title: WPF RichTextBox's conditionally scroll?
body: <p>I've a <code>RichTextBox</code> in my app which is getting new content on certain events.<br>
  When new content is added, I'd like to scroll to the bottom, <strong>only if</strong> the scroll was at the bottom <strong>before</strong>.<br>
  How do I do this?<br>
  More specifically, the part that gives me trouble is determining the scroll position.  </p>
  
  <p>If it matters, the <code>RichTextBox</code> is using the default style and template, a few brushes changed or set to null, vertical scrollbar visibility is Auto and it's read-only.</p>