two monitor system. Using:
'xrandr --output DP-0 --mode 3840x2160 --pos 2880x0 --output DVI-I-1 --mode 1920x1200 --scale 1.5x1.5'
to enable both display. To have similar font sizes on DVI-I-1 scaling used.
The primary monitor left x location starts at offset 2880 pixels (1920* 1.5= 2880). The secondary display with the lower resolution is placed left of the primary display.
Dolphin opens on secondary display. After moving it to the primary display (DP-0) mouse actions (eg. chose a file) are ignored (menu items still accessible). Move it back to the secondary display returns to normal behaviour. I guess the mouse pointers location event returns a wrong value not taking into account the x- offset (2880 in this case) of the display.