Undo in main editor after edit in linked subdocument causes corruption

[njx]

Tested in Mac Chrome 24.0.1312.57

Run the gist at https://gist.github.com/njx/4964012 (next to the lib folder)

Result: content is out of order after the undo. The exact corruption that happens seems to depend on where the edit occurs. Also, interestingly, if the linked subdocument only goes from line 1 to line 3, the bug doesn't happen.