Grouped ComboBox shows group headers, but not items

Question

I'm attempting a grouped ComboBox using GroupStyle:

<ComboBox ItemsSource="{Binding GroupedItems}">
    <ComboBox.GroupStyle>
        <GroupStyle>
            <GroupStyle.HeaderTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}" />
                </DataTemplate>
            </GroupStyle.HeaderTemplate>
        </GroupStyle>
    </ComboBox.GroupStyle>
</ComboBox>

Here the "GroupedItems" property is a ListCollectionView with a "GroupDescription" applied.

This almost works: the headers are correct, but the items themselves do not appear in the popup. Why?

Note: I'm using the a modified version of the ComboBox styles/templates on MSDN.

Answer 1

It turns out that the ComboBox control template on MSDN is wrong, and the documentation is also wrong. I realized this after I removed the styles/templates from my resources, and then the grouping showed up correctly.

After extracting the actual built-in control template using Blend, I found that the grouping functionality of the ComboBox relies on these named parts: "Popup" (not "PART_Popup" as is listed on MSDN), "DropDown", "DropDownBorder", "DropDownScrollViewer", and "ItemsPresenter".

<Popup x:Name="Popup">
    <Grid x:Name="DropDown">
        <Border x:Name="DropDownBorder">
            <ScrollViewer x:Name="DropDownScrollViewer">
                <ItemsPresenter x:Name="ItemsPresenter" />
            </ScrollViewer>
        </Border>
    </Grid>
</Popup>

Thanks Obama! Microsoft!