The source project of this merge request has been removed.
Adwaita: Style separator.sidebar
Let separators be declared as sidebars to have the same style as those drawn by GtkStackSidebar. This also let them handle the selection-mode class, whether they are assigned it or they descend from something in selection mode.
This is convenient when building a custom sidebar using a GtkSeparator and to extend a sidebar to the title bar.