Add WndProcs to handle extended frame windows.

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1305,6 +1305,9 @@ namespace Avalonia.Win32.Interop
         [DllImport("dwmapi.dll")]
         public static extern int DwmIsCompositionEnabled(out bool enabled);
 
+        [DllImport("dwmapi.dll")]
+        public static extern bool DwmDefWindowProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam, ref IntPtr plResult);
+        
         [DllImport("dwmapi.dll")]
         public static extern void DwmEnableBlurBehindWindow(IntPtr hwnd, ref DWM_BLURBEHIND blurBehind);
 

src/Windows/Avalonia.Win32/WindowImpl.WndProc.cs

@@ -12,11 +12,164 @@ using static Avalonia.Win32.Interop.UnmanagedMethods;
 
 namespace Avalonia.Win32
 {
+    public partial class WindowImpl
+    {
+        private int LEFTEXTENDWIDTH = 4;
+        private int RIGHTEXTENDWIDTH = 4;
+        private int BOTTOMEXTENDWIDTH = 4;
+        private int TOPEXTENDWIDTH = 100;
+
+        // Hit test the frame for resizing and moving.
+        HitTestValues HitTestNCA(IntPtr hWnd, IntPtr wParam, IntPtr lParam)
+        {
+            // Get the point coordinates for the hit test.
+            var ptMouse = PointFromLParam(lParam);
+
+            // Get the window rectangle.            
+            GetWindowRect(hWnd, out var rcWindow);
+
+            // Get the frame rectangle, adjusted for the style without a caption.
+            RECT rcFrame = new RECT();
+            AdjustWindowRectEx(ref rcFrame, (uint)(WindowStyles.WS_OVERLAPPEDWINDOW & ~WindowStyles.WS_CAPTION), false, 0);
+
+            // Determine if the hit test is for resizing. Default middle (1,1).
+            ushort uRow = 1;
+            ushort uCol = 1;
+            bool fOnResizeBorder = false;
+
+            // Determine if the point is at the top or bottom of the window.
+            if (ptMouse.Y >= rcWindow.top && ptMouse.Y < rcWindow.top + TOPEXTENDWIDTH)
+            {
+                fOnResizeBorder = (ptMouse.Y < (rcWindow.top - rcFrame.top));
+                uRow = 0;
+            }
+            else if (ptMouse.Y < rcWindow.bottom && ptMouse.Y >= rcWindow.bottom - BOTTOMEXTENDWIDTH)
+            {
+                uRow = 2;
+            }
+
+            // Determine if the point is at the left or right of the window.
+            if (ptMouse.X >= rcWindow.left && ptMouse.X < rcWindow.left + LEFTEXTENDWIDTH)
+            {
+                uCol = 0; // left side
+            }
+            else if (ptMouse.X < rcWindow.right && ptMouse.X >= rcWindow.right - RIGHTEXTENDWIDTH)
+            {
+                uCol = 2; // right side
+            }
+
+            // Hit test (HTTOPLEFT, ... HTBOTTOMRIGHT)
+            HitTestValues[][] hitTests = new[]
+            {
+                new []{ HitTestValues.HTTOPLEFT,    fOnResizeBorder ? HitTestValues.HTTOP : HitTestValues.HTCAPTION,    HitTestValues.HTTOPRIGHT },
+                new []{ HitTestValues.HTLEFT,      HitTestValues.HTNOWHERE,     HitTestValues.HTRIGHT },
+                new []{ HitTestValues.HTBOTTOMLEFT, HitTestValues.HTBOTTOM, HitTestValues.HTBOTTOMRIGHT },
+            };
+
+            System.Diagnostics.Debug.WriteLine(hitTests[uRow][uCol]);
+
+            return hitTests[uRow][uCol];
+        }
+
+        protected virtual unsafe IntPtr CustomCaptionProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam, ref bool callDwp)
+        {
+            IntPtr lRet = IntPtr.Zero;
+
+            callDwp = !DwmDefWindowProc(hWnd, msg, wParam, lParam, ref lRet);
+
+            MARGINS margins = new MARGINS { cxLeftWidth = 0, cxRightWidth = 0, cyBottomHeight = 0, cyTopHeight = 100 };
+            RECT border_thickness = new RECT();
+
+            switch ((WindowsMessage)msg)
+            {
+                case WindowsMessage.WM_ACTIVATE:
+                    {
+                        if (GetStyle().HasFlag(WindowStyles.WS_THICKFRAME))
+                        {
+                            AdjustWindowRectEx(ref border_thickness, (uint)(GetStyle()), false, 0);
+                            border_thickness.left *= -1;
+                            border_thickness.top *= -1;
+                        }
+                        else if (GetStyle().HasFlag(WindowStyles.WS_BORDER))
+                        {
+                            border_thickness = new RECT { bottom = 1, left = 1, right = 1, top = 1 };
+                        }
+
+                        LEFTEXTENDWIDTH = border_thickness.left;
+                        RIGHTEXTENDWIDTH = border_thickness.right;
+                        BOTTOMEXTENDWIDTH = border_thickness.bottom;
+
+                        // Extend the frame into the client area.                        
+                        margins.cxLeftWidth = border_thickness.left;
+                        margins.cxRightWidth = border_thickness.right;
+                        margins.cyBottomHeight = border_thickness.bottom;
+                        margins.cyTopHeight = border_thickness.top;
+                         
+                        var hr = DwmExtendFrameIntoClientArea(hWnd, ref margins);
+
+                        //if (hr < 0)
+                        {
+                            // Handle the error.
+                        }
+
+                        lRet = IntPtr.Zero;
+                        callDwp = true;
+                        break;
+                    }
+
+                case WindowsMessage.WM_NCCALCSIZE:
+                    {
+                        if (ToInt32(wParam) == 1)
+                        {
+                            lRet = IntPtr.Zero;
+                            callDwp = false;
+                        }
+                        break;
+                    }
+
+                case WindowsMessage.WM_NCHITTEST:
+                    if (lRet == IntPtr.Zero)
+                    {
+                        lRet = (IntPtr)HitTestNCA(hWnd, wParam, lParam);
+
+                        if (((HitTestValues)lRet) != HitTestValues.HTNOWHERE)
+                        {
+                            callDwp = false;
+                        }
+                    }
+                    break;
+            }
+
+            return lRet;
+        }
+    }
+
+    public partial class WindowImpl
+    {
+        protected virtual unsafe IntPtr WndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
+        {
+            IntPtr lRet = IntPtr.Zero;
+            bool callDwp = true;
+
+            if (DwmIsCompositionEnabled(out bool enabled) == 0)
+            {
+                lRet = CustomCaptionProc(hWnd, msg, wParam, lParam, ref callDwp);
+            }
+
+            if (callDwp)
+            {
+                lRet = AppWndProc(hWnd, msg, wParam, lParam);
+            }
+
+            return lRet;
+        }
+    }
+
     public partial class WindowImpl
     {
         [SuppressMessage("Microsoft.StyleCop.CSharp.NamingRules", "SA1305:FieldNamesMustNotUseHungarianNotation",
             Justification = "Using Win32 naming for consistency.")]
-        protected virtual unsafe IntPtr WndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
+        protected virtual unsafe IntPtr AppWndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
         {
             const double wheelDelta = 120.0;
             uint timestamp = unchecked((uint)GetMessageTime());