prototype, allowing to draw in non-client area.

without breaking win32 frame functionality.

samples/ControlCatalog/MainWindow.xaml

@@ -1,6 +1,8 @@
 <Window xmlns="https://github.com/avaloniaui" MinWidth="500" MinHeight="300"
         Title="Avalonia Control Gallery"
         Icon="resm:ControlCatalog.Assets.test_icon.ico?assembly=ControlCatalog"
-        xmlns:local="clr-namespace:ControlCatalog">
-    <local:MainView/>
-</Window>
+        xmlns:local="clr-namespace:ControlCatalog" Background="Transparent">
+    <Grid HorizontalAlignment="Left" Width="200" Background="Red">
+        <TextBlock Text="Drawing in Non-client Area Win32" />
+    </Grid>
+ </Window>

src/Avalonia.Visuals/Rect.cs

@@ -120,6 +120,8 @@ namespace Avalonia
         /// </summary>
         public Size Size => new Size(_width, _height);
 
+        public double Left => _x;
+
         /// <summary>
         /// Gets the right position of the rectangle.
         /// </summary>
@@ -130,6 +132,11 @@ namespace Avalonia
         /// </summary>
         public double Bottom => _y + _height;
 
+        /// <summary>
+        /// Gets the top position of the rectangle.
+        /// </summary>
+        public double Top => _y;
+
         /// <summary>
         /// Gets the top left point of the rectangle.
         /// </summary>

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

@@ -907,6 +907,18 @@ namespace Avalonia.Win32.Interop
         [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Unicode, EntryPoint = "SetWindowTextW")]
         public static extern bool SetWindowText(IntPtr hwnd, string lpString);
 
+        [StructLayout(LayoutKind.Sequential)]
+        public struct MARGINS
+        {
+            public int leftWidth;
+            public int rightWidth;
+            public int topHeight;
+            public int bottomHeight;
+        }
+
+        [DllImport("dwmapi.dll")]
+        public static extern int DwmExtendFrameIntoClientArea(IntPtr hwnd, ref MARGINS margins);
+
         public enum ClassLongIndex : int
         {
             GCL_HCURSOR = -12,

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -455,6 +455,54 @@ namespace Avalonia.Win32
                 IntPtr.Zero);
         }
 
+
+        protected int BorderTop { get; private set; }
+        protected int BorderLeft { get; private set; }
+        protected int BorderRight { get; private set; }
+        protected int BorderBottom { get; private set; }
+
+        private HitTestValues NCHitText(uint m, IntPtr LParam, IntPtr wParam)
+        {
+            int lParam = (int)LParam;
+            Point pt = new Point(lParam & 0xffff, lParam >> 16);
+
+            var rc = new Rect(Position, ClientSize);
+
+            int row = 1;
+            int col = 1;
+            bool onResizeBorder = false;
+
+            // Determine if we are on the top or bottom border
+            if (pt.Y >= rc.Top && pt.Y < rc.Top + BorderTop)
+            {
+                onResizeBorder = pt.Y < (rc.Top + BorderBottom);
+                row = 0;
+            }
+            else if (pt.Y < rc.Bottom && pt.Y > rc.Bottom - BorderBottom)
+            {
+                row = 2;
+            }
+
+            // Determine if we are on the left border or the right border
+            if (pt.X >= rc.Left && pt.X < rc.Left + BorderLeft)
+            {
+                col = 0;
+            }
+            else if (pt.X < rc.Right && pt.X >= rc.Right - BorderRight)
+            {
+                col = 2;
+            }
+
+            HitTestValues[,] hitTests = new HitTestValues[,]
+            {
+                {HitTestValues.HTTOPLEFT, onResizeBorder ? HitTestValues.HTTOP : HitTestValues.HTCAPTION, HitTestValues.HTTOPRIGHT},
+                {HitTestValues.HTLEFT, HitTestValues.HTNOWHERE, HitTestValues.HTRIGHT},
+                {HitTestValues.HTBOTTOMLEFT, HitTestValues.HTBOTTOM, HitTestValues.HTBOTTOMRIGHT}
+            };
+
+            return hitTests[row, col];
+        }
+
         [SuppressMessage("Microsoft.StyleCop.CSharp.NamingRules", "SA1305:FieldNamesMustNotUseHungarianNotation", Justification = "Using Win32 naming for consistency.")]
         protected virtual IntPtr WndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
         {
@@ -465,6 +513,8 @@ namespace Avalonia.Win32
 
             RawInputEventArgs e = null;
 
+            bool callDWP = false;
+
             WindowsMouseDevice.Instance.CurrentWindow = this;
 
             switch ((UnmanagedMethods.WindowsMessage)msg)
@@ -484,8 +534,54 @@ namespace Avalonia.Win32
                             break;
                     }
 
+                    MARGINS margins = new MARGINS();
+
+                    margins.leftWidth = Math.Abs(BorderLeft);
+                    margins.rightWidth = Math.Abs(BorderRight);
+                    margins.bottomHeight = Math.Abs(BorderBottom);
+                    margins.topHeight = Math.Abs(BorderTop);
+
+                   // int hr = UnmanagedMethods.DwmExtendFrameIntoClientArea(hWnd, ref margins);
+
                     return IntPtr.Zero;
 
+                case WindowsMessage.WM_NCCALCSIZE:
+                    return IntPtr.Zero;
+                //break;
+
+                case WindowsMessage.WM_CREATE:
+                    RECT rcClient;
+                    UnmanagedMethods.GetWindowRect(hWnd, out rcClient);
+
+
+                    RECT rc = new RECT();
+                    uint style = UnmanagedMethods.GetWindowLong(hWnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE);
+                    uint styleEx = UnmanagedMethods.GetWindowLong(hWnd, (int)UnmanagedMethods.WindowLongParam.GWL_EXSTYLE);
+                    UnmanagedMethods.AdjustWindowRectEx(ref rc, style, false, styleEx);
+
+                    BorderTop = Math.Abs(rc.top);
+                    BorderLeft = Math.Abs(0);
+                    BorderRight = Math.Abs(0);
+                    BorderBottom = Math.Abs(0);
+                    break;
+
+                case WindowsMessage.WM_NCHITTEST:
+                    var ht = NCHitText(msg, lParam, wParam);
+
+
+                    callDWP = (ht == HitTestValues.HTNOWHERE);
+
+                    if (callDWP)
+                    {
+
+                    }
+                    else
+                    {
+                        return new IntPtr((int)ht);
+                    }
+                    break;
+
+
                 case UnmanagedMethods.WindowsMessage.WM_CLOSE:
                     bool? preventClosing = Closing?.Invoke();
                     if (preventClosing == true)