Comment 11 for bug 1293252

Then you can try to remove workspace switcher and/or show desktop applets - they are libwnck applets too.

Can you report this bug upstream?
https://bugzilla.gnome.org/enter_bug.cgi?product=libwnck

pixman errors are fixed in git master:
https://git.gnome.org/browse/libwnck/commit/?id=2fc0f058f00daf76c06e485c9d39491513f48345