See: http://wstaw.org/m/2012/12/05/plasma-desktopeX2766.png
Background is properly rendered, (unlike in your screenshot) and all buttons are present before hovering.
So that I am clueless.

Just a shot in the dark - Eclipse has a similar problem with menus (menus are not displaying properly until "walked over" by a cursor) under KDE with the compositing enabled. It is a GTK bug according to the Eclipse bug tracker. With the compositing/effects off, everything works OK. Could this be the same issue?

Jan