add client area extending api to window.

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -94,5 +94,11 @@ namespace Avalonia.Platform
         /// </summary>
         /// 
         void SetMinMaxSize(Size minSize, Size maxSize);
+
+        bool ExtendClientAreaToDecorationsHint { get; set; }
+
+        Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
+        
+        Thickness ExtendedMargins { get; }
     }
 }

src/Avalonia.Controls/Window.cs

@@ -71,6 +71,8 @@ namespace Avalonia.Controls
     {
         private readonly List<(Window child, bool isDialog)> _children = new List<(Window, bool)>();
 
+        private bool _isExtendedIntoWindowDecorations;
+
         /// <summary>
         /// Defines the <see cref="SizeToContent"/> property.
         /// </summary>
@@ -87,6 +89,29 @@ namespace Avalonia.Controls
                 o => o.HasSystemDecorations,
                 (o, v) => o.HasSystemDecorations = v);
 
+        /// <summary>
+        /// Defines the <see cref="ExtendClientAreaToDecorationsHint"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> ExtendClientAreaToDecorationsHintProperty =
+            AvaloniaProperty.Register<Window, bool>(nameof(ExtendClientAreaToDecorationsHint), false);
+
+
+        /// <summary>
+        /// Defines the <see cref="IsExtendedIntoWindowDecorations"/> property.
+        /// </summary>
+        public static readonly DirectProperty<Window, bool> IsExtendedIntoWindowDecorationsProperty =
+            AvaloniaProperty.RegisterDirect<Window, bool>(nameof(IsExtendedIntoWindowDecorations),
+                o => o.IsExtendedIntoWindowDecorations,
+                unsetValue: false);
+
+        /// <summary>
+        /// Defines the <see cref="WindowDecorationMargins"/> property.
+        /// </summary>
+        public static readonly DirectProperty<Window, Thickness> WindowDecorationMarginsProperty =
+            AvaloniaProperty.RegisterDirect<Window, Thickness>(nameof(WindowDecorationMargins),
+                o => o.WindowDecorationMargins);
+        
+
         /// <summary>
         /// Defines the <see cref="SystemDecorations"/> property.
         /// </summary>
@@ -164,6 +189,9 @@ namespace Avalonia.Controls
             WindowStateProperty.Changed.AddClassHandler<Window>(
                 (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.WindowState = (WindowState)e.NewValue; });
 
+            ExtendClientAreaToDecorationsHintProperty.Changed.AddClassHandler<Window>(
+                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.ExtendClientAreaToDecorationsHint = (bool)e.NewValue; });
+
             MinWidthProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size((double)e.NewValue, w.MinHeight), new Size(w.MaxWidth, w.MaxHeight)));
             MinHeightProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, (double)e.NewValue), new Size(w.MaxWidth, w.MaxHeight)));
             MaxWidthProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size((double)e.NewValue, w.MaxHeight)));
@@ -189,6 +217,7 @@ namespace Avalonia.Controls
             impl.GotInputWhenDisabled = OnGotInputWhenDisabled;
             impl.WindowStateChanged = HandleWindowStateChanged;
             _maxPlatformClientSize = PlatformImpl?.MaxAutoSizeHint ?? default(Size);
+            impl.ExtendClientAreaToDecorationsChanged = ExtendClientAreaToDecorationsChanged;            
             this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl?.Resize(x));
 
             PlatformImpl?.ShowTaskbarIcon(ShowInTaskbar);
@@ -237,6 +266,32 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Gets or sets if the ClientArea is Extended into the Window Decorations (chrome or border).
+        /// </summary>
+        public bool ExtendClientAreaToDecorationsHint
+        {
+            get { return GetValue(ExtendClientAreaToDecorationsHintProperty); }
+            set { SetValue(ExtendClientAreaToDecorationsHintProperty, value); }
+        }
+
+        /// <summary>
+        /// Gets if the ClientArea is Extended into the Window Decorations.
+        /// </summary>
+        public bool IsExtendedIntoWindowDecorations
+        {
+            get => _isExtendedIntoWindowDecorations;
+            private set => SetAndRaise(IsExtendedIntoWindowDecorationsProperty, ref _isExtendedIntoWindowDecorations, value);
+        }
+
+        private Thickness _windowDecorationMargins;
+
+        public Thickness WindowDecorationMargins
+        {
+            get => _windowDecorationMargins;
+            private set => SetAndRaise(WindowDecorationMarginsProperty, ref _windowDecorationMargins, value);
+        }
+
         /// <summary>
         /// Sets the system decorations (title bar, border, etc)
         /// </summary>
@@ -435,6 +490,13 @@ namespace Avalonia.Controls
             }
         }
 
