fix smooth toggling between decorated and non-decorated.

samples/ControlCatalog/DecoratedWindow.xaml

@@ -3,7 +3,7 @@
         x:Class="ControlCatalog.DecoratedWindow"
         Title="Avalonia Control Gallery"
         Icon="/Assets/test_icon.ico"
-        xmlns:local="clr-namespace:ControlCatalog" HasSystemDecorations="False">
+        xmlns:local="clr-namespace:ControlCatalog" HasSystemDecorations="False" Name="Window">
     <Grid RowDefinitions="5,*,5" ColumnDefinitions="5,*,5">
         <DockPanel  Grid.Column="1"  Grid.Row="1" >
             <Grid Name="TitleBar" Background="LightBlue" DockPanel.Dock="Top" ColumnDefinitions="Auto,*,Auto">
@@ -20,7 +20,11 @@
                 </StackPanel>
             </Grid>
             <Border Background="White" Margin="5">
+                <StackPanel>
                 <TextBlock>Hello world!</TextBlock>
+
+                <CheckBox IsChecked="{Binding ElementName=Window, Path=HasSystemDecorations}">Decorated</CheckBox>
+                </StackPanel>
             </Border>
         </DockPanel>
         <Border Name="TopLeft" Background="Red"/>

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -279,23 +279,36 @@ namespace Avalonia.Win32
             
             var oldThickness = BorderThickness;
             
-            var thickness = BorderThickness;
-            
             _decorated = value;
 
-            var newRect = new Rect(
-                windowRect.left - thickness.Left,
-                windowRect.top - thickness.Top,
-                (windowRect.right - windowRect.left) + (thickness.Left + thickness.Right),
-                (windowRect.bottom - windowRect.top) + (thickness.Top + thickness.Bottom));
+            Rect newRect;
+
+            if (value)
+            {
+                var thickness = BorderThickness;
+
+                newRect = new Rect(
+                    windowRect.left - thickness.Left,
+                    windowRect.top - thickness.Top,
+                    (windowRect.right - windowRect.left) + (thickness.Left + thickness.Right),
+                    (windowRect.bottom - windowRect.top) + (thickness.Top + thickness.Bottom));
+            }
+            else
+            {
+                newRect = new Rect(
+                    windowRect.left + oldThickness.Left,
+                    windowRect.top + oldThickness.Top,
+                    (windowRect.right - windowRect.left) - (oldThickness.Left + oldThickness.Right),
+                    (windowRect.bottom - windowRect.top) - (oldThickness.Top + oldThickness.Bottom));
+            }
 
             UnmanagedMethods.SetWindowPos(_hwnd, IntPtr.Zero, (int)newRect.X, (int)newRect.Y, (int)newRect.Width,
                 (int)newRect.Height,
-                UnmanagedMethods.SetWindowPosFlags.SWP_NOZORDER | UnmanagedMethods.SetWindowPosFlags.SWP_NOACTIVATE);
+                UnmanagedMethods.SetWindowPosFlags.SWP_NOZORDER | UnmanagedMethods.SetWindowPosFlags.SWP_NOACTIVATE | SetWindowPosFlags.SWP_FRAMECHANGED);
         }
 
         public void Invalidate(Rect rect)
-        {
+        {   
             var f = Scaling;
             var r = new UnmanagedMethods.RECT
             {