When I set the primary screen to my internal laptop screen, ubuntu uses the external monitor as primary screen and vice versa. When I unplug the external screen, I have no primary screen left.
In more detail:
I recently installed ubuntu 16.04 on a new laptop, using kde, and copied most of my settings and files from lower versions on an older laptop (14.04 was the last one I used). (My system language is German, so I am only guessing how things are called in English.)
I sometimes use an external monitor as a second screen. Whenever I activate it, the primary screen switches to the external monitor, meaning the launcher and task bar are shown on the external and all newly opened applications are shown there. This was not to big an issue as it reverted back when I unplugged the external screen and changed it back to only one monitor. Changing the setting for the primary screen in the kde settings window seemed to have mostly random effect.
Since a few days now I cannot get it to work with only the laptop screen anymore. No matter what I tried, I can only get the launcher and task bar to show on the laptop screen, when the external monitor is plugged in. I tried rebooting with and without external monitor plugged in, did not help.
I tried changing the setting for primary screen in the settings window as well as using xrandr, for both I found the same effect:
This is the result when I check for connected monitors (only copied connected ones):
$ xrandr | grep connected eDP1 connected 1366x768+()+() (normal left inverted right x axis y axis) 344 mm x 194 mm HDMI1 connected 1920x1080+()+() (normal left inverted right x axis y axis) 160mm x 90 mm
When I use
xrandr --output eDPI --primary
the primary screen (launcher, taskbar, ...) switches to the external monitor.
xrandr --output HDMI1 --primary
Makes the primary screen switch to the laptop screen. Once, I unplug the external monitor now, I have no primary screen left.
I would like to fix the laptop screen as primary, no matter what else I use as secondary screen.