title: WPF BookMark/Indexes support on FixedDocument
body: <p>In our application that is using <strong>Fixed document</strong> and richtextbox for text boxes, we're trying to add bookmarks and indexes. For that we're relying on Tag property on runs class to store the metadata i.e whether a run is bookmark/index information is stored in the tag property. But these tags gets deleted when we apply some text attributes over a selection having some run with bookmarks and some normal runs without bookmarks/indexes.</p>
  
  <p>Is there any way we could restrict the deletion of our tag data? The tags are user data and it should not be deleted without any notification (callback), do we have any callback to notify such deletions?</p>
  
  <p>We are stuck with tags. Any lead or alternate solution would be appreciated.</p>