Rendering problems in kwin and active

[Marco Martin]

Hi all,

Recently some rendering problems are starting to appear in KWin but just in 
Plasma  Active:
http://wstaw.org/m/2013/01/03/plasma-desktopeW5084.png

the windows strip leaves a trace of old pixmaps when the user scrolls.
this happens both when composite is on or off above is a screenshot without 
composite in VirtualBox, it happens also on device both with composite enabled 
or disabled (but it seems the problem doesn't occur if composite is enabled 
but using xrender)

i didn't manage to reproduce it so far on any system that is not Mer, so some 
hypothesis may be:
* something that happens only when built on mobile profile and as "kwinactive"
* a problem depending of the particular Qt version used in Mer
* graphics driver related (unlikely since happens in virtualbox as well?)
* other?

any ideas what it may be?

Cheers,
Marco Martin