I just read Martin Graesslins blog about fixes in 4.9.2 (*) and stumbled upon this issue https://bugs.kde.org/show_bug.cgi?id=306260 I could not see that it is fixed in gentoo (I did not get this freeze but could not find any patches/rebundled source tarballs), so I better report it. (*) http://blog.martin-graesslin.com/blog/2012/09/this-week-in-kwin-2012-week-36/
05 Sep 2012; Johannes Huber <johu@gentoo.org> +files/kwin-4.9.1-tab-freeze.patch, +kwin-4.9.1-r1.ebuild, -kwin-4.9.1.ebuild: Revision bump adds upstream patch which fixes an regression, that can causes freezes on tabbing by packagers ml.
Oops, sorry :/ I even synced, but then was searching on the PC I was SSHed in - with a too old tree :(