I cannot display the drop-doown-menus of any KDE applications like kile, okular texmaker. However, I can click on the menus, but it does not show all the elements that it should show. ubuntu 18.04.1 (gnome3) LTS
Any hint is appreciated!
Solved: There was a problem with the display that I do not know why prevented to correctly display the menus. So I turned off one of the 3 monitor in the setting displays menu and then it worked! Thanks cadash!!!
Just in case, I would try setting colors, window style and window decoration to defaults. Since you mention it happenned during a system upgrade, also try doing:
sudo apt-get update --fix-missing
If it installs anything, reboot and give it a try. If none of this works and you can backup your personal info, doing a fresh install might be the best option as you might encounter additional issues caused by the upgrade in the future.