The portal only allows an external application to request the service from the compositor. He portal does not allow an external application to provide additional functionality beyond what is already provided by the compositor.
In my use case, I want to screen share a specific window, not the whole screen; this is functionality previously provided by a third-party X11-based tool, which cannot be provided from an XDG portal (even on those compositors that support portal at all).
In my use case, I want to screen share a specific window, not the whole screen; this is functionality previously provided by a third-party X11-based tool, which cannot be provided from an XDG portal (even on those compositors that support portal at all).