To replicate this (as it is rather difficult to explain) go to http://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx
Click on the HTML tab and replace all the content with:

<div>
  <div>
    <div style="width: 100%;">foo bar
    </div>
  </div>
</div>

Click back on the Design tab and click at then end of the first line.  When you press enter, the new line will be filled with non breaking spaces equal to the amount the inner-most div was indented.