xdg-shell: set_parent should be no-op if there are no children and the surface is null

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston-desktop/internal.h

@@ -192,6 +192,8 @@ weston_desktop_surface_set_app_id(struct weston_desktop_surface *surface,
 void
 weston_desktop_surface_set_pid(struct weston_desktop_surface *surface,
 			       pid_t pid);
+bool
+weston_desktop_surface_children_list_empty(struct weston_desktop_surface *surface);
 void
 weston_desktop_surface_set_geometry(struct weston_desktop_surface *surface,
 				    struct weston_geometry geometry);

libweston-desktop/surface.c

@@ -724,6 +724,12 @@ weston_desktop_surface_set_pid(struct weston_desktop_surface *surface,
 	surface->pid = pid;
 }
 
+bool
+weston_desktop_surface_children_list_empty(struct weston_desktop_surface *surface)
+{
+	return wl_list_empty(&surface->children_list);
+}
+
 void
 weston_desktop_surface_set_geometry(struct weston_desktop_surface *surface,
 				    struct weston_geometry geometry)

libweston-desktop/xdg-shell.c

@@ -346,8 +346,12 @@ weston_desktop_xdg_toplevel_protocol_set_parent(struct wl_client *wl_client,
 		weston_desktop_surface_get_implementation_data(dsurface);
 	struct weston_desktop_surface *parent = NULL;
 
-	if (parent_resource != NULL)
+	if (parent_resource != NULL) {
 		parent = wl_resource_get_user_data(parent_resource);
+	} else {
+		if (weston_desktop_surface_children_list_empty(dsurface))
+			return;
+	}
 
 	weston_desktop_xdg_toplevel_ensure_added(toplevel);
 	weston_desktop_api_set_parent(toplevel->base.desktop, dsurface, parent);