Layer surfaces are a way in wayland to create windows without decoration and to be positioned anywere on the screen, you can also put anchors on the sides, corners or center of the screen, make the layer be displayed on top or behind other windows or if they're exclusive (reserves space for the layer, useful for bars for example so the windows do not go behind the bar), they're used for bars, widgets, overlays, background, context menus, etc... All wayland compositors excluding gnome already support them and it's a shame that this project is not using layer surfaces because you don't need to position the window manually anymore, you can pass through the mouse clicks and the window also has no decorations.
You can look at this library to use layer surfaces in gtk to start: https://github.com/wmww/gtk4-layer-shell/tree/main