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: XLIB_SKIP_ARGB_VISUALS=1 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
Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.