I noticed, that XShape, despite being primitive alpha channel replacement, really affects default shadows, produced by compton. However, in gentoo xfce4-notifyd doesn't even depend on any x11 stuff, so I'm not sure is special x11 support would be ok. I just completed PR for Dunst, which already used xshape, being not rgba-enabled, but it directly uses x11/cairo combo without toolkits like gtk. I noticed at least some mentions or X11 (among flags/libs in build system files like Makefile.am). It would be some chance to have good shadows with rounded notifications in x11 (no idea, what is in wayland, since never tried it).
Frankly, I'm not very interested in further improving the look without compositing. If you want to provide a patch and maintain that part of the code feel free to submit a merge request on gitlab.
Well, I'm not sure if I will continue it either). I could not even get border geometry/radius values from GtkStyleProvider. Best I could get for dunst: https://user-images.githubusercontent.com/3896190/81013253-439dff80-8e74-11ea-9569-fd80f7bb3a5a.png (unlike uncomposited black artifacts, there are now white artifacts, with black shadow and white windows behind. Strange, that xfwm4 overlay borders, appearing on Alt+Tab, have full-window shadow, just inside of frame ))). Theoretically could be used as option with themes not providing own themes, if I could get necessary border info :/ . Such an interesting challenge, after first coding attempt with xlib.
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