or to flip the default modes (shift+enter creates double space) then add the following lines to program/js/editor.js conf var starting at line 23
forced_root_block : false,
force_br_newlines : true,
force_p_newlines : false,
(note, remove that last comma if these are the final three conf entries)
(second note, I've only tested this in v0.8.5)
thanks Roundcube team, well done.