scene: schedule an output frame on wl_surface.frame

Some clients (e.g. mpv, Firefox) request a new wl_surface.frame
callback without damaging their surface. When this happens,
schedule a new output frame.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3350

types/scene/wlr_scene.c

@@ -236,10 +236,6 @@ static void scene_surface_handle_surface_commit(struct wl_listener *listener,
 		wl_container_of(listener, scene_surface, surface_commit);
 	struct wlr_surface *surface = scene_surface->surface;
 
-	if (!pixman_region32_not_empty(&surface->buffer_damage)) {
-		return;
-	}
-
 	struct wlr_scene *scene = scene_node_get_root(&scene_surface->node);
 
 	int lx, ly;
@@ -256,6 +252,17 @@ static void scene_surface_handle_surface_commit(struct wl_listener *listener,
 		return;
 	}
 
+	// Even if the surface hasn't submitted damage, schedule a new frame if
+	// the client has requested a wl_surface.frame callback.
+	if (!wl_list_empty(&surface->current.frame_callback_list) &&
+			scene_surface->primary_output != NULL) {
+		wlr_output_schedule_frame(scene_surface->primary_output);
+	}
+
+	if (!pixman_region32_not_empty(&surface->buffer_damage)) {
+		return;
+	}
+
 	struct wlr_scene_output *scene_output;
 	wl_list_for_each(scene_output, &scene->outputs, link) {
 		struct wlr_output *output = scene_output->output;