It appears that GTK+ 3.21.6 requires XDG-Shell version 6 for Wayland support, improves scrollbars, updates the cross-fade() function to correctly apply the cross-fading effect, and modernizes the example apps for newcomers. In some other good news for GTK app developers, the GTK+ 3.21.6 release restores the old sizing behavior of the GtkScrolledWindow widget, and implements a new API (Application Programming Interface) to let you change it in the future, and addresses various size allocation issues reported by users with the GtkPopover widget.