WPF MenuItem.IsHighlighted == false when ContextMenu and MenuItems are Transparent - wpf

Question

I am trying to create a context menu which allows the user to see what is under it, and so I have made the Background for both the ContextMenu, and its MenuItems Transparent. When the user moves the mouse over the MenuItems(hovers) they should show up due to a ControlTemplate Trigger on IsHighlighted. The problem is that when the mouse is over the transparent part of the MenuItems the Trigger is not working. If I change the background color of either the ContextMenu, or its MenuItems to something other than Transparent the trigger works. I thought a Transparent Background was supposed to behave like a solid color, but in this case it is not. Any ideas how to make this work?
<Window x:Class="TransparentContextMenu.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Height="600" Width="800">
<Window.Resources>
<ControlTemplate x:Key="contextMenuItemTemplate" TargetType="{x:Type MenuItem}">
<Grid>
<Polygon Points="0,0 180,0 180,30 0,30" Fill="Transparent" x:Name="Background"/>
<Polygon Points="0,0 180,0 180,30 0,30" Opacity="0" x:Name="HoverBorder" Fill="Gray"/>
<ContentPresenter ContentSource="Header" HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsHighlighted" Value="true">
<Setter Property="Opacity" Value="1" TargetName="HoverBorder"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style x:Key="contextMenuStyle" TargetType="{x:Type ContextMenu}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ContextMenu}">
<Border Background="Transparent">
<StackPanel IsItemsHost="True"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid Background="Transparent">
<Grid.ContextMenu>
<ContextMenu Style="{StaticResource contextMenuStyle}">
<MenuItem Header="Item A" Template="{StaticResource contextMenuItemTemplate}"/>
<MenuItem Header="Item B" Template="{StaticResource contextMenuItemTemplate}"/>
<MenuItem Header="Item C" Template="{StaticResource contextMenuItemTemplate}"/>
<MenuItem Header="Item D" Template="{StaticResource contextMenuItemTemplate}"/>
<MenuItem Header="Item E" Template="{StaticResource contextMenuItemTemplate}"/>
</ContextMenu>
</Grid.ContextMenu>
</Grid>
</Window>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
}

Answer 1

Try setting background to 'null' instead.