Fix Menu Popup placement

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

@@ -37,6 +37,7 @@
                    MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"
                    IsLightDismissEnabled="True"
                    IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}"
+                   PlacementMode="BottomEdgeAlignedLeft"
                    OverlayInputPassThroughElement="{Binding $parent[Menu]}">
               <Border Background="{DynamicResource MenuFlyoutPresenterBackground}"
                       BorderBrush="{DynamicResource MenuFlyoutPresenterBorderBrush}"

src/Avalonia.Themes.Simple/Controls/Menu.xaml

@@ -26,8 +26,8 @@
             </ContentPresenter>
             <Popup Name="PART_Popup"
                    IsLightDismissEnabled="True"
-                   IsOpen="{TemplateBinding IsSubMenuOpen,
+                   IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}"
-                                            Mode=TwoWay}"
+                   PlacementMode="BottomEdgeAlignedLeft"
                    OverlayInputPassThroughElement="{Binding $parent[Menu]}">
               <Border Background="{DynamicResource ThemeBackgroundBrush}"
                       BorderBrush="{DynamicResource ThemeBorderMidBrush}"