+        protected virtual void ExtendClientAreaToDecorationsChanged(bool isExtended)
+        {
+            IsExtendedIntoWindowDecorations = isExtended;
+
+            WindowDecorationMargins = PlatformImpl.ExtendedMargins;
+        }
+
         /// <summary>
         /// Hides the window but does not close it.
         /// </summary>

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -84,6 +84,12 @@ namespace Avalonia.DesignerSupport.Remote
 
         public IScreenImpl Screen { get; } = new ScreenStub();
         public Action GotInputWhenDisabled { get; set; }
+        
+        public bool ExtendClientAreaToDecorationsHint { get; set; }
+        
+        public Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
+
+        public Thickness ExtendedMargins { get; } = new Thickness();
 
         public void Activate()
         {

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -39,6 +39,12 @@ namespace Avalonia.DesignerSupport.Remote
 
         public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
 
+        public bool ExtendClientAreaToDecorationsHint { get; set; }
+
+        public Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
+
+        public Thickness ExtendedMargins { get; } = new Thickness();
+
         public WindowStub(IWindowImpl parent = null)
         {
             if (parent != null)

src/Avalonia.Native/WindowImpl.cs

@@ -98,6 +98,12 @@ namespace Avalonia.Native
 
         public Action<WindowState> WindowStateChanged { get; set; }
 
+        public bool ExtendClientAreaToDecorationsHint { get; set; }
+
+        public Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
+
+        public Thickness ExtendedMargins { get; } = new Thickness();
+
         public void ShowTaskbarIcon(bool value)
         {
             // NO OP On OSX

src/Avalonia.X11/X11Window.cs

@@ -311,6 +311,12 @@ namespace Avalonia.X11
             set => _transparencyHelper.TransparencyLevelChanged = value;
         }
 
+        public bool ExtendClientAreaToDecorationsHint { get; set; }
+
+        public Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
+
+        public Thickness ExtendedMargins { get; } = new Thickness();
+
         public Action Closed { get; set; }
         public Action<PixelPoint> PositionChanged { get; set; }
 

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

@@ -14,11 +14,6 @@ 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)
         {
@@ -32,28 +27,40 @@ namespace Avalonia.Win32
             RECT rcFrame = new RECT();
             AdjustWindowRectEx(ref rcFrame, (uint)(WindowStyles.WS_OVERLAPPEDWINDOW & ~WindowStyles.WS_CAPTION), false, 0);
 
+            RECT border_thickness = new RECT();
+            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 };
+            }
+
             // 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)
+            if (ptMouse.Y >= rcWindow.top && ptMouse.Y < rcWindow.top + border_thickness.top)
             {
                 fOnResizeBorder = (ptMouse.Y < (rcWindow.top - rcFrame.top));
                 uRow = 0;
             }
-            else if (ptMouse.Y < rcWindow.bottom && ptMouse.Y >= rcWindow.bottom - BOTTOMEXTENDWIDTH)
+            else if (ptMouse.Y < rcWindow.bottom && ptMouse.Y >= rcWindow.bottom - border_thickness.bottom)
             {
                 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)
+            if (ptMouse.X >= rcWindow.left && ptMouse.X < rcWindow.left + border_thickness.left)
             {
                 uCol = 0; // left side
             }
-            else if (ptMouse.X < rcWindow.right && ptMouse.X >= rcWindow.right - RIGHTEXTENDWIDTH)
+            else if (ptMouse.X < rcWindow.right && ptMouse.X >= rcWindow.right - border_thickness.right)
             {
                 uCol = 2; // right side
             }
@@ -66,8 +73,6 @@ namespace Avalonia.Win32
                 new []{ HitTestValues.HTBOTTOMLEFT, HitTestValues.HTBOTTOM, HitTestValues.HTBOTTOMRIGHT },
             };
 
-            System.Diagnostics.Debug.WriteLine(hitTests[uRow][uCol]);
-
             return hitTests[uRow][uCol];
         }
 
@@ -84,35 +89,37 @@ namespace Avalonia.Win32
             {
                 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 };
-                        }
+                        if (!_isClientAreaExtended)
+                        {                            
+                            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;
-                        TOPEXTENDWIDTH = border_thickness.top;
+                            // 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;
 
-                        // 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);
 
-                        var hr = DwmExtendFrameIntoClientArea(hWnd, ref margins);
+                            //if (hr < 0)
+                            {
+                                // Handle the error.
+                            }
 
-                        //if (hr < 0)
-                        {
-                            // Handle the error.
-                        }
+                            _isClientAreaExtended = true;
+                            _extendedMargins = new Thickness(margins.cxLeftWidth / Scaling, margins.cyTopHeight / Scaling, margins.cxRightWidth / Scaling, margins.cyBottomHeight / Scaling);
+                            ExtendClientAreaToDecorationsChanged?.Invoke(true);
 
+                        }
                         lRet = IntPtr.Zero;
                         callDwp = true;
                         break;