I just bought a Dell Monitor yesterday. When I hot-plugged in with my Alienware 15 R3 through HDMI cable, running Ubuntu 16.04 with NVIDIA G-Force 1070, and Nvidia's driver, it worked in expanded display mode as expected.
However, when I rebooted the system, the built-in display and the external Dell monitor were merged into one display, I could not use mouse to move or manipulate the windows, or menu, with poor resolution. Eventually, after some repeated reboot, sometimes, I got into the displayed mode of mirrored that the external monitor showed the same content as the built-in monitor, rendering it useless for my requirement. If I used System Settings -> Displays panel, with the external monitor plugged in, it would automatically configured as mirrored display mode, even if I explicitly unchecked the option of "mirrored", it would automatically revert back to mirrored. Sometimes, I even saw an error message, something like "failure with CRTC 520" (the exact content, I don't remember now.)
I also tried to delete ~/.configure/monitors.xml and logout, and re-login, without any improvement.
I had removed the Nouvean drivers, and installed the latest Nvidia drivers, before plugging the new external monitor. Without the external monitor connected, it has been working fine.
It seems that my problem is similar to that reported Dual monitor on Ubuntu 16.04.
It is also similar to this: external monitor self-acting reset to mirrored monitors under 16.04.
I also tried the suggestion listed here Setting up dual monitors in Ubuntu (my laptop and one external monitor)
It made situation even worse. After executing the command:
I could only have my external monitor working, the built-in one was either not visible, or only 640x480 resolution, and mirrored of the external one.
It seems to me that I might ran into a bug of Ubuntu 14.04 reported here:
(There are also many dual/external display issues reported in this forum, but few has any useful answer.)
I'm new to Ubuntu, please give me some pointers on how to investigate and resolve the problem.
Thanks in advance!