remove menu shadows from fluent theme as this is not compatible with the rounded corners.

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

@@ -39,6 +39,7 @@
   <Setter Property="HorizontalAlignment" Value="Stretch" />
   <Setter Property="TextBlock.FontSize" Value="{DynamicResource ContentControlFontSize}" />
   <Setter Property="TextBlock.FontWeight" Value="Normal" />
+  <Setter Property="WindowManagerAddShadowHint" Value="False" />
   <Setter Property="Template">
     <ControlTemplate>
       <Border Background="{TemplateBinding Background}"

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

@@ -110,7 +110,7 @@
             </Grid>
           </Border>
           <Popup Name="PART_Popup"
-                 WindowManagerAddShadowHint="True"
+                 WindowManagerAddShadowHint="False"
                  PlacementMode="Right"
                  HorizontalOffset="{StaticResource MenuFlyoutSubItemPopupHorizontalOffset}"
                  IsLightDismissEnabled="True"