Question

I am using listbox grouping, I want to avoid vertical spaces left between header and subelements, as shown below:

Here is the xaml for the same

<Grid>
    <Grid.Resources>
        <XmlDataProvider x:Key="Company" XPath="/Company">
            <x:XData>
                <Company xmlns="">
                    <Person Name="Jack" Role="CEO"/>
                    <Person Name="Tim" Role="PL" />
                    <Person Name="Jil" Role="PL" />
                    <Person Name="Jimmy" Role="PM" />
                    <Person Name="Joy" Role="PM" />
                    <Person Name="Jim" Role="PL" />
                    <Person Name="Jack" Role="PM" />
                </Company>
            </x:XData>
        </XmlDataProvider>

        <DataTemplate x:Key="categoryTemplate">
            <TextBlock Text="{Binding Path=Name}" FontWeight="Bold" Background="ForestGreen" Margin="0,5,0,0"/>
        </DataTemplate>
        <DataTemplate x:Key="template">
            <TextBlock Text="{Binding XPath=@Name}"/>
        </DataTemplate>

        <CollectionViewSource x:Key="cvs" Source="{Binding Source={StaticResource Company},XPath=Person}">
            <CollectionViewSource.GroupDescriptions>
                <PropertyGroupDescription PropertyName="@Role"/>
            </CollectionViewSource.GroupDescriptions>
        </CollectionViewSource>
    </Grid.Resources>


    <ListBox Name="lst" ItemTemplate="{StaticResource template}" ItemsSource="{Binding Source={StaticResource cvs}}">
        <ListBox.GroupStyle>
            <GroupStyle HeaderTemplate="{StaticResource categoryTemplate}" />
        </ListBox.GroupStyle>
    </ListBox>
</Grid>

Answer 1

The ListBox will use GroupItems when you group and they have a default Margin set to 5,0,0,0. Also, ListBoxItem comes with a default Padding of 2,0,0,0. You can change either one, or both, of them like this

<ListBox ...>
    <ListBox.Resources>
        <Style TargetType="{x:Type GroupItem}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type GroupItem}">
                        <StackPanel>
                            <ContentPresenter/>
                            <ItemsPresenter Margin="0,0,0,0"/>
                        </StackPanel>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </ListBox.Resources>
    <ListBox.ItemContainerStyle>
        <Style TargetType="ListBoxItem">
            <Setter Property="Padding" Value="0"/>
        </Style>
    </ListBox.ItemContainerStyle>
    <!--...-->
</ListBox>