Fluent MenuItem does not align right.

[Rijno]

Description

In plain old WPF, a MenuItem with HorizontalAlignment="Right" is aligned to the right of the menu bar. With Fluent, it does not do this anymore.

Default	Fluent

Reproduction Steps

App.xaml

<Application x:Class="TestWpfFluentMenuAlignment.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml" >
   <Application.Resources>
      <ResourceDictionary>
         <ResourceDictionary.MergedDictionaries>
            <!-- Load WPF Fluent theme (introduced in NET 9.0) -->
            <ResourceDictionary Source="pack://application:,,,/PresentationFramework.Fluent;component/Themes/Fluent.xaml" />
         </ResourceDictionary.MergedDictionaries>
      </ResourceDictionary>
   </Application.Resources>
</Application>

MainWindow.xaml

<Window x:Class="TestWpfFluentMenuAlignment.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
   <DockPanel>
      <Menu DockPanel.Dock="Top">
         <Menu.ItemsPanel>
            <ItemsPanelTemplate>
               <DockPanel HorizontalAlignment="Stretch"/>
            </ItemsPanelTemplate>
         </Menu.ItemsPanel>
         <MenuItem Header="_File">
            <MenuItem Header="New"/>
         </MenuItem>
         <MenuItem Header="_Help" HorizontalAlignment="Right">
            <MenuItem Header="About"/>
         </MenuItem>
      </Menu>

      <Grid></Grid>
   </DockPanel>
</Window>

Expected behavior

Aligns the MenuItem with HorizontalAlignment="Right" to the right of the menu bar.

Actual behavior

MenuItem with HorizontalAlignment="Right" remains aligned to the left.

Regression?

No response

Known Workarounds

No response

Impact

No response

Configuration

Which version of .NET is the code running on?
NET 9.0.200
What OS and version, and what distro if applicable?
Windows 10 19045.4291
What is the architecture (x64, x86, ARM, ARM64)?
x64
Do you know whether it is specific to that configuration?
Not specific to configuration

Other information

No response

[lindexi]

Reference: #9980