title: How to avoid an open Silverlight ComboBox DropDown to be on top of everything
body: <p>I have a ComboBox which I managed to keep its DropDown open after LostFocus. It's populated with a number of CheckBoxes. Whenever the user toggles one of these CheckBoxes, the whole application is updated by calling a DataService. During this time I'm blocking further user input using a transparent overlay rectangle (on top of the whole Silverlight content) with a wait cursor.</p>
  
  <p>I tried increasing the Canvas.ZIndex of that Rectangle to get it on top of everything. But the open DropDown of my ComboBox is always on top of it.</p>
  
  <p>A very much simplified example shows what I mean:</p>
  
  <pre><code>&lt;UserControl x:Class="ComboDropDownTest.Page"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  Width="400" Height="300"&gt;
  &lt;Grid x:Name="LayoutRoot" Background="White"&gt;
  &lt;ComboBox Canvas.ZIndex="0" Height="40" VerticalAlignment="Top"&gt;
      &lt;ComboBoxItem Content="ComboBoxItem1"/&gt;
      &lt;ComboBoxItem Content="ComboBoxItem2"/&gt;
      &lt;ComboBoxItem Content="ComboBoxItem3"/&gt;
      &lt;ComboBoxItem Content="ComboBoxItem4"/&gt;
  &lt;/ComboBox&gt;
  &lt;Button Canvas.ZIndex="9999" Height="55" Margin="18,66,19,0" VerticalAlignment="Top" Content="Button"/&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p></p>
  
  <p>When the ComboBox is open, the DropDown covers the Button ignoring the ZIndex order.</p>