remove disabledBy array on win32.

src/Avalonia.Controls/Window.cs

@@ -69,6 +69,9 @@ namespace Avalonia.Controls
     /// </summary>
     public class Window : WindowBase, IStyleable, IFocusScope, ILayoutRoot
     {
+        private List<Window> _children = new List<Window>();
+        private Window _owner;
+
         /// <summary>
         /// Defines the <see cref="SizeToContent"/> property.
         /// </summary>
@@ -366,6 +369,12 @@ namespace Avalonia.Controls
             {
                 if (close)
                 {
+                    if(_owner != null)
+                    {
+                        _owner._children.Remove(this);
+                        _owner = null;
+                    }
+
                     PlatformImpl?.Dispose();
                 }
             }
@@ -408,6 +417,14 @@ namespace Avalonia.Controls
             using (BeginAutoSizing())
             {
                 Renderer?.Stop();
+
+                if (_owner != null)
+                {
+                    _owner._children.Remove(this);
+                    // update enabled state of parent.
+                    _owner = null;
+                }
+
                 PlatformImpl?.Hide();
             }
 
@@ -521,7 +538,8 @@ namespace Avalonia.Controls
             using (BeginAutoSizing())
             {
                 PlatformImpl.SetParent(owner.PlatformImpl);
-                owner.PlatformImpl.SetEnabled(false);
+                owner._children.Add(this);
+                owner.PlatformImpl.SetEnabled(false);                
                 PlatformImpl?.Show();
 
                 Renderer?.Start();

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -44,8 +44,7 @@ namespace Avalonia.Win32
         private readonly ManagedWindowResizeDragHelper _managedDrag;
 #endif
 
-        private const WindowStyles WindowStateMask = (WindowStyles.WS_MAXIMIZE | WindowStyles.WS_MINIMIZE);
-        private readonly List<WindowImpl> _disabledBy;
+        private const WindowStyles WindowStateMask = (WindowStyles.WS_MAXIMIZE | WindowStyles.WS_MINIMIZE);        
         private readonly TouchDevice _touchDevice;
         private readonly MouseDevice _mouseDevice;
         private readonly ManagedDeferredRendererLock _rendererLock;
@@ -70,7 +69,6 @@ namespace Avalonia.Win32
 
         public WindowImpl()
         {
-            _disabledBy = new List<WindowImpl>();
             _touchDevice = new TouchDevice();
             _mouseDevice = new WindowsMouseDevice();
 
@@ -342,13 +340,6 @@ namespace Avalonia.Win32
 
         public void Hide()
         {
-            if (_parent != null)
-            {
-                _parent._disabledBy.Remove(this);
-                _parent.UpdateEnabled();
-                _parent = null;
-            }
-
             UnmanagedMethods.ShowWindow(_hwnd, ShowWindowCommand.Hide);
         }
 
@@ -363,7 +354,7 @@ namespace Avalonia.Win32
         public void SetParent(IWindowImpl parent)
         {
             _parent = (WindowImpl)parent;
-            _parent._disabledBy.Add(this);
+            SetOwnerHandle(_parent._hwnd);
         }
 
         public void SetEnabled(bool enable) => EnableWindow(_hwnd, enable);
@@ -665,7 +656,9 @@ namespace Avalonia.Win32
                     SetWindowPos(_hwnd, WindowPosZOrder.HWND_NOTOPMOST, x, y, cx, cy, SetWindowPosFlags.SWP_SHOWWINDOW);
                 }
             }
-        }        
+        }
+
+        private void SetOwnerHandle(IntPtr handle) => SetWindowLongPtr(_hwnd, (int)WindowLongParam.GWL_HWNDPARENT, handle);
 
         private WindowStyles GetWindowStateStyles ()
         {
@@ -722,11 +715,6 @@ namespace Avalonia.Win32
             }
         }
 
-        private void UpdateEnabled()
-        {
-            SetEnabled(_disabledBy.Count == 0);
-        }
-
         private void UpdateWindowProperties(WindowProperties newProperties, bool forceChanges = false)
         {
             var oldProperties = _windowProperties;