Today I implemented a new niri screencasting feature: the dynamic cast target.
It shows up as a special "window" in the portal dialog. Once you select it, you can change what it casts with niri binds! Switch between windows, monitors, and in the future maybe workspaces and other stuff. Works with OBS, browsers, anything else.
I also wrote a wiki page with all our screencasting-related features: https://github.com/YaLTeR/niri/wiki/Screencasting
Thanks @elkowar for coming up with the dynamic cast idea!