dedicated window decoration library
For Wayland applications that would like to use GTK-styled window decorations without depending on full GTK, it would be very useful to separate the basic decoration functionality into a dedicated library. This should only concern the default window decoration and its functionality (minimise, maximise, close) as shown by mutter on X11/Xwayland.
Such a small and dedicated library would be very helpful to applications that do not want to depend on a full toolkit for drawing CSD on Wayland, and which are usually not concerned with drawing window decoration and its logic on other platforms like Windows, macOS or Android. Examples of such projects are SDL, GLFW, and mpv, since these projects draw to raw buffers or use OpenGL directly.
Projects like libdecoration attempt to support projects with CSD on Wayland, but would also depend on a GTK-decorations library to match the style as found in original GTK applications.
Furthermore, GTK itself could depend on such a GTK-decoration library to make sure that GTK as a toolkit, applications that use this GTK-decoration library and applications that use the library via libdecoration, would all look the same and integrate well.