Wikimedia Bugzilla is closed!

Problem: FCKeditorParser.body.php inserts unwanted "\n".
This destroys the page layout.

Test: go to http://mediawiki.fckeditor.net/Sandbox 
      and enter some lines of text

Now switch to [Rich Editor] mode and back [Disable rich editor] multiple times and see the additional unwanted line breaks and the self-modifying layout =:(

Solution: change FCKeditorParser.body.php, line 685 from 
  if( $paragraphStack === false ) {
    $output .= $t . "\n";
  }

into:

  if( $paragraphStack === false ) {
    $output .= $t;
  }

Note: This workaround does the job for me, but is not really tested in more detail.