Ref T2009. Currently, the code figures out if a comment is on the left or right by looking at the <th /> preceeding the enclosing <td />.
This gets the right result in 2-up, but in 1-up rows are always <th />, <th />, <td />, so it always detects every inline as being in the new file.
Because "old" and "new" cells aren't inherently distingushable in the 1up view, we can't use a DOM test for this at all. Instead, just track this state explicitly.