How do I get WPF GridSplitter controls working inside an ItemsControl?

Could anyone possibly explain to me why the following simple example works:

<ItemsControl x:Class="UserControl1"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x=开发者_JAVA百科"http://schemas.microsoft.com/winfx/2006/xaml">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <Grid />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="5" />
            <RowDefinition />
        </Grid.RowDefinitions>

        <Grid Background="Yellow" />

        <GridSplitter Grid.Row="1"
                      HorizontalAlignment="Stretch"
                      VerticalAlignment="Stretch" />

        <Grid Grid.Row="2"
              Background="Orange" />
    </Grid>
</ItemsControl>

...but when I make the ItemsPanelTemplate's the main one, it does not:

<ItemsControl x:Class="UserControl1"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition />
                    <RowDefinition Height="5" />
                    <RowDefinition />
                </Grid.RowDefinitions>
            </Grid>
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>

    <Grid Background="Yellow" />

    <GridSplitter Grid.Row="1"
                  HorizontalAlignment="Stretch" 
                  VerticalAlignment="Stretch" />

    <Grid Grid.Row="2" Background="Orange" />
</ItemsControl>

They both display a Yellow box on top of an Orange box, with a horizontal splitter between them. In the first example the splitter works correctly, allowing you to resize the two areas. In the second example (that produces a near identical visual tree), the splitter is locked, it won't allow me to drag it to resize the two areas!

This is a very simplified example of what I'm trying to achieve - but it demonstrates the issue I'm getting in my actual app. I must be missing something, what's stopping the splitter from functioning? All three children get added to the ItemsPanelTemplate grid ok....

Any explanation or fix would be much appreciated!

Regards, Dave

oh no one replied? it is because the GridSplitter must be the immediate child of the parent Grid to know the actual size of the columns.