fcitx5-qt is the Qt im-module for fcitx5 and it needs to be installed in order to use fcitx5 with Qt-based applications. It currently supports Qt4, Qt5 and Qt6. You'll need several Qt modules if you're going to enable Qt5/Qt6 support, namely QtCore, QtDbus, QtWidgets and QtX11Extras. Qt5 is bundled in Slackware, and if you want Qt6 support you're on your own; but any app that requires Qt6 on SBo at this point should have already pulled all the dependencies in. Relevant CMake options are ENABLE_QT4, ENABLE_QT5 and ENABLE_QT6. Only ENABLE_QT5 is turned on by default. You might want to turn the flags on/off depending on the Qt version you are using. To enable Qt4, you need to modify -DENABLE_QT4=Off \ from "Off" to "On". Similarly, to turn on Qt6 support, turn -DENABLE_QT6=On \ -DENABLE_QT6_WAYLAND_WORKAROUND=On .. from "Off" to "On".