ItemsControl is preventing item content from expanding to fill its container - wpf

Question

I am creating a WPF custom control that inherits from ItemsControl. In the ControlTemplate of my control I have an ItemsPresenter. The problem is that I need to be able to get content in the items presenter to fill the entire content area. In the code below I have a simplified example of the broken code and also what I am trying to accomplish. BTW setting HorizontalAlignment and VerticalAlignment to Stretch causes the content expand horizontally but not vertically.
<Window x:Class="yada"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="768" Width="1024">
<Grid>
<Grid.Resources>
<Style TargetType="Border" x:Key="stepBorder">
<Setter Property="Background" Value="CadetBlue"></Setter>
<Setter Property="BorderBrush" Value="Green" ></Setter>
<Setter Property="BorderThickness" Value="2"></Setter>
<Setter Property="Padding" Value="25"></Setter>
</Style>
</Grid.Resources>
<TabControl Margin="0,20,0,0">
<TabItem Header="Broken">
<Border>
<ItemsControl>
<ItemsControl.Items>
<Control>
<Control.Template>
<ControlTemplate>
<Border Style="{StaticResource stepBorder}">
<TextBlock Text="Border fills small region near the top"></TextBlock>
</Border>
</ControlTemplate>
</Control.Template>
</Control>
</ItemsControl.Items>
</ItemsControl>
</Border>
</TabItem>
<TabItem Header="Works">
<Border>
<Control>
<Control.Template>
<ControlTemplate>
<Border Style="{StaticResource stepBorder}" >
<TextBlock Text="Border fills entire control"></TextBlock>
</Border>
</ControlTemplate>
</Control.Template>
</Control>
</Border>
</TabItem>
</TabControl>
</Grid>
</Window>
Here is some code from my control. I cant include it all, there is too much. I think this is the relevant part however:
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:Wizard}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" x:Name="ContentRow" />
<RowDefinition Height="{Binding ElementName= NavButtonPanel,Path=ActualHeight}"></RowDefinition>
</Grid.RowDefinitions>
<ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto" x:Name="ContentScrollViewer" MaxHeight="{Binding ElementName=ContentRowHeight, Path=ActualHeight}">
<ItemsPresenter
x:Name="Presenter"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}">
</ItemsPresenter>
</ScrollViewer>
<Border Style="{TemplateBinding NavButtonPanelStyle}" Grid.Row="1" x:Name="NavButtonPanel">
<StackPanel Orientation="Horizontal" HorizontalAlignment="Right" >
</StackPanel>
</Border>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate></DataTemplate>
</Setter.Value>
</Setter>

Answer 1

As usual, just posting on SO gets me halfway to my answer.
It turns out the ItemsControl is using an evil stackpanel as a container for items. Fortunately there is a top-secret property called ItemsPanel that lets you change this. Just set it to a grid as shown below and you are in business.
In my case I am writing a custom control that inherits from ItemsControl so in the Style I add this property setter:
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<Grid></Grid>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>