I want to correct my initial take above. The JavaScript code I was looking at actually appears to show/hide children in threads.

The indentation appears to be handled separately by invisible GIFs:

  <td class="ind" indent="0">
    <img src="s.gif" height="1" width="0">
  </td>

  <td class="ind" indent="1">
    <img src="s.gif" height="1" width="40">
  </td>

  <td class="ind" indent="2">
    <img src="s.gif" height="1" width="80">
  </td>