Propagate mouse pointer capture to Win32 API

src/Avalonia.Input/MouseDevice.cs

@@ -20,8 +20,13 @@ namespace Avalonia.Input
         private Rect _lastClickRect;
         private ulong _lastClickTime;
 
-        private readonly Pointer _pointer = new Pointer(Pointer.GetNextFreeId(), PointerType.Mouse, true);
+        private readonly Pointer _pointer;
 
+        public MouseDevice(Pointer pointer = null)
+        {
+            _pointer = pointer ?? new Pointer(Pointer.GetNextFreeId(), PointerType.Mouse, true);
+        }
+        
         /// <summary>
         /// Gets the control that is currently capturing by the mouse, if any.
         /// </summary>
@@ -51,7 +56,7 @@ namespace Avalonia.Input
         /// within the control's bounds or not. The current mouse capture control is exposed
         /// by the <see cref="Captured"/> property.
         /// </remarks>
-        public virtual void Capture(IInputElement control)
+        public void Capture(IInputElement control)
         {
             _pointer.Capture(control);
         }

src/Avalonia.Input/Pointer.cs

@@ -27,6 +27,11 @@ namespace Avalonia.Input
             var seen = new HashSet<IInputElement>(control1.GetSelfAndVisualAncestors().OfType<IInputElement>());
             return control2.GetSelfAndVisualAncestors().OfType<IInputElement>().FirstOrDefault(seen.Contains);
         }
+
+        protected virtual void PlatformCapture(IInputElement element)
+        {
+            
+        }
         
         public void Capture(IInputElement control)
         {
@@ -34,6 +39,7 @@ namespace Avalonia.Input
                 Captured.DetachedFromVisualTree -= OnCaptureDetached;
             var oldCapture = control;
             Captured = control;
+            PlatformCapture(control);
             if (oldCapture != null)
             {
                 var commonParent = FindCommonParent(control, oldCapture);

src/Windows/Avalonia.Win32.Interop/Wpf/WpfMouseDevice.cs

@@ -9,22 +9,32 @@ namespace Avalonia.Win32.Interop.Wpf
     {
         private readonly WpfTopLevelImpl _impl;
 
-        public WpfMouseDevice(WpfTopLevelImpl impl)
+        public WpfMouseDevice(WpfTopLevelImpl impl) : base(new WpfMousePointer(impl))
         {
             _impl = impl;
         }
 
-        public override void Capture(IInputElement control)
+        class WpfMousePointer : Pointer
         {
-            if (control == null)
+            private readonly WpfTopLevelImpl _impl;
+
+            public WpfMousePointer(WpfTopLevelImpl impl) : base(Pointer.GetNextFreeId(), PointerType.Mouse, true)
+            {
+                _impl = impl;
+            }
+
+            protected override void PlatformCapture(IInputElement control)
             {
-                System.Windows.Input.Mouse.Capture(null);
+                if (control == null)
+                {
+                    System.Windows.Input.Mouse.Capture(null);
+                }
+                else if ((control.GetVisualRoot() as EmbeddableControlRoot)?.PlatformImpl != _impl)
+                    throw new ArgumentException("Visual belongs to unknown toplevel");
+                else
+                    System.Windows.Input.Mouse.Capture(_impl);
             }
-            else if ((control.GetVisualRoot() as EmbeddableControlRoot)?.PlatformImpl != _impl)
-                throw new ArgumentException("Visual belongs to unknown toplevel");
-            else
-                System.Windows.Input.Mouse.Capture(_impl);
-            base.Capture(control);
         }
+        
     }
 }

src/Windows/Avalonia.Win32/Input/WindowsMouseDevice.cs

@@ -1,7 +1,10 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
+using Avalonia.Controls;
 using Avalonia.Input;
+using Avalonia.VisualTree;
 using Avalonia.Win32.Interop;
 
 namespace Avalonia.Win32.Input
@@ -10,23 +13,32 @@ namespace Avalonia.Win32.Input
     {
         public static WindowsMouseDevice Instance { get; } = new WindowsMouseDevice();
 
+        public WindowsMouseDevice() : base(new WindowsMousePointer())
+        {
+            
+        }
+        
         public WindowImpl CurrentWindow
         {
             get;
             set;
         }
 
-        public override void Capture(IInputElement control)
+        class WindowsMousePointer : Pointer
         {
-            base.Capture(control);
-
-            if (control != null)
+            public WindowsMousePointer() : base(Pointer.GetNextFreeId(),PointerType.Mouse, true)
             {
-                UnmanagedMethods.SetCapture(CurrentWindow.Handle.Handle);
             }
-            else
+
+            protected override void PlatformCapture(IInputElement element)
             {
-                UnmanagedMethods.ReleaseCapture();
+                var hwnd = ((element?.GetVisualRoot() as TopLevel)?.PlatformImpl as WindowImpl)
+                    ?.Handle.Handle;
+
+                if (hwnd.HasValue && hwnd != IntPtr.Zero)
+                    UnmanagedMethods.SetCapture(hwnd.Value);
+                else
+                    UnmanagedMethods.ReleaseCapture();
             }
         }
     }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -336,6 +336,7 @@ namespace Avalonia.Win32
 
         public void BeginMoveDrag()
         {
+            WindowsMouseDevice.Instance.Capture(null);
             UnmanagedMethods.DefWindowProc(_hwnd, (int)UnmanagedMethods.WindowsMessage.WM_NCLBUTTONDOWN,
                 new IntPtr((int)UnmanagedMethods.HitTestValues.HTCAPTION), IntPtr.Zero);
         }
@@ -357,6 +358,7 @@ namespace Avalonia.Win32
 #if USE_MANAGED_DRAG
             _managedDrag.BeginResizeDrag(edge, ScreenToClient(MouseDevice.Position));
 #else
+            WindowsMouseDevice.Instance.Capture(null);
             UnmanagedMethods.DefWindowProc(_hwnd, (int)UnmanagedMethods.WindowsMessage.WM_NCLBUTTONDOWN,
                 new IntPtr((int)EdgeDic[edge]), IntPtr.Zero);
 #endif