Libadwaita aims to help unify GNOME applications and advance their human interface guidelines while still allowing GTK4 to advance and be used by developers independent of GNOME.
Looking ahead, libadwaita will likely be used by GNOME GTK4 applications for providing better integration or a more unified UI/UX across these GNOME applications. While Adwaita has been the GNOME “visual language” for a while, the new Libadwaita library will be “the missing code” to it.
Libadwaita is based on the libhandy user interface library while being a GTK4 port. Libadwaita will now ship the Adwaita stylesheet along with its variants. Existing libhandy developers are involved with the libadwaita effort and moving forward that will be their focus. Development on libhandy itself is expected to now slow down greatly with libadwaita to be the primary focus looking forward.
More details on these plans with libadwaita 1.0 aiming for the GNOME 41 release can be found via this blog post by developer Adrien Plazas.