Textblock is overlapping with combobox items (WPF)

<Grid
    Width="130"
    VerticalAlignment="Center">
    <Grid.Resources>
        <CollectionViewSource x:Key="Columns" Source="{Binding MySource}" />
        <DataTemplate DataType="{x:Type models:MyLabel}">
            <CheckBox
                Checked="CheckBox_CheckChanged"
                Content="{Binding Label}"
                IsChecked="{Binding IsSelected}"
                Unchecked="CheckBox_CheckChanged" />
        </DataTemplate>
    </Grid.Resources>
    <ComboBox
        IsEditable="False"
        IsReadOnly="True"
        SelectedIndex="-1">
        <ComboBox.ItemsSource>
            <CompositeCollection>
                <CheckBox
                    x:Name="SelectAllCb"
                    Checked="SelectAllCb_CheckedChanged"
                    Content="Select All"
                    Unchecked="SelectAllCb_CheckedChanged" />
                <CollectionContainer Collection="{Binding Source={StaticResource Columns}}" />
            </CompositeCollection>
        </ComboBox.ItemsSource>
    </ComboBox>
    <TextBlock
        Margin="5,0,0,0"
        VerticalAlignment="Center"
        IsHitTestVisible="False"
        Text="Show/Hide Columns" />
</Grid>