Reports

found the source of the problem, it's because i nested Menu element within the MenuItem element, which is unnecessary and a mistake on my part

adjusted Titlebar.xaml code

<Menu
    Grid.Column="0"
    HorizontalAlignment="Left"
    VerticalAlignment="Center"
    Style="{StaticResource MenuStyle1}">
    <MenuItem Header="File" Style="{StaticResource MenuItemStyle1}">
        <MenuItem
            Command="{Binding OpenCommand}"
            Header="Open"
            Style="{StaticResource MenuItemStyle1}" />
        <MenuItem
            Command="{Binding SaveCommand}"
            Header="Save"
            Style="{StaticResource MenuItemStyle1}" />
        <MenuItem Header="Close" Style="{StaticResource MenuItemStyle1}" />
    </MenuItem>

</Menu>

79628689