I have the same problem.
at work i use one HDMI monitor with the laptop monitor as primary.
If i use OPERA on my laptop monitor when i go home i can use opera with no problem, but if at work i use OPERA on my external HDMI Monitor and then shutdown my laptop, when i will use it at home Opera will not open on the laptop screen... and it's not possible to move the window back to the laptop screen.
The solution to make the main display it's not working and let me say even if it works it's not possible for a user to change the main display.

I have the same problem.
at work i use one HDMI monitor with the laptop monitor as primary.
If i use OPERA on my laptop monitor when i go home i can use opera with no problem, but if at work i use OPERA on my external HDMI Monitor and then shutdown my laptop, when i will use it at home Opera will not open on the laptop screen... and it's not possible to move the window back to the laptop screen.
The solution to make the main display it's not working and let me say even if it works it's not possible for a user to change the main display.