While messing with the dekstop containment layouting mechanism I found that it would re-layout sometimes without obvious reason; closing some applet popups (e.g. kickoff) would cause a re-evaluation of the preferredWidth of the panel which eventually would end up calling setLength with a slightly different size causing us to emit this signal.
Not completely sure why that is, perhaps some Layout weirdness but in any case this is an optimization that makes sens imho.