I use KDE Plasma and recently I noticed that the drop-down menu in the system configuration application is different. Usually When I click on a menu it slides to the side and its submenu takes its place. Now the menu remains and the submenu is displayed next to it. I searched everywhere and couldn’t find how to reset it. Does anyone know how?
Try resizing the application window a little. When wide enough to provide the space, the menus will do what you describe. It probably won’t take very much to make it just narrow enough to collapse it the way you want.