1) Your picture shows an LG monitor. Why does it not show the screen of your notebook?
2) If you are purposefully showing on an external monitor could it have something to do with the monitor resolution i.e. is it cutting off the bottom? Are there and scroll bars on the side so it can move down...
You are asking some very large and very complex questions. This is probably not the best place to seek answers.
Have you tried the Mono Community? Look here https://www.mono-project.com/community/
Scroll down to "Get Help".
A Thin Client is a very lightweight computer, normally attached to a Server so it will need a very lightweight distro.https://en.wikipedia.org/wiki/Thin_client
Does your Thin Client have a make and model? How much RAM has it got?
Something like Peppermint? https://peppermintos.com/