title: WPF Window Drop-Shadow effect has square corners
body: <p>I've been trying to get this drop shadow effect to work, but I don't know whats wrong. </p>
  
  <p>I've tried giving the same effect to <code>Grid</code> and <code>Border</code>, but they all give the same effect.</p>
  
  <p><img src="https://imgur.com/5GE0e1W.png" alt="Example"></p>
  
  <pre><code>&lt;Window x:Class="New_EZexeat.ReturnMessageDialogueBox"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
          xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
          xmlns:local="clr-namespace:New_EZexeat"
          mc:Ignorable="d"
          WindowStyle="None" WindowStartupLocation="CenterScreen" AllowsTransparency="True" Background="Transparent"
          Title="ReturnMessageDialogueBox" Height="150" Width="300"&gt;
      &lt;Window.Effect&gt;
          &lt;DropShadowEffect Direction="-75" ShadowDepth="2" Opacity="0.8" BlurRadius="25" Color="Black"/&gt;
      &lt;/Window.Effect&gt;
  
  
      &lt;Grid Background="Transparent"&gt;
          &lt;Border Background="White" CornerRadius="20"&gt;
              &lt;StackPanel HorizontalAlignment="Center" VerticalAlignment="Center"&gt;
                  &lt;TextBlock x:Name="itstext"
                             FontSize="15"
                             FontWeight="SemiBold"
                             Margin="0 0 0 10"
                             TextAlignment="Center"&gt;&lt;Run Text="Username or Password is incorrect."/&gt;&lt;LineBreak/&gt;&lt;Run Text=" Please try again"/&gt;&lt;/TextBlock&gt;
  
                  &lt;Button Content="OK"
                          Style="{StaticResource OrangeButtonTemplate}"
                          FontWeight="SemiBold"
                          FontSize="20"
                          Background="#FFFFD411" 
                          Margin="20 0 20 0"
                          IsCancel="True"
                          FocusVisualStyle="{x:Null}"
                          BorderBrush="{x:Null}" 
                          Height="45"/&gt;
              &lt;/StackPanel&gt;
          &lt;/Border&gt;
  
      &lt;/Grid&gt;
  &lt;/Window&gt;
  </code></pre>