Final, final fix that preserves shadow and aero rectangle.

WpfApp1/NativeMethods.cs

@@ -1,35 +1,61 @@
 using System;
 using System.Runtime.InteropServices;
-using System.Windows;
-using System.Windows.Interop;
 
 namespace WpfApp1
 {
     public static class NativeMethods
     {
-        [DllImport("user32.dll", EntryPoint = "SetWindowLong")]
-        public static extern int SetWindowLong32(HandleRef hWnd, WindowLongFlags nIndex, int dwNewLong);
+        public const int WM_NCCALCSIZE = 0x83;
+        public const int WM_NCPAINT = 0x85;
 
-        [DllImport("user32.dll", EntryPoint = "SetWindowLongPtr")]
-        public static extern IntPtr SetWindowLongPtr64(HandleRef hWnd, WindowLongFlags nIndex, IntPtr dwNewLong);
+        [DllImport("kernel32", SetLastError = true)]
+        private static extern IntPtr LoadLibrary(string lpFileName);
 
-        public static IntPtr SetWindowLongPtr(HandleRef hWnd, WindowLongFlags nIndex, IntPtr dwNewLong)
+        [DllImport("dwmapi.dll", PreserveSig = false)]
+        public static extern bool DwmIsCompositionEnabled();
+
+        [DllImport("kernel32", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
+        private static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
+
+        [StructLayout(LayoutKind.Sequential)]
+        public struct MARGINS
+        {
+            public int leftWidth;
+            public int rightWidth;
+            public int topHeight;
+            public int bottomHeight;
+        }
+
+        private delegate int DwmExtendFrameIntoClientAreaDelegate(IntPtr hwnd, ref MARGINS margins);
+
+        public static int DwmExtendFrameIntoClientArea(IntPtr hwnd, ref MARGINS margins)
         {
-            return IntPtr.Size == 8 ? SetWindowLongPtr64(hWnd, nIndex, dwNewLong) : new IntPtr(SetWindowLong32(hWnd, nIndex, dwNewLong.ToInt32()));
+            var hModule = LoadLibrary("dwmapi");
+
+            if (hModule == IntPtr.Zero)
+            {
+                return 0;
+            }
+
+            var procAddress = GetProcAddress(hModule, "DwmExtendFrameIntoClientArea");
+
+            if (procAddress == IntPtr.Zero)
+            {
+                return 0;
+            }
+
+            var delegateForFunctionPointer = (DwmExtendFrameIntoClientAreaDelegate)Marshal.GetDelegateForFunctionPointer(procAddress, typeof(DwmExtendFrameIntoClientAreaDelegate));
+
+            return delegateForFunctionPointer(hwnd, ref margins);
         }
 
-        public enum WindowLongFlags
+        public static bool IsDwmAvailable()
         {
-            GWL_EXSTYLE = -20,
-            GWLP_HINSTANCE = -6,
-            GWLP_HWNDPARENT = -8,
-            GWL_ID = -12,
-            GWL_STYLE = -16,
-            GWL_USERDATA = -21,
-            GWL_WNDPROC = -4,
-            DWLP_USER = 0x8,
-            DWLP_MSGRESULT = 0x0,
-            DWLP_DLGPROC = 0x4
+            if (LoadLibrary("dwmapi") == IntPtr.Zero)
+            {
+                return false;
+            }
+            return true;
         }
 
         internal enum WVR

WpfApp1/WindowChromeLoadedBehavior.cs

@@ -11,6 +11,8 @@ namespace WpfApp1
 {
     public class WindowChromeLoadedBehavior : Behavior<FrameworkElement>
     {
+        private Window window;
+
         protected override void OnAttached()
         {
             base.OnAttached();
@@ -25,7 +27,7 @@ protected override void OnDetaching()
 
         private void OnLoaded(object sender, RoutedEventArgs e)
         {
-            var window = Window.GetWindow(AssociatedObject);
+            window = Window.GetWindow(AssociatedObject);
 
             if (window == null) return;
 
@@ -59,29 +61,50 @@ private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref b
         {
             switch (msg)
             {
-                // WM_NCCALCSIZE
-                case 0x83:
+                case NativeMethods.WM_NCPAINT:
+                    RemoveFrame();
+                    handled = false;
+                    break;
+
+                case NativeMethods.WM_NCCALCSIZE:
+
+                    handled = false;
+
                     var rcClientArea = (RECT)Marshal.PtrToStructure(lParam, typeof(RECT));
-
                     rcClientArea.Bottom += (int)(WindowChromeHelper.WindowResizeBorderThickness.Bottom / 2);
-
                     Marshal.StructureToPtr(rcClientArea, lParam, false);
-
-                    handled = true;
-
+
                     var retVal = IntPtr.Zero;
-
                     if (wParam == new IntPtr(1))
                     {
                         retVal = new IntPtr((int)NativeMethods.WVR.REDRAW);
                     }
-
                     return retVal;
             }
 
             return IntPtr.Zero;
         }
 
+        private void RemoveFrame()
+        {
+            if (Environment.OSVersion.Version.Major >= 6 && NativeMethods.IsDwmAvailable())
+            {
+                if (NativeMethods.DwmIsCompositionEnabled() && SystemParameters.DropShadow)
+                {
+                    NativeMethods.MARGINS margins;
+
+                    margins.bottomHeight = -1;
+                    margins.leftWidth = 0;
+                    margins.rightWidth = 0;
+                    margins.topHeight = 0;
+
+                    var helper = new WindowInteropHelper(window);
+
+                    NativeMethods.DwmExtendFrameIntoClientArea(helper.Handle, ref margins);
+                }
+            }
+        }
+
         [Serializable]
         [StructLayout(LayoutKind.Sequential)]
         public struct RECT