I am using a ThinkPad X1 with an Intel HD Graphics 520 card and have installed Ubuntu 16.04 (and upgraded linux kernel to 4.4.21). I installed ubuntu-gnome-desktop, then configured gdm as the graphics manager instead of lightdm.
Additionally, I followed the steps described here to download the Intel installer for Ubuntu 16.04, from which I updated the driver and rebooted. This did not change anything with the inability to use the second display and did not cause
xrandr to begin detecting the HDMI port.
I am unable to get the system to detect a second display via any connections. I've tried through the OneLink dock via DislayPort cables. I've tried a direct mini DisplayPort to a DisplayPort on the monitor. And I've tried with an HDMI cable between the two. In all cases, I can't detect the external monitor, and
xrandr only lists a single screen, 0, corresponding to the LCD screen on the laptop itself.
If I go into BIOS and I change the boot display setting so that HDMI is the boot display, then the HDMI connection does work, and the system boots and displays on the external monitor. But if I unplug the monitor, the built-in LCD screen on the laptop does not turn on, and remains black. And I still cannot detect any secondary display, not even the built-in screen.
How can I support both monitors (built-in screen and external display)? The system does not detect additional displays at all.
Previously I had tried poking around with nvidia settings and drivers, but this did not help (and let to frustrations with the nvidia driver login loop problem). It's an Intel graphics card, as mentioned earlier.
Basically, the output of
xrandr --prop worries me the most, since it does not detect any other displays apart from whatever is chosen as the boot display. It should show that there is a dock DisplayPort connection, a mini DisplayPort connection, and an HDMI connection all on this machine, all disconnected (or something). But it doesn't show them at all -- leading me to believe the graphics card itself is configured in some way that only permits a single display.