extend api so that when a parent is set you can state if the current window becomes modal.

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -29,7 +29,8 @@ namespace Avalonia.Platform
         /// Sets the parent of the window.
         /// </summary>
         /// <param name="parent">The parent <see cref="IWindowImpl"/>.</param>
-        void SetParent(IWindowImpl parent);
+        /// <param name="isModal">If this window is modal or not.</param>
+        void SetParent(IWindowImpl parent, bool isModal);
         
         /// <summary>
         /// Disables the window for example when a modal dialog is open.

src/Avalonia.Controls/Window.cs

@@ -539,7 +539,7 @@ namespace Avalonia.Controls
 
             using (BeginAutoSizing())
             {
-                PlatformImpl.SetParent(owner.PlatformImpl);
+                PlatformImpl.SetParent(owner.PlatformImpl, true);
                 Owner = owner;
                 owner.AddChild(this);
                 PlatformImpl?.Show();

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -117,7 +117,7 @@ namespace Avalonia.DesignerSupport.Remote
         {
         }
 
-        public void SetParent(IWindowImpl parent)
+        public void SetParent(IWindowImpl parent, bool isModal)
         {
         }
 

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -130,7 +130,7 @@ namespace Avalonia.DesignerSupport.Remote
         {
         }
 
-        public void SetParent(IWindowImpl parent)
+        public void SetParent(IWindowImpl parent, bool isModal)
         {
         }
 

src/Avalonia.Native/WindowImpl.cs

@@ -114,7 +114,7 @@ namespace Avalonia.Native
 
         public Action GotInputWhenDisabled { get; set; }
 
-        public void SetParent(IWindowImpl parent)
+        public void SetParent(IWindowImpl parent, bool isModal)
         {
         }
 

src/Avalonia.X11/X11Window.cs

@@ -1033,7 +1033,7 @@ namespace Avalonia.X11
             ChangeWMAtoms(value, _x11.Atoms._NET_WM_STATE_ABOVE);
         }
 
-        public void SetParent(IWindowImpl parent)
+        public void SetParent(IWindowImpl parent, bool isModal)
         {
             SetTransientParent((X11Window)parent);
         }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -351,7 +351,7 @@ namespace Avalonia.Win32
 
         public Action GotInputWhenDisabled { get; set; }
 
-        public void SetParent(IWindowImpl parent)
+        public void SetParent(IWindowImpl parent, bool isModal)
         {
             _parent = (WindowImpl)parent;
             SetOwnerHandle(_parent._hwnd);