Clipscreen is a straightforward application that creates a virtual monitor mirroring a specified part of your screen, indicated by a green rectangle. This handy tool allows you to share this virtual monitor instead of your whole screen during screen sharing sessions. To use Clipscreen, ensure you have X11 development libraries, Cairo graphics library, and a C++ compiler installed. The application has only been tested on Linux and xorg systems. To run Clipscreen, use the command ./clipscreen to create an overlay window at the desired position on your screen. To terminate the application, press Ctrl+C. The software is provided under a free license, granting users permission to modify, distribute, and sell copies.
https://github.com/splitbrain/clipscreen