Editing a long post seems to be very painful. This could even prevent many - at least users who are new to the editor - not to edit anything complex anymore.
You need to double scroll - scroll between source and result pane, and scroll inside source pane. Why not use left-right layout?
Why not provide a source-result synchronization feature? Which means, whenever you click somewhere in the source or result pane, the other is auto scrolled (need left-right layout to do this) and the corresponding place is high lighted.