This line of code https://git.xfce.org/apps/xfce4-notifyd/tree/xfce4-notifyd/xfce-notify-window.c#n421 doesn't allow the border-radius of the notification window to be 0, but I don't see why this is required, as Cairo handles gracefully the case of a 0px radius. Ok, setting 1px as radius achieves basically the same result as 0, so this is not serious; I just wanted to point it out.
I guess we can drop that part, not sure if it's necessary. However, is there a concrete use-case you're after?
I found out this because I'm using Compton as compositor, which doesn't properly handle the case of rounded cornered windows. I could disable shadows and blurring for the notification, but I think the shadow makes the notification stand out a bit more from the "background". Having a 1px radius rounded corners is already enough "visually appealing" to me, but at first I couldn't understand why I kept seeing heavily rounded corners also when setting 0 as radius.
I just checked again and what sucks is that without setting the border-radius to 6px (or something that is >0) it will look bad with Adwaita, which is the Gtk default. You'll simply get rectangular corners. So I'd rather leave the code as is for now (or port it all over to gtk_render_background and gtk_render_frame).
This is fixed in git master. We now respect 0px border radius and at the same time Adwaita's notifications still look ok (we use a different fallback class now). https://git.xfce.org/apps/xfce4-notifyd/commit/?id=7595209b43b6f7be29cc7c304abf892f2bfad3fc https://git.xfce.org/apps/xfce4-notifyd/commit/?id=e4df993c1130ada6258b9fabf7d725671b4d03e2