On my originally-gaming model computer with Nvidia RTX 2070, running Ubuntu 18 and Xfce everything runs smoothly, but when I plug in a HDMI monitor (model LG Flatron E2281, nothing special) I cannot select it.

For the screen I am using another graphics card than the Nvidia: it's an Intel UHD 630, 3e9b 'coffee lake', says hardinfo. The Nvidia card is mainly for using with cuda.

The 'displays' dialog appears but the extra display is not in there (so I can only select the existing situation). The situation is similar to this one but the drivers work for everything (including cuda etc) and I'm not using GDM but Xfce.

The combination of getting the notification of the display, but not the option of using it is strange? Also xrandr does not show extra screens. The only test that I could come up with was doing

for (( ;; ));do xrandr >> /tmp/test.txt; sleep .1; done

while switching the screen on. The only screen that shows up is screen 0 which is e-DP1, the laptop screen.

Could this be an Xfce specific issue?

EDIT: it could also be an issue with this Intel video card, Linux and HDMI, as suggested by this post on a Mint forum, which suggests adding i915.alpha_support=1 to the boot parameters.

It does look like something that will stop the Nvidia card working (as in: installing nouveau).

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