title: Datagrid SelectedItem lost on window.IsEnabled = false
body: <p>I have a (main)<code>Window</code> containing a <code>Frame</code>. A <code>Page</code> is loaded to that <code>Frame</code>, with some viewmodel as its datacontext. </p>
  
  <p>The View has some datagrids, where one is bound to the viewmodel and the other is bound to the selected.item (so that you get a master-details view..)</p>
  
  <p>the problem occurs when I display a popup box, and set the mainwindow IsEnabled = false. When that happens, the selected item from the datagrid bound to the viewmodel gets unselected, and of course the other datagrids selected items get unselected as well. </p>
  
  <p>How can I preserve the "state" of the UI when my mainwindow.IsEnabled get set to false?</p>