Xaml designer behaves differently in VS 2012 and VS 2010 - visual-studio-2010

Question

I recently switched to Visual Studio 2012 from VS2010.
I have some user controls which are templates for other user controls.
The first ones have some ContentControl whose content is defined in the second ones.
Follows the definition of a user control based on a template control (ImpegniBaseView)
<localView:ImpegniBaseView>
<localView:ImpegniBaseView.ActionButtons>
<StackPanel Height="54" HorizontalAlignment="Left" VerticalAlignment="Top" Width="668" Orientation="Horizontal">
<Button Content="Salva" Height="31" Width="88" Command="{Binding Path=SaveCommand}" IsEnabled="{Binding Editable}" Margin="5,0" />
<Button Command="{Binding Path=CreateCommand}" Content="Nuovo impegno master" Height="31" Width="Auto"
Visibility="{Binding IsFirstLevelView, Converter={StaticResource BoolToVisibility}}"
IsEnabled="{Binding Path=Editable, Converter={StaticResource InverseBooleanConverter}}" Margin="5,0" />
</StackPanel>
</localView:ImpegniBaseView.ActionButtons>
</localView:ImpegniBaseView>
In Vs 2010 I could easily select, for example, the button 'Salva' in the designer. Now I cannot. I suppose I have to change an option in the designer, but I don't know which one...
Update
Follows the relevant part of the user control used as template
<UserControl x:Class="Ragioneria.View.ImpegniBaseView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:Ragioneria"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="700">
<DockPanel DataContext="{Binding ImpegnoSelezionato}">
<DockPanel DockPanel.Dock="Top">
<StackPanel Orientation="Horizontal" DockPanel.Dock="Left">
<Label Content="Anno/Num-Sub" Height="28" HorizontalAlignment="Left" Margin="5" Name="label1" VerticalAlignment="Top" Width="Auto" />
<ContentControl Content="{Binding Path=IdentificazioneImpegnoSection, RelativeSource={RelativeSource AncestorType=UserControl}}" Width="400" Height="28" Margin="5" />
</StackPanel>
<TextBox Height="28" Width="100" Name="textBox9" Text="{Binding Path=DataRegistrazione, StringFormat=d}" DockPanel.Dock="Right" Style="{DynamicResource tb_readonly}" TextAlignment="Right" Margin="5" />
<Label Content="Data" Height="28" Margin="5" Name="label7" Width="50" DockPanel.Dock="Right"/>
</DockPanel>
<ContentControl DockPanel.Dock="Bottom"
DataContext="{Binding DataContext.ActiveWorkspace, RelativeSource={RelativeSource AncestorType=Window}}"
Content="{Binding Path=ActionButtons, RelativeSource={RelativeSource AncestorType=UserControl}}" />
</DockPanel>
Thanks
Filippo

Answer 1

You must add [AlternateContentProperty] to ActionButtons.
Unfortunately, unlike in Silverlight, you won't find this attribute in standard library.
Instead, you will need to use Microsoft.Windows.Design.PropertyEditing.AlternateContentProperty from Microsoft.Windows.Design.Interaction.dll.
Since this is a non-redistributable dll from Visual Studio, you can't reference it directly and have to create designer metadata assembly.
See this bug
and vote here.