Setting ThemeMode on a window causes a button to be positioned differently

[adegeo]

Description

Setting ThemeMode on a window causes a button to be completely repositioned.

Reproduction Steps

1. Brand new WPF project.
2. Add a button to the grid and set the width/height to something small.
3. Set the ThemeMode property on the window to anything other than None.

<Window x:Class="Names.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"
        xmlns:local="clr-namespace:Names"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="400" ThemeMode="System">
    <Grid>
        <Button Width="200" Height="30" />
    </Grid>
</Window>

Before ThemeMode is set:

After ThemeMode is set:

Expected behavior

Don't alter the layout.

Actual behavior

Layout is altered.

Regression?

No response

Known Workarounds

No response

Impact

No response

Configuration

• Visual Studio 2022 17.12 GA
• .NET 9 GA

Other information

No response

[lindexi]

@adegeo

The style define is in \src\Themes\PresentationFramework.Fluent\Styles\Button.xaml

wpf/src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Styles/Button.xaml

Line 25 in d4bb347

<Setter Property="HorizontalAlignment" Value="Left" />

See #8870

And in WinUI, the default button style is also left-aligned, so I think this style is appropriate here. What do you think?