System settings -> Application Style -> Window decorations Change from Breeze to Plastik, hit Apply. Hit " < All Settings " (back) button. Now the visual content does not change, but mouse cursor is working with [invisible] "all settings" page. Either it is rendered under the window decorations graphics (which is not interactive any more, but maybe actively rendered above?), or nothing is rendered at all and the window decorations is just remaining artefact? Restarting system settings will fix it.
Is this Wayland or X? Can you provide a screenshot?
I guess I'm on X? (in `ps` output "/usr/lib/xorg/Xorg -nolisten tcp ..."). I will add final state screenshot, showing screen after hitting the back button (" < All settings "), where you can see the main window area keeps it's rendering from the Style->window decorations tab, but the mouse cursor shows active hint for the "style" icon, which was supposed to be under cursor on the all-settings page. If I click it blindly, it goes to the style settings (active areas for mouse are different), but rendering is still stuck on the window decorations tab. I.e. the app is active and works, even some visuals works (like the mouse-over hint), but the main window content is stuck.
Created attachment 110389 [details] Screenshot after the system settings window content gets stuck, but the mouse-over hint works
If bug occurs on Wayland - maybe duplicate of https://bugs.kde.org/show_bug.cgi?id=385921
With KDE plasma 5.14.4, frameworks 5.52.0, Qt 5.11.2 on kernel 4.19.5: I can NOT reproduce this any more. Can't tell exactly which update fixed it, as I did try it first time now after reporting it, and there were several versions of everything between.
ok, please reopen if it reappears.