title: What is the difference between ITrackingPoint, ITrackingSpan, SnapshotPoint, SnapshotSpan, ITextViewLine and when to use?
body: <p>I'm trying to figure out how to get the visible lines in a IWpfTextView to place a ViewPort and/or Text Adornment based on visible lines.</p>
  
  <p>It seems that these 4 are involved in some way, at least the Spans.  And to make sure I understand, a Span is just a series of characters in the TextView right?  </p>