title: WPF popup location issue
body: <p>I am experiencing a strange WPF popup placement issue.</p>
  
  <p>I have defined this XAML: </p>
  
  <pre><code>&lt;Window x:Class="PositionBug.MainWindow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      Title="MainWindow" Height="300" Width="525"&gt;
  &lt;Grid&gt;
      &lt;TextBlock Name="Textblock1" Height="60" Width="300" Background="LightGray" HorizontalAlignment="Center"
                 VerticalAlignment="Bottom"&gt;The PlacementTarget&lt;/TextBlock&gt;
      &lt;Popup Name="Popup1" PlacementTarget="{Binding ElementName=Textblock1}" Placement="Top" Width="120" Margin="198,0,199,0" IsOpen="True"&gt;
          &lt;TextBlock Background="LightBlue" FontSize="18"&gt;This is a Popup&lt;/TextBlock&gt;
      &lt;/Popup&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p></p>
  
  <p>On most computers this is the result, as expected: 
  <img src="https://imgur.com/jcFJtw9.jpg" alt="correct"></p>
  
  <p>However, on multiple units of one specific computer model, the result is presented like this: 
  <img src="https://imgur.com/FOCyglT.jpg" alt="incorrect"></p>
  
  <p>Is there any way to force Placement to both Top AND Left?</p>