In 6.8.0 kuickshow will only show a blank window.
Actually, it happens only if the Composite extension is enabled.
With or without xcompmgr actually running.
This is a result of kuickshow (or whatever library it uses to draw) not
handling the ARGB visuals exported by Composite correctly.
As a workaround, for now you can set the environment variable:
before running kuickshow. In the long run, kuickshow (or the library it uses)
needs to be rewritten to properly handle the composite visual.
See the KDE bug here: http://bugs.kde.org/show_bug.cgi?id=89313