title: Item replacement in ObservableCollection doesn't update WPF ItemsControl
body: <p>I have (I thought) a fairly conventional WPF app.</p>
  
  <p>I've </p>
  
  <pre><code>&lt;ListBox ItemsSource="{Binding MyList}" ...
  </code></pre>
  
  <p>MyList is an </p>
  
  <pre><code>ObservableCollection&lt;MyItem&gt;
  </code></pre>
  
  <p>MyItem does not implement <code>INotifiyPropertyChanged</code></p>
  
  <p>when I <code>Add</code> items to the OC, the UI updates with out issue.
  When I <strong>replace</strong> items, via </p>
  
  <pre><code>MyList[index] = newItem;
  </code></pre>
  
  <p>the UI only updates occasionally.
  <strong>Sometimes</strong> selecting another control on the window helps, but often not.Updates are arriving at about 5 per second. If it matters, this is WPF 4 on a Win7 x64 machine.</p>