implent popup routing events to parent toplevel.

src/Avalonia.Base/Input/PointerEventArgs.cs

@@ -70,7 +70,16 @@ namespace Avalonia.Input
             if (relativeTo == null)
                 return pt;
 
-            return pt * _rootVisual.TransformToVisual(relativeTo) ?? default;
+            if (!ReferenceEquals(_rootVisual, relativeTo.VisualRoot))
+            {
+                var screenPt = _rootVisual.PointToScreen(pt);
+
+                return relativeTo.PointToClient(screenPt);
+            }
+            else
+            {
+                return pt * _rootVisual.TransformToVisual(relativeTo) ?? default;
+            }
         }
 
         /// <summary>

src/Avalonia.Themes.Fluent/Controls/PopupRoot.xaml

@@ -13,15 +13,15 @@
         <LayoutTransformControl LayoutTransform="{TemplateBinding Transform}">
           <Panel>
             <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
-            <Border Background="Red">
-            <VisualLayerManager IsPopup="True">
-              <ContentPresenter Name="PART_ContentPresenter"
-                                Background="{TemplateBinding Background}"
-                                ContentTemplate="{TemplateBinding ContentTemplate}"
-                                Content="{TemplateBinding Content}"
-                                Padding="{TemplateBinding Padding}" />
-            </VisualLayerManager>
-              </Border>
+            <Border Background="Transparent">
+              <VisualLayerManager IsPopup="True">
+                <ContentPresenter Name="PART_ContentPresenter"
+                                  Background="{TemplateBinding Background}"
+                                  ContentTemplate="{TemplateBinding ContentTemplate}"
+                                  Content="{TemplateBinding Content}"
+                                  Padding="{TemplateBinding Padding}" />
+              </VisualLayerManager>
+            </Border>
           </Panel>
         </LayoutTransformControl>
       </ControlTemplate>