Gridsplitter does not allow user to resize grid - wpf

Question

My code is as follows:
<Grid
ShowGridLines="True">
<Grid.RowDefinitions>
<RowDefinition Height="*"></RowDefinition>
<RowDefinition Height="*"></RowDefinition>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="150"></ColumnDefinition>
<ColumnDefinition Width="5"></ColumnDefinition>
<ColumnDefinition Width="*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<GridSplitter
Grid.Row="0"
Grid.Column="1"
Grid.ColumnSpan="2"
Width="5"
Background="Purple"
HorizontalAlignment="left" />
<ListBox
Grid.Row="0"
Grid.Column="0"
ItemsSource="{Binding Tests}"
ScrollViewer.HorizontalScrollBarVisibility="Visible"
Grid.IsSharedSizeScope="True">
<ListBox.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"></ColumnDefinition>
<ColumnDefinition Width="10*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<TextBlock
Text="{Binding Name}"
Grid.Column="0">
</TextBlock>
<Image
Grid.Column="1">
</Image>
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
<TextBlock
Grid.Row="0"
Grid.Column="1"
Text="{Binding CommandPromptOutput}">
</TextBlock>
<Button
Grid.Row="1"
Grid.Column="2"
Content="Run" VerticalAlignment="Bottom" HorizontalContentAlignment="Center" HorizontalAlignment="Stretch" />
</Grid>
The grid splitter is showing up but it has no control over resizing anything.
I have also tried adding:
<GridSplitter
Grid.Row="0"
Grid.Column="1"
Grid.ColumnSpan="2"
Width="5"
Background="Purple"
HorizontalAlignment="left"
ResizeDirection="Columns"
ResizeBehavior="PreviousAndNext" />