Among other VNC clients I've checked the only one that realy worked for me and I've managed to compile was this one:

https://github.com/matteodelabre/vnsee

The author is also very nice and responsive to bug reports.

I've managed to make it work as second monitor for linux virtual machine.

I wish I could use it as second monitor for Mac and was unable to make it work with MacOS so far.

Anyone made it work with Mac? Who knows how to make it work with MacOS ?