fix transparency fallback border.

src/Avalonia.Themes.Default/EmbeddableControlRoot.xaml

@@ -3,7 +3,8 @@
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
   <Setter Property="Template">
     <ControlTemplate>
-      <Border Name="PART_TransparencyFallback">
+      <Panel>
+        <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
         <Border Background="{TemplateBinding Background}">
           <VisualLayerManager>
             <ContentPresenter Name="PART_ContentPresenter"
@@ -12,7 +13,7 @@
                               Margin="{TemplateBinding Padding}"/>
           </VisualLayerManager>
         </Border>
-      </Border>
+      </Panel>
     </ControlTemplate>
   </Setter>
 </Style>

src/Avalonia.Themes.Default/OverlayPopupHost.xaml

@@ -2,7 +2,8 @@
   <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
   <Setter Property="Template">
     <ControlTemplate>
-      <Border Name="PART_TransparencyFallback">
+      <Panel>
+        <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
         <VisualLayerManager IsPopup="True">
           <ContentPresenter Name="PART_ContentPresenter"
                             Background="{TemplateBinding Background}"
@@ -10,7 +11,7 @@
                             Content="{TemplateBinding Content}"
                             Padding="{TemplateBinding Padding}"/>
         </VisualLayerManager>
-      </Border>
+      </Panel>
     </ControlTemplate>
   </Setter>
 </Style>

src/Avalonia.Themes.Default/PopupRoot.xaml

@@ -2,7 +2,8 @@
   <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
   <Setter Property="Template">
     <ControlTemplate>
-      <Border Name="PART_TransparencyFallback">
+      <Panel>
+        <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
         <VisualLayerManager IsPopup="True">
           <ContentPresenter Name="PART_ContentPresenter"
                             Background="{TemplateBinding Background}"
@@ -10,7 +11,7 @@
                             Content="{TemplateBinding Content}"
                             Padding="{TemplateBinding Padding}"/>
         </VisualLayerManager>
-      </Border>
+      </Panel>
     </ControlTemplate>
   </Setter>
 </Style>

src/Avalonia.Themes.Default/Window.xaml

@@ -4,7 +4,8 @@
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
   <Setter Property="Template">
     <ControlTemplate>
-      <Border Name="PART_TransparencyFallback">
+      <Panel>
+        <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
         <Border Background="{TemplateBinding Background}">
           <VisualLayerManager>
             <ContentPresenter Name="PART_ContentPresenter"
@@ -15,7 +16,7 @@
                               VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
           </VisualLayerManager>
         </Border>
-      </Border>
+      </Panel>
     </ControlTemplate>
   </Setter>
 </Style>