The issue with Xfce specifically was that "gdk_monitor_get_model()" would return NULL instead of the proper name and all the multi-monitor handling would fall apart in turn, that seems like an gtk issue, not an Xfce issue.
As for "propagated-inputs", I put it there because that's where all the other X11 libraries already are. Don't know exactly why, but it seems to cause xrandr show up in the pkgconfig file which it wouldn't otherwise:
pkgconfig/gdk-3.0.pc:
Requires.private: ... xrandr ...