title: Whole line highlighting in AvalonEdit
body: <p>I'm creating a view for patch files using AvalonEdit, and I want to make it so that diffs are highlighted across the entire line, not just the text background - similar to what GitHub for Windows does today:</p>
  
  <p><img src="https://cl.ly/image/0h3A1Z3I3z2K/content#png" alt=""></p>
  
  <p>I'm new to AvalonEdit, so I'm not sure the best way to do this. Here's what I've found so far:</p>
  
  <ul>
  <li><p>Override <code>VisualLineElementGenerator</code> in order to create an additional TextSpan that is the length of the control. This seems Tricky.</p></li>
  <li><p>Create a new control to add to <code>TextView.Layers</code> in the background and OnRender in the green/red by hand - this seems more promising, but it's not super clear what event I need to hook in order to detect when to re-render.</p></li>
  <li><p>Override TextView - this seems like overkill.</p></li>
  </ul>
  
  <p><strong>Edit:</strong> Here's what happens with a simple syntax highlighter, which is what I <strong>don't</strong> want:</p>
  
  <p><img src="https://cl.ly/image/2j0O2w1V331U/content#png" alt=""></p>