title: On Windows 10 (1803), all applications lost touch or stylus if a WPF transparent window covers on them
body: <p>If I create a new WPF application with a simple empty window like the code shown below, I find that all applications which are covered by the WPF app lost touch or stylus reaction. This can only be reproduced when Windows 10 is upgraded to 1803 (10.0.17134.0).</p>
  
  <pre><code>&lt;Window x:Class="TheWPFCoveringWindow.MainWindow"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          WindowStyle="None" WindowState="Maximized"
          AllowsTransparency="True" Background="Transparent"
          Topmost="True"&gt;
      &lt;Button Content="Test" Width="200" Height="100" /&gt;
  &lt;/Window&gt;
  </code></pre>
  
  <p>I wrote another WPF application to find out what happened. So I add a StylusDown event to the Window like the code shown below:</p>
  
  <pre><code>// This code is in another WPF application.
  private void OnStylusDown(object sender, StylusDownEventArgs e)
  {
      // Set a breakpoint here.
  }
  </code></pre>
  
  <p>But the breakpoint never reached until I closed the transparent WPF window which is on top.</p>
  
  <p>I pushed the very simple code to GitHub: <a href="https://github.com/dotnet-campus/TouchIssueOnWindows10.0.17134" rel="nofollow noreferrer">dotnet-campus/TouchIssueOnWindows10.0.17134</a>. Cloning it might help a little.</p>
  
  <p>Why does this happen and how to solve it? Any reply is appreciated.</p>