Previewer: reuse last render scaling

src/Avalonia.DesignerSupport/DesignWindowLoader.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Reflection;
 using System.Text;
 using Avalonia.Controls;
+using Avalonia.Controls.Embedding.Offscreen;
 using Avalonia.Controls.Platform;
 using Avalonia.Markup.Xaml;
 using Avalonia.Styling;
@@ -13,6 +14,9 @@ namespace Avalonia.DesignerSupport
     public class DesignWindowLoader
     {
         public static Window LoadDesignerWindow(string xaml, string assemblyPath, string xamlFileProjectPath)
+            => LoadDesignerWindow(xaml, assemblyPath, xamlFileProjectPath, 1.0);
+
+        public static Window LoadDesignerWindow(string xaml, string assemblyPath, string xamlFileProjectPath, double renderScaling)
         {
             Window window;
             Control control;
@@ -96,6 +100,9 @@ namespace Avalonia.DesignerSupport
                     window = new Window() {Content = (Control)control};
                 }
 
+                if (window.PlatformImpl is OffscreenTopLevelImplBase offscreenImpl)
+                    offscreenImpl.RenderScaling = renderScaling;
+
                 Design.ApplyDesignModeProperties(window, control);
 
                 if (!window.IsSet(Window.SizeToContentProperty))

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -67,8 +67,8 @@ namespace Avalonia.DesignerSupport.Remote
         {
             _transport.Send(new RequestViewportResizeMessage
             {
-                Width = clientSize.Width,
-                Height = clientSize.Height
+                Width = Math.Ceiling(clientSize.Width * RenderScaling),
+                Height = Math.Ceiling(clientSize.Height * RenderScaling)
             });
             ClientSize = clientSize;
             RenderAndSendFrameIfNeeded();

src/Avalonia.DesignerSupport/Remote/RemoteDesignerEntryPoint.cs

@@ -1,13 +1,10 @@
 using System;
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Net;
 using System.Reflection;
 using System.Threading;
-using System.Xml;
 using Avalonia.Controls;
 using Avalonia.DesignerSupport.Remote.HtmlTransport;
-using Avalonia.Input;
 using Avalonia.Remote.Protocol;
 using Avalonia.Remote.Protocol.Designer;
 using Avalonia.Remote.Protocol.Viewport;
@@ -20,6 +17,7 @@ namespace Avalonia.DesignerSupport.Remote
         private static ClientSupportedPixelFormatsMessage s_supportedPixelFormats;
         private static ClientViewportAllocatedMessage s_viewportAllocatedMessage;
         private static ClientRenderInfoMessage s_renderInfoMessage;
+        private static double s_lastRenderScaling = 1.0;
 
         private static IAvaloniaRemoteTransportConnection s_transport;
         class CommandLineArgs
@@ -226,6 +224,9 @@ namespace Avalonia.DesignerSupport.Remote
             }
             if (obj is UpdateXamlMessage xaml)
             {
+                if (s_currentWindow is not null)
+                    s_lastRenderScaling = s_currentWindow.RenderScaling;
+
                 try
                 {
                     s_currentWindow?.Close();
@@ -237,7 +238,7 @@ namespace Avalonia.DesignerSupport.Remote
                 s_currentWindow = null;
                 try
                 {
-                    s_currentWindow = DesignWindowLoader.LoadDesignerWindow(xaml.Xaml, xaml.AssemblyPath, xaml.XamlFileProjectPath);
+                    s_currentWindow = DesignWindowLoader.LoadDesignerWindow(xaml.Xaml, xaml.AssemblyPath, xaml.XamlFileProjectPath, s_lastRenderScaling);
                     s_transport.Send(new UpdateXamlResultMessage(){Handle = s_currentWindow.PlatformImpl?.Handle?.Handle.ToString()});
                 }
                 catch (Exception e)