WPF Items control doesnot show VerticalScrollBar automatically

Question

Here is the intention,

When items get added to ItemsControl and if the content is more than the available size I wish to see scrolling in ItemsControl. I bet this is the default behavior for any ItemsControl type.

So I generated a sample replicates my intention. Suggest me what I need to fix

<StackPanel>
        <StackPanel.Resources>
            <Style TargetType="{x:Type ItemsControl}">
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type ItemsControl}">
                            <Border SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}">
                                <ScrollViewer VerticalScrollBarVisibility="Visible">
                                    <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                                </ScrollViewer>
                            </Border>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </StackPanel.Resources>
        <ToggleButton Name="SomeToggle" Content="Show/Hide"/>
        <Button Content="Add Item" Click="ButtonBase_OnClick" />
        <TextBlock Text="Hide/Show on toggle" >
            <TextBlock.Style>
                <Style TargetType="TextBlock">
                    <Setter Property="Visibility" Value="Collapsed"/>
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding Path=IsChecked,ElementName=SomeToggle}" Value="True">
                            <Setter Property="Visibility" Value="Visible" />
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </TextBlock.Style>
        </TextBlock>

    <ItemsControl Name="ItemsCollection" >
    </ItemsControl>

</StackPanel>

and Code behind:

private int counter = 0;
    private ObservableCollection<string> coll; 
    public MainWindow()
    {
        coll= new ObservableCollection<string>();
        //ItemsCollection.ItemsSource = coll;
        InitializeComponent();
    }

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
    {
        ItemsCollection.Items.Add(string.Format("Item {0}", ++counter));
    }

Answer 1

This is basically occurs each time you put a ScrollViewer inside a Stackpanel, and this is due to the way the Stackpanel works, take a look here

you could either put the Stackpanel inside a ScrollView, or better change your main panel to a Grid

   <Grid >
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="Auto"/>            
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Grid.Resources>
        <Style TargetType="{x:Type ItemsControl}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ItemsControl}">
                        <Border SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}">
                            <ScrollViewer VerticalScrollBarVisibility="Visible">
                                <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                            </ScrollViewer>
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Grid.Resources>
    <ToggleButton Name="SomeToggle" Content="Show/Hide" Grid.Row="0"/>
    <Button Content="Add Item" Click="ButtonBase_OnClick" Grid.Row="1"/>
    <TextBlock Text="Hide/Show on toggle" Grid.Row="2">
        <TextBlock.Style>
            <Style TargetType="TextBlock">
                <Setter Property="Visibility" Value="Collapsed"/>
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Path=IsChecked,ElementName=SomeToggle}" Value="True">
                        <Setter Property="Visibility" Value="Visible" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </TextBlock.Style>
    </TextBlock>
    <ItemsControl Name="ItemsCollection" Grid.Row="3">
    </ItemsControl>
</Grid>

Answer 2

The only reason you are not able to see the scrollbars cause you have not set the height of your ItemsControl.

Every time you add a item it's height will expand and StackPanel will expand with it to contain the content. You have already defined ScrollViewer to your ItemsControl template you have to also decide at what point it has to work.

<ItemsControl Name="ItemsCollection" Height="200" >
</ItemsControl>

Then it will also not matter either you containing it in a StackPanel or any other control.

initial scroll bar

After adding items

with code

And just from understanding point of view. this will also work:

Remove style for item control & set the height of scroll viewer.

To handle the height of all elements:

<ScrollViewer MaxHeight="{Binding RelativeSource={RelativeSource AncestorType={x:Type Border}},Path=ActualHeight,Mode=OneWay}">
    <StackPanel>
    <StackPanel.Resources>
        <Style TargetType="{x:Type ItemsControl}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ItemsControl}">
                        <Border SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}">
                            <ScrollViewer VerticalScrollBarVisibility="Visible">
                                <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                            </ScrollViewer>
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </StackPanel.Resources>
    <ToggleButton Name="SomeToggle" Content="Show/Hide"/>
    <Button Content="Add Item" Click="Button_Click" />
    <TextBlock Text="Hide/Show on toggle" >
        <TextBlock.Style>
            <Style TargetType="TextBlock">
                <Setter Property="Visibility" Value="Collapsed"/>
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Path=IsChecked,ElementName=SomeToggle}" Value="True">
                        <Setter Property="Visibility" Value="Visible" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </TextBlock.Style>
    </TextBlock>

    <ItemsControl Name="ItemsCollection" ItemsSource="{Binding list}" 
                  MaxHeight="{Binding RelativeSource={RelativeSource AncestorType={x:Type Border}},Path=ActualHeight,Mode=OneWay}">
    </ItemsControl>

</StackPanel>
</ScrollViewer>