Complex controls in a Popup

Question

I am building a control allowing to manage some items and select one. It comprises a TextBox and a Button allowing to open a Popup where it all happens.

I would like to have a list of items with details in a System.Windows.Controls.Primitives.Popup class. I am using a Grid with 2 columns, the first one contains the list of items, the second one its details.

Now I want to use a System.Windows.Controls.GridSplitter control to allow the user to resize the area of details.

The problem is that this works everywhere except of inside a Popup control. My question is, should I use some other control simulating Popup's behavior (Window..) or is there any other way to achieve this.

EDITED: If I set Width Property of the Popup it works, because the Popup is not trying to adapt to its content. But I want to preserve this behaviour..

What I'm planning is to add complex animations to the controls in the Popup as well.

CODE:

...
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>

    <TextBox Grid.Column="0" Grid.Row="0" Text="Selected Item" />
    <ToggleButton Grid.Row="0" Grid.Column="1" Focusable="False">
        <Grid>
            <TextBlock Text="Select" />

            <Popup StaysOpen="False" Width="700" VerticalAlignment="Bottom" IsOpen="{Binding Path=IsChecked, 
                                                    RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ToggleButton, AncestorLevel=1},
                                                    Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" PlacementTarget="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ToggleButton, AncestorLevel=1}}">

                <local:MyManagementControl />

            </Popup>
        </Grid>
    </ToggleButton>

</Grid>
...

MyManagementControl contains:

...
<Grid>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />            
        </Grid.RowDefinitions>

        <ListView Grid.Row="0" Grid.Column="0">            
            ...
        </ListView>

        <GridSplitter Grid.Column="0" Width="10" HorizontalAlignment="Right" VerticalAlignment="Stretch" />

        <Grid Grid.Row="0" Grid.Column="1">
        ...Details...
        </Grid>
    </Grid>
...