If I open in Systemsettings in "Display and Monitor" the "Compositor" tab, the systemsettings window doesn't show the content until I resize the window. Tested on a Intel and AMD GPU. Plasma 5.11.4 Frameworks 5.41 Qt 5.10
Reproducible even with master (future Plasma 5.12).
This is even worse as you even cannot change a thing in that window. E.g. opening KScreen configuration when I choose a different primary screen, then the combobox doesn't repaint and keeps showing initial value. Another example is opening KCM for network connections, where you cannot switch connections or config tabs as the content doesn't get repaint. This seems to be somehow affecting KCMs which are using both QML and QWidgets (KScreen and plasma-nm KCM). When I open them separately using kcmshell5, then both mentioned KCMs work without any problem.
Another insteresting issue connected to thie behaviour is that sometimes such misbehaving systemsettings is leaking parts of its gui through other windows even if it is under them (visual flashes). Similar issues are with e.g. kmail. I thought that is is because of qt5-webengine but I'm not so sure right now. If these apps (kmail, systemsettings5) are run with QT_QPA_PLATFORM=xcb -> mentioned problems are gone.
For me this has been fixed with Plasma 5.12.3. Based on the release notes I think it was this change https://phabricator.kde.org/D11045. Thanks