strange freeze in active w/master kde-workspace

[Marco Martin]

Hi all,
we have a strange problem in active when building with kde-workspace master, 
built with mobile profile and -DKWIN_BUILD_WITH_OPENGLES=ON sometimes when 
clicking on a window switcher item (so KWinfdowSystem::activateWindow) a 
complete x freeze occurs (still possible to go out of it with ctrl+alt+f1 
tough)

what made me wonder if it was something kwin related, is that if after some 
time the system is running i do a kwin --replace or kwin_gles --replace with 
the new instance the problem disappears (happens for both kwin and kwin_gles 
when they are autostarted)
i tried to do something pretty blasphemous: configuring to autostart the xfce 
windowmanager instead of kwin, and also in this case the problem doesn't 
happen.

any idea what it could be?

Cheers,
Marco Martin