Hi Pierre, here is a video that will help as I know it's hard to reproduce things.  It shows issues logged elsewhere in mantis, in order of appearance in the vid:

a) Sometimes when you create a new item it double spaces by default (as if it's HTML), other times not. 
b) Hitting tab goes down a line, so does shift-tab.
c) inserting a table causes double spacing also.

You are right that hitting 'div' gets rid of them- unless there's a reason not to, we should make div's the default as it's in line with what a user would expect by default.

Hope it helps!