Q: Using Windows laptop as 2nd screen for Ubuntu desktop. possible? I know its trivial to use your laptop as a 2nd monitor if your desktop is also running Windows.
But what if you are running Linux(Ubuntu) on your desktop.
Is is still possible to somehow use your laptop as an external monitor?

A: As far as I know, there is no way to use Windows to extend your Linux desktop.
However, this is way you can kind of do it.  A hack of sorts.
First, you would install Synergy on both machines.  This will allow you to control both machines from one mouse and keyboard.  There are both free and paid versions of Synergy.
Then install an X server, such as VCXsrv, on Windows and then forward X11 applications from your Linux machine to the Windows machine.  Your Linux apps will run on the Linux machine, but their application windows appear on the Windows machine.
The Windows machine's mouse and keyboard control the forwarded X11 applications.  However, with Synergy you have control of the Windows machine's mouse and keyboard.
Obviously, this is not seamless integration, but it does give you more desktop real estate.  You cant drag Windows back and forth from one machine to the other.  Not every application is guaranteed to look, or even work, properly when forwarded, but it just might be enough for you.

A: Virtual Monitor Over VNC
With this solution you can use any OS that is able to connect via VNC to your Ubuntu machine like a second screen. This is done by using x11vnc, the following is derived from the VNC Virtual Display Linker python code. I had a couple of issues with this code but I managed to get it working as shown below.
Simplified Intructions:
Please be aware of the notes and considerations at the end of this answer.

*

*Definitions: The "server" will be the computer which screen you want to extend, the "client" will be the computer (or tablet, any device able to deploy a VNC client) that you want to use as a screen.

*Required Software: On the server install x11vnc and gtf, on the client install a vnc client.

*You need to know client's screen size (we'll call them: CLIENT_WIDTH, CLIENT_HEIGHT) and the servers' screen size (we'll call them: SERVER_WIDTH, SERVER_HEIGHT).

*In the server, run in a terminal gtf CLIENT_WIDTH CLIENT_HEIGHT 60 (replace the CLIENT_WIDTH and CLIENT_HEIGHT with the corresponding values). As an example for gtf CLIENT_WIDTH CLIENT_HEIGHT 60 is

# 1384x768 @ 60.00 Hz (GTF) hsync: 47.70 kHz; pclk: 86.62 MHz
Modeline "1384x768_60.00"  86.62  1384 1456 1600 1816  768 769 772 795  -HSync +Vsync



*From the previous result copy what is in front of Modeline, (in this case "1384x768_60.00"  86.62  1384 1456 1600 1816  768 769 772 795  -HSync +Vsync) and use that as a parameter to xrandr --newmode command. For example, in this case, we'll have to run in the server xrandr --newmode "1368x768_60.00"  85.86  1368 1440 1584 1800  768 769 772 795  -HSync +Vsync

*In 5 we've created a newmode in xrandr, now we need to add it to the virtual screen, as follows xrandr --addmode VIRTUAL1 "1368x768_60.00" (please remember to use your own newly created mode name, which is the value including quotes in 5).

*Run x11vnc -usepw -nocursorshape -nocursorpos -noxinerama -solid -repeat -forever -clip CLIENT_WIDTHxCLIENT_HEIGHT+SERVER_WIDTH+0. As an example: x11vnc -usepw -nocursorshape -nocursorpos -noxinerama -solid -repeat -forever -clip 1368x768+1920+0. This will create a VNC server in port 5900 you can connect to.

*Lastly, connect to the server from you other computer (or any VNC client, recommended krdc for linux machines). You should be able to enjoy your extended screen now.

Notes and considerations:

*

*In step 8, with krdc, it is recommended to use the "Medium Quality" option, even if you are on cable: both the "Low" and "High" quality options produce errors.

*I tried this over a WiFi network and it works properly, however on cable it's smoother.

*I had problems when starting the VNC server in step 7, the screen started to flicker and I couldn't see what I was doing on the screen. To fix this, I connected via the VNC server (the extended screen already works at this point), opened the Displays tool (the one that comes with the system where you can see the displays that are connected) and Unify Outputs then Break Unified Outputs
(an example of this option in Kubuntu is shown below). Then everything worked.


