Screen sharing is different thing, there is no fragmentation there.
But in order to take screenshot I had to write different code:
For KDE. It also requires special line in desktop file for security reasons.
Global shortcuts are even worse. It also DE specific and users have to manually register them in DE settings. In order for your application to support this, it should export such functions via the Dbus interface. And all this incompatible with Windows (my app is cross-platform), so I had to provide in-app interface for global shortcuts too that works for Windows and X11 users.
What prevents you from using org.freedesktop.portal.Screenshot with GNOME and KDE as well? They both support taking screenshots using that method.
Unfortunately, you on GNOME and KDE you don’t have org.freedesktop.portal.Screenshot method in /org/freedesktop/portal/desktop.
Not trying to be a menace, but I just tried it out using xdg-portal-test-kde and the screenshot portal definitely works on KDE Plasma 5.27.10. If you are experiencing issues with that, please create a bug report for xdg-desktop-portal-kde so it can be fixed.
