Setting default display mode for additional monitor

Question

I have 2 monitors, but my second display mirrors my first display, instead of extending it.

Pressing super+p fixes this - but I have to press it every single time I boot.

How do I set the default behaviour?

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When I press super+p to fix the problem my xrandr outputs the following:

Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 598mm x 336mm
   1920x1080     60.00*+
   1680x1050     59.95  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1280x800      59.81  
   1280x720      60.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   640x480       75.00    72.81    59.94  
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DVI-D-0 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 598mm x 336mm
   1920x1080     60.00*+
   1680x1050     59.95  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1280x800      59.81  
   1280x720      60.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   640x480       75.00    72.81    59.94  

The difference between this output and the wrong one are these lines:

Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 16384 x 16384

...

DVI-D-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 598mm x 336mm

Answer 1

screen setup should be remembered, but in cases like this, it is mostly the graphics driver and the screen that do not communicate accurately in an early stage after log in, causing the screen setup jump back to default.

How to solve

It can easily be solved by running an xrandr command a few seconds after log in, and both screens are recognized.

Add the following command to Startup Applications: Dash > Startup Applications > Add:

/bin/bash -c "sleep 10 && xrandr --output DVI-I-1 --pos 0x0 && xrandr --output DVI-D-0 --pos 1920x0"

Notes

Please first test the command

xrandr --output DVI-I-1 --pos 0x0 && xrandr --output DVI-D-0 --pos 1920x0

before adding it to startup applications to make sure I made no typos.

Explanation

See this answer for an extended explanation.