Notifications

[mischnic]

Are supported by Windows and macOS but not GTK+ (only with libnotify) ?

[andlabs]

GLib apparently has GNotification; I did not know this. Other than that, yes; there's also libappindicator and other stuff. There's also the deprecated GtkStatusIcon which is more advanced but less guaranteed to be available on all Linux desktop environments.

I still don't know if this is in scope for libui.

[rubyFeedback]

I still don't know if this is in scope for libui.

Perhaps as a secondary design goal. Or to have some kind of plugin infrastructure
so people could share and re-use code parts, for the things that are not directly
part of libui itself. Personally though I would be fine to use functionality only on
linux; my GUIs don't necessarily need to use the very same parts as-is. But I understand
that more and more code also makes libui harder to maintain, so being conservative
in feature additions is a good design goal IMO.