At work I have a docking station, that allows me to use two external screens with my laptop. And I find this extremely useful. I put a terminal window here, some source doe there, a similar source file over there, and I need to write a unit test while looking up things on Stack Overflow. So those three screens are easily filled.

Sometimes I work from home. I don't have a docking station at my place, and I can use only one external screen. This works as well, but often I miss the extra screen real estate. We do have a tablet though, mainly used by our children. Those are typically away or asleep when I code... So how difficult could it be to use the tablet as an external screen?

Of course this turned out to be way more difficult than I initially thought it would be. (A detail I might have failed to mention is that I am running Ubuntu 18.10 on my laptop.) It took some fiddling, but in the end I have a working system. It is not the world's most beautiful solution, but it will do for me.

Thank you, Mike

I found my inspiration in a blog post on Mike's Coding Oddities. It needed some tinkering, because I need a 3rd screen, Mr. Mike needs a 2nd screen.

The proposed solution does not work for Wayland. Which is a shame, becuase Waylands sounds so much cooler than the outdated Xorg. But if you want more screen space, you need to make sacrifices.

My configuration

The on-board screen of my laptop is 'called' eDP-1. The external screen is HDMI-1. I always put the external screen at the right side of my laptop, and I want the tablet to the right of the external screen. Both laptop and external screen have a resolution of 1920x1080. The tablet has a screen resolution of 1920x1200. Of which 1920x120 pixels will not be used because of the solution I choose, but it is the best I can do for now.

(Finding the names of the outputs, eDP-1 and HDMI-1, was not easy. I ended up installing and running arandr to find out how the outputs are called, but there are probably better ways to do this.)

The script

This is the script I use to get my setup right:

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#!/bin/bash

sudo xrandr --fb 5760x1080 --output HDMI-1 --panning 3840x1080+1920+0/3840x1080+1920+0
sleep 3
sudo xrandr --fb 5760x1080 --output HDMI-1 --panning 1920x1080+1920+0/3840x1080+1920+0
# WARNING! Unprotected server, connection not encrypted. Don't use as-is on
# a public or untrusted network!
x11vnc -clip 1920x1080+3840+0 -nocursorshape -nocursorpos

You need to use sudo. Omitting sudo does not produce error messages, but it will not work either.

As I understand it, the first line ensures that the screen space for my 3 screens all together becomes 5740x1080 pixels. The external screen (HDMI-1) became twice as wide (3840x1080), and is located to the right of the laptop screen (+1920+0).

If you only execute the first line of the script, the external screen 'slides' as a 'view port' over the wider logical screen when you move the mouse cursor to the left side or right side of the screen.

sleep 3 just waits for three seconds, and the next line limits the area of the logical screen that can be displayed on the physical screen by 1920x1080 pixels. That way, you can move your mouse cursor outside the visible part of the screen, without having the view port sliding.

At last I start a VNC server, to serve the invisible area of the external screen. The options -nocursorshape and -nocursorpos take care of the mouse cursor being controlled by the VNC server (laptop), and not by the client (tablet).

Now you need to install a VNC client on the tablet, to connect to your PC. I use VNC viewer, but other viewers are availabe.

Please note that the VNC connection created this way, is not secured, neither encrypted. That's fine for testing, or when you are in complete control of your network. But if that is not the case, use ssl to encrypt your connection, and put a password on your VNC connection. (I don't know yet how to tho this, but I guess if you look at the man page of x11vnc, you will find some hints if you search for -ssl or -rfbauth )

Room for improvement

This solution works readonably well, but it has its limitations. First of all, Linux handles the external screen and the tablet as one logical screen. The annoying consequence is that when you maximize a window, the window is scattered over both the external screen and the tablet. To work around this issue, you can - instead of maximizing a window - use 'dock left' and 'dock right'. Gnome has shortcut keys for this: super key + left arrow and super key + right arrow. For other display managers, this may vary.

I think there should be a way to create a real virtual screen for the tablet to display, so that its resolution, orientation and position can be configured independently of the other screens. A dummy video driver for xorg, exists, but I have not tried yet to make that work. I am also a bit reluctant to tinker with the Xorg configuration file, especially because these days this is file is not explicitly needed any more. Maybe a similar solution for Wayland exists, of which I'm not aware.

A second problem is that the display on the tablet comes with some delay. I'm not sure whether this is due to network latency, or maybe the tablet is too slow to render the stream. If you put a terminal window on the tablet, the delay is acceptable, but for fancy things like Hipchat (Hipchat? Fancy? I do not understand why an IM application can be so hard to render. But that is a different story), it is not that convenient.

A third thing I noticed, is that the VNC server sometimes turns off the 'key repeat functionality'. I mean, if you press e.g. backspace for a long time, it only deletes one character, instead of continuing to delete until the key is released. To fix this, type xset r on. If it does not work, type it again. And again. In the end, it will probably work. (Look at the log messages of your VNC server to find out how many times you need to enter the command.)

Conclusion

A tablet as a third screen: it works with xrandr and x11vnc, but with a few restrictions. Just try it, and if you have any tips to improve this solution, let me know.