title: WPF DataGridCell watermark style. Cannot edit the cell
body: <p>I wanted to add a style to a <code>DataGridCell</code> in my app to have the effect of a watermarked "hint" in case the cell is empty to give the user a hint about what he needs to type in the cell.  </p>
  
  <pre><code>&lt;Window.Resources&gt;
      &lt;Style x:Key="WatermarkTextBox" TargetType="{x:Type DataGridCell}"&gt;
          &lt;Setter Property="Template"&gt;
              &lt;Setter.Value&gt;
                  &lt;ControlTemplate TargetType="{x:Type DataGridCell}"&gt;
                      &lt;Grid&gt;
                          &lt;Border x:Name="BorderBase" Background="White" BorderThickness="1.4,1.4,1,1" BorderBrush="Red"&gt;
                              &lt;Label x:Name="TextPrompt" 
                              Content="{Binding RelativeSource={RelativeSource  Mode=TemplatedParent}, Path=Tag}" 
                              Background="{TemplateBinding Background}" Visibility="Visible" 
                              Focusable="False" Foreground="Silver"/&gt;
                          &lt;/Border&gt;
                          &lt;!--&lt;ScrollViewer Margin="0" x:Name="PART_ContentHost" Foreground="Black"/&gt;--&gt;
                      &lt;/Grid&gt;
                      &lt;ControlTemplate.Triggers&gt;
                          &lt;MultiTrigger&gt;
                              &lt;MultiTrigger.Conditions&gt;
                                  &lt;Condition Property="IsFocused" Value="False"/&gt;
                                  &lt;Condition Property="Content" Value=""/&gt;
                              &lt;/MultiTrigger.Conditions&gt;
                              &lt;Setter Property="Visibility" TargetName="TextPrompt" Value="Visible"/&gt;
                          &lt;/MultiTrigger&gt;
                          &lt;Trigger Property="IsFocused" Value="True"&gt;
                              &lt;Setter Property="BorderBrush" TargetName="BorderBase" Value="Black"/&gt;
                              &lt;Setter Property="Visibility" TargetName="TextPrompt" Value="Collapsed"/&gt;
                          &lt;/Trigger&gt;
                          &lt;Trigger Property="IsEnabled" Value="False"&gt;
                              &lt;Setter Property="Foreground" Value="DimGray" /&gt;
                          &lt;/Trigger&gt;
                      &lt;/ControlTemplate.Triggers&gt;
                  &lt;/ControlTemplate&gt;
              &lt;/Setter.Value&gt;
          &lt;/Setter&gt;
      &lt;/Style&gt;
  &lt;/Window.Resources&gt;
  </code></pre>
  
  <p>And this is my code:  </p>
  
  <pre><code>Style style = this.FindResource("WatermarkTextBox") as Style;
  myCell.Tag = "input position here please";
  myCell.Style = style;
  </code></pre>
  
  <p>And it works fine. When this code is executed, the watermark style is applied correctly. But when I focus on the cell, I cannot write anything.<br>
  I mean, the "<code>IsFocused</code>" trigger is executed and the <code>Label</code> is collapsed (it disappears) but I cannot input anything inside the cell. </p>