title: PreviewKeyDown is not seeing Alt-modifiers
body: <p>I have some code which is (supposed to be) capturing keystrokes. The top level window has a</p>
  
  <pre><code>Keyboard.PreviewKeyDown="Window_PreviewKeyDown"
  </code></pre>
  
  <p>clause and the backing CS file contains:</p>
  
  <pre><code>private void Window_PreviewKeyDown(object sender, KeyEventArgs e) {
      if (e.KeyboardDevice.Modifiers == ModifierKeys.Control) {
          switch (e.Key) {
              case Key.L:
                  btnPrev_Click(sender, new RoutedEventArgs());
                  e.Handled = true;
                  break;
              case Key.R:
                  btnNext_Click(sender, new RoutedEventArgs());
                  e.Handled = true;
                  break;
          }
      }
  }
  </code></pre>
  
  <p>Now that works fine, both <kbd>CTRL</kbd><kbd>L</kbd> and <kbd>CTRL</kbd><kbd>R</kbd> call the relevant functions.</p>
  
  <p>As soon as I change the modifer check to use <code>ModifierKeys.Alt</code>, it stops working. In other words, neither <kbd>ALT</kbd><kbd>L</kbd> and <kbd>ALT</kbd><kbd>R</kbd> call the functions.</p>
  
  <p>What am I missing here?</p>