Note that there's a similar bug at the beginning of the file--if the very first line is blank and you try to edit a rule that starts on the second line, you'll see the blank line above it. I can't figure out a way to hide that properly given CM's current behavior. I have a question out to Marijn on the CM Google Group about it. In any case, that edge case isn't likely to be hit often. (The only reason we're hitting this one now is that we happen to add rules at the end of a document with a blank line after them.)