Sorry, english is not my native langage and I don't know witch product is affected by this problem. I use gnumeric on a platform with kubuntu 18.04 (beta1) ant plasma 5.12.3. Using plasma session, all seems ok. Using plasma WAYLAND session, gnumeric have no "windows" arround the program. So i can't change the size and to close gnumeric, I have to use the main menu.
This is a bug in GTK.