The gtk2_preview and gtk3_preview helper applications are installed into ${LIBEXEC_INSTALL_DIR}, but looked up with QStandardPaths::findExecutable, which only looks at qgetenv(PATH). The libexec directory is not part of PATH so the previews are not available.
Git commit fdb0df568637358d27e2dd315eda2aa32d2d551b by Aleix Pol. Committed on 09/08/2017 at 16:17. Pushed by apol into branch 'master'. Fix look-up of gtk preview modules M +2 -0 CMakeLists.txt A +1 -0 config.h.cmake M +1 -1 gtk3proxies/CMakeLists.txt M +1 -1 gtkproxies/CMakeLists.txt M +1 -1 gtkproxies/preview.c M +2 -1 src/appearancegtk2.cpp M +3 -2 src/gtkconfigkcmodule.cpp M +1 -0 tests/CMakeLists.txt https://commits.kde.org/kde-gtk-config/fdb0df568637358d27e2dd315eda2aa32d2d551b