Mirroring a Wayland desktop region for easy screen sharing

I'm a desktop Linux user — specifically Debian GNU/Linux with GNOME and Wayland. I also do a fair number of video calls involving screen sharing. And I have an ultra-wide screen monitor (32:9). This can sometimes be a challenging combination. I want to share a portion of my screen and any window shown there, but most of the software I use only supports sharing either a single window, or an entire monitor. If I share a single window, it's annoying to share a different window every time I need to show something else. If I share the entire monitor, the aspect ratio means the other participants will have a pretty bad time watching it. Year of screen sharing on the Linux Desktop Wayland has been around for almost 20 years, but the support in the wider ecosystem is still spotty. One reason is enhanced security (or — introducing any security at all) compared to the old X11 protocol, which breaks some use cases, screen sharing being one of them. The Wayland way to do screen sharing is through…