nvidia - Ubuntu 16.04; external monitor; after reboot, no longer working as dual monitor, only working as mirrored - Ask Ubuntu

i bought dell monitor yesterday. when hot-plugged in alienware 15 r3 through hdmi cable, running ubuntu 16.04 nvidia g-force 1070, , nvidia's driver, worked in expanded display mode expected.

however, when rebooted system, built-in display , external dell monitor merged 1 display, not use mouse move or manipulate windows, or menu, poor resolution. eventually, after repeated reboot, sometimes, got displayed mode of mirrored external monitor showed same content built-in monitor, rendering useless requirement. if used system settings -> displays panel, external monitor plugged in, automatically configured mirrored display mode, if explicitly unchecked option of "mirrored", automatically revert mirrored. sometimes, saw error message, "failure crtc 520" (the exact content, don't remember now.)

i tried delete ~/.configure/monitors.xml , logout, , re-login, without improvement.

i had removed nouvean drivers, , installed latest nvidia drivers, before plugging new external monitor. without external monitor connected, has been working fine.

it seems problem similar reported dual monitor on ubuntu 16.04.

it similar this: external monitor self-acting reset mirrored monitors under 16.04.

i tried suggestion listed here setting dual monitors in ubuntu (my laptop , 1 external monitor)

it made situation worse. after executing command:

sudo nvidia-xconfig 

i have external monitor working, built-in 1 either not visible, or 640x480 resolution, , mirrored of external one.

it seems me might ran bug of ubuntu 14.04 reported here:

https://bugs.launchpad.net/ubuntu/+source/nvidia-prime/+bug/1394665 

(there many dual/external display issues reported in forum, few has useful answer.)

i'm new ubuntu, please give me pointers on how investigate , resolve problem.

thanks in advance!

here work-around found working. before booting up, remove hdmi cable external monitor, boot built-in display, plug in hdmi cable, turn on power external monitor, never use nvidia x sever setting app, expanded display work expected.

must removing cable monitor. removing cable alienware laptop, while connecting monitor, without power supply monitor, still let external monitor influence booting procedure, causing above mentioned display problems.

nvidia x server setting app has been found messing display setup.

resuming suspend, should still work, without removing cable.