After <http://trac.webkit.org/changeset/143299>, you can no longer edit text on in source code mode on the jWYSIWYG editor.
This was first reported to the Chromium project: https://code.google.com/p/chromium/issues/detail?id=246738
*** Bug 117888 has been marked as a duplicate of this bug. ***
A fix was landed in Blink.
(In reply to comment #3)
> A fix was landed in Blink.
Blink's fix was just to roll out r143299. I don't want to do that before taking the time to investigate the issue. We might be able to fix it without regressing webkit.org/b/110172.