Hello. After I installed Nvidia 390 via mhwd I noticed that KDE settings did not have my display resolution so I had to set it manually via https://wiki.manjaro.org/index.php?title=Did_X.server_recognise_your_monitor_correctly%3F After that I can choose right resolution (1920 x 1080) but I noticed when I open any Kde panel or window it looks bigger and I think uses different resolution than my desktop, plus my cursor becomes bigger when I move it to any Kde window.
P.s. Is it right that xdpyinfo | grep -B2 resolution shows:
screen #0: dimensions: 1920x1080 pixels (345x194 millimeters) resolution: 141x141 dots per inch