.Diagnostics.Debug.WriteLine("Wpf MouseLeave");
        }

        private void On_UIReady(object sender, EventArgs e)
        {
            ctrlHost = new ControlHost(600, 600);
            ControlHostElement.Child = ctrlHost;
        }
    }
}

// HostWindow.xaml
<Window x:Class="WpfHostingWin32Control.HostWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Name="mainWindow"
    Width="1250"
    Height="700"
    Background="LightGreen" 
    Loaded="On_UIReady">
    <StackPanel Margin="20" Orientation="Horizontal">
        <Border Name="ControlHostElement" Width="600" Height="600"/>
        <TextBox Margin="5,0,0,0" x:Name="txtbox1" Width="600" Height="600"/>
    </StackPanel>
</Window>  

// ControlHost.cs
using System;
using System.Windows;
using System.Windows.Interop;
using System.Runtime.InteropServices;

namespace WpfHostingWin32Control
{
    public class ControlHost : HwndHost
    {
        IntPtr hwndHost;
        int hostHeight, hostWidth;
        bool m_MouseHasEntered;

        public ControlHost(double height, double width)
        {
            hostHeight = (int)height;
            hostWidth = (int)width;
        }

        protected override HandleRef BuildWindowCore(HandleRef hwndParent)
        {
            hwndHost = IntPtr.Zero;
            hwndHost = CreateWindowEx(0, "listbox", "",WS_CHILD | WS_VISIBLE | WS_BORDER, 0, 0, 
                hostWidth, hostHeight, hwndParent.Handle, IntPtr.Zero, IntPtr.Zero, 0);

            return new HandleRef(this, hwndHost);
        }

        protected override IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
        {
            switch (msg)
            {
                case WM_MOUSEMOVE:
                    {
                        if (m_MouseHasEntered == false)
                        {
                            RequestMouesLeaveNotification();
                            m_MouseHasEntered = true;
                            System.Diagnostics.Debug.WriteLine("WM_MOUSEMOVE/ENTER");
                        }
                    }
                    break;

                case WM_MOUSELEAVE:
                    {
                        m_MouseHasEntered = false;
                        System.Diagnostics.Debug.WriteLine("WM_MOUSELEAVE");
                    }
                    break;
            }

            handled = false;
            return IntPtr.Zero;
        }

        private void RequestMouesLeaveNotification()
        {
            var tme = new TRACKMOUSEEVENT()
            {
                cbSize = (uint)Marshal.SizeOf(typeof(TRACKMOUSEEVENT)),
                dwFlags = TME_LEAVE,
                hwndTrack = hwndHost,
                dwHoverTime = 0
            };

            if (!TrackMouseEvent(out tme))
                System.Diagnostics.Debug.WriteLine("TrackMouseEvent failed!");
        }

        protected override void DestroyWindowCore(HandleRef hwnd)
        {
            DestroyWindow(hwnd.Handle);
        }

        //PInvoke declarations
        [DllImport("user32.dll", EntryPoint = "CreateWindowEx", CharSet = CharSet.Unicode)]
        internal static extern IntPtr CreateWindowEx(int dwExStyle,
                                                      string lpszClassName,
                                                      string lpszWindowName,
                                                      int style,
                                                      int x, int y,
                                                      int width, int height,
                                                      IntPtr hwndParent,
                                                      IntPtr hMenu,
                                                      IntPtr hInst,
                                                      [MarshalAs(UnmanagedType.AsAny)] object pvParam);

        [DllImport("user32.dll", EntryPoint = "DestroyWindow", CharSet = CharSet.Unicode)]
        internal static extern bool DestroyWindow(IntPtr hwnd);

        private const int WM_MOUSEMOVE = 0x0200;
        private const int WM_MOUSELEAVE = 0x02A3;
        private const int TME_LEAVE = 0x00000002;
        private const int WS_CHILD = 0x40000000;
        private const int WS_VISIBLE = 0x10000000;
        private const int WS_BORDER = 0x00800000;

        [StructLayout(LayoutKind.Sequential)]
        private struct TRACKMOUSEEVENT
        {
            public uint cbSize;
            public uint dwFlags;
            public IntPtr hwndTrack;
            public uint dwHoverTime;
        }

        [DllImport("user32.dll")]
        private static extern bool TrackMouseEvent(out TRACKMOUSEEVENT tme);
    }
}