From a83e06067b191648065bd4a03c7fe99ce1ee1331 Mon Sep 17 00:00:00 2001
From: Julien Lebosquain <julien@lebosquain.net>
Date: Mon, 13 Apr 2026 18:29:02 +0200
Subject: [PATCH 1/2] Add back Design.SetPreviewWith(AvaloniaObject, Control)
 overload

---
 src/Avalonia.Controls/Design.cs               | 49 +++++++++++++++++++
 .../Xaml/DesignModeTests.cs                   | 12 ++---
 2 files changed, 53 insertions(+), 8 deletions(-)

diff --git a/src/Avalonia.Controls/Design.cs b/src/Avalonia.Controls/Design.cs
index 1f7dd4d934b..0e7d5365733 100644
--- a/src/Avalonia.Controls/Design.cs
+++ b/src/Avalonia.Controls/Design.cs
@@ -15,6 +15,20 @@ public static class Design
         private static Dictionary<object, ITemplate<Control>?> s_previewWith = [];
         private static Dictionary<object, object?> s_templateDataContext = [];
 
+        private static ITemplate<Control> InvalidVisualTemplate
+            => field ??= new FuncTemplate<Control>(() => new StackPanel
+            {
+                Children =
+                {
+                    new TextBlock { Text = "A Template must be specified to use Design.PreviewWith on a Visual. Example:" },
+                    new TextBlock { Text = "<Design.PreviewWith>" },
+                    new TextBlock { Text = "    <Template>" },
+                    new TextBlock { Text = "        <Border Padding=\"20\"><!-- YOUR CONTROL FOR PREVIEW HERE --></Border>" },
+                    new TextBlock { Text = "    </Template>" },
+                    new TextBlock { Text = "</Design.PreviewWith>" }
+                }
+            });
+
         /// <summary>
         /// Gets a value indicating whether the application is running in design mode.
         /// </summary>
@@ -142,6 +156,41 @@ public static void SetPreviewWith(AvaloniaObject target, ITemplate<Control>? tem
             s_previewWith[target] = template;
         }
 
+        /// <summary>
+        /// Sets a preview control for the specified <see cref="AvaloniaObject"/> at design-time.
+        /// </summary>
+        /// <param name="target">The target object.</param>
+        /// <param name="control">The preview control.</param>
+        public static void SetPreviewWith(AvaloniaObject target, Control? control)
+        {
+            if (control is null)
+            {
+                s_previewWith[target] = null;
+                return;
+            }
+
+            switch (target)
+            {
+                case ResourceDictionary resourceDictionary:
+                    SetPreviewWith(resourceDictionary, control);
+                    break;
+                case IDataTemplate dataTemplate:
+                    SetPreviewWith(dataTemplate, control);
+                    break;
+                case IStyle style:
+                    SetPreviewWith(style, control);
+                    break;
+                case Visual visual:
+                    s_previewWith[target] = null;
+                    // TODO: restore once ITemplate overloads are correctly handled in the XAML compiler.
+                    //SetPreviewWith(visual, InvalidVisualTemplate);
+                    break;
+                default:
+                    SetPreviewWith(target, new FuncTemplate<Control>(() => control));
+                    break;
+            }
+        }
+
         /// <summary>
         /// Sets a preview template for the specified <see cref="ResourceDictionary"/> at design-time.
         /// </summary>
diff --git a/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/DesignModeTests.cs b/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/DesignModeTests.cs
index 42776b236c1..cb2b25f0d54 100644
--- a/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/DesignModeTests.cs
+++ b/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/DesignModeTests.cs
@@ -33,23 +33,19 @@ public void Design_Mode_PreviewWith_Should_Be_Ignored_Without_Design_Mode()
     }
 
     [Fact]
-    public void Design_Mode_PreviewWith_Works_With_Control_Template()
+    public void Design_Mode_PreviewWith_Returns_Original_Control()
     {
         using (UnitTestApplication.Start(TestServices.MockWindowingPlatform))
         {
             var obj = (Control)AvaloniaRuntimeXamlLoader.Load(@"
 <Button xmlns='https://github.com/avaloniaui'>
     <Design.PreviewWith>
-        <Template>
-            <Border>
-                <Button />
-            </Border>
-        </Template>
+        <Border />
     </Design.PreviewWith>
 </Button>", designMode: true);
             var preview = Design.CreatePreviewWithControl(obj);
-            var previewBorder = Assert.IsType<Border>(preview);
-            Assert.IsType<Button>(previewBorder.Child);
+            // Should return the original control, not the preview, as this is not supported to avoid stack overflows.
+            Assert.Same(obj, preview);
         }
     }
 

From 119305e91c14f37f4c928de1fc08ee0b804d6960 Mon Sep 17 00:00:00 2001
From: Julien Lebosquain <julien@lebosquain.net>
Date: Mon, 13 Apr 2026 18:57:25 +0200
Subject: [PATCH 2/2] Remove dead code

---
 src/Avalonia.Controls/Design.cs | 19 ++-----------------
 1 file changed, 2 insertions(+), 17 deletions(-)

diff --git a/src/Avalonia.Controls/Design.cs b/src/Avalonia.Controls/Design.cs
index 0e7d5365733..890d7077c84 100644
--- a/src/Avalonia.Controls/Design.cs
+++ b/src/Avalonia.Controls/Design.cs
@@ -15,20 +15,6 @@ public static class Design
         private static Dictionary<object, ITemplate<Control>?> s_previewWith = [];
         private static Dictionary<object, object?> s_templateDataContext = [];
 
-        private static ITemplate<Control> InvalidVisualTemplate
-            => field ??= new FuncTemplate<Control>(() => new StackPanel
-            {
-                Children =
-                {
-                    new TextBlock { Text = "A Template must be specified to use Design.PreviewWith on a Visual. Example:" },
-                    new TextBlock { Text = "<Design.PreviewWith>" },
-                    new TextBlock { Text = "    <Template>" },
-                    new TextBlock { Text = "        <Border Padding=\"20\"><!-- YOUR CONTROL FOR PREVIEW HERE --></Border>" },
-                    new TextBlock { Text = "    </Template>" },
-                    new TextBlock { Text = "</Design.PreviewWith>" }
-                }
-            });
-
         /// <summary>
         /// Gets a value indicating whether the application is running in design mode.
         /// </summary>
@@ -180,10 +166,9 @@ public static void SetPreviewWith(AvaloniaObject target, Control? control)
                 case IStyle style:
                     SetPreviewWith(style, control);
                     break;
-                case Visual visual:
+                case Visual:
+                    // Not a supported scenario without templates; causes stack overflows.
                     s_previewWith[target] = null;
-                    // TODO: restore once ITemplate overloads are correctly handled in the XAML compiler.
-                    //SetPreviewWith(visual, InvalidVisualTemplate);
                     break;
                 default:
                     SetPreviewWith(target, new FuncTemplate<Control>(() => control));