Make St aware of the UI group.

Inside the Shell, all the UI (including chrome, the overview, and
the actual windows) is not a child of the stage but of a special
ClutterGroup, which is cloned inside the magnifier.
Add function for setting this special actor so that actors added by
St are visible in the magnifier. Nothing yet uses this, but the
tooltip will soon.

https://bugzilla.gnome.org/show_bug.cgi?id=635100

js/ui/main.js

@@ -136,6 +136,7 @@ function start() {
 
     // Set up stage hierarchy to group all UI actors under one container.
     uiGroup = new Clutter.Group();
+    St.set_ui_root(global.stage, uiGroup);
     global.window_group.reparent(uiGroup);
     global.overlay_group.reparent(uiGroup);
     global.stage.add_actor(uiGroup);

src/st/st-widget.c

@@ -2222,3 +2222,71 @@ on_can_focus_notify (GObject    *gobject,
   atk_object_notify_state_change (ATK_OBJECT (data),
                                   ATK_STATE_FOCUSABLE, can_focus);
 }
+
+static GQuark
+st_ui_root_quark (void)
+{
+  static GQuark value = 0;
+  if (G_UNLIKELY (value == 0))
+    value = g_quark_from_static_string ("st-ui-root");
+  return value;
+}
+
+static void
+st_ui_root_destroyed (ClutterActor *actor,
+                      ClutterStage *stage)
+{
+  st_set_ui_root (stage, NULL);
+  g_signal_handlers_disconnect_by_func (actor, st_ui_root_destroyed, stage);
+}
+
+/**
+ * st_get_ui_root:
+ * @stage: a #ClutterStage
+ * @container: (allow-none): the new UI root
+ *
+ * Sets a #ClutterContainer to be the parent of all UI in the program.
+ * This container is used when St needs to add new content outside the
+ * widget hierarchy, for example, when it shows a tooltip over a widget.
+ */
+void
+st_set_ui_root (ClutterStage     *stage,
+                ClutterContainer *container)
+{
+  ClutterContainer *previous;
+
+  g_return_if_fail (CLUTTER_IS_STAGE (stage));
+  g_return_if_fail (CLUTTER_IS_CONTAINER (container));
+
+  previous = st_get_ui_root (stage);
+  if (previous)
+    g_signal_handlers_disconnect_by_func (container, st_ui_root_destroyed, stage);
+
+  if (container)
+    {
+      g_signal_connect (container, "destroy", G_CALLBACK (st_ui_root_destroyed), stage);
+      g_object_set_qdata_full (G_OBJECT (stage), st_ui_root_quark (), g_object_ref (container), g_object_unref);
+    }
+}
+
+/**
+ * st_get_ui_root:
+ * @stage: a #ClutterStage
+ *
+ * Returns: (transfer none): the container which should be the parent of all user interface,
+ *   which can be set with st_set_ui_root(). If not set, returns @stage
+ */
+ClutterContainer *
+st_get_ui_root (ClutterStage *stage)
+{
+  ClutterContainer *root;
+
+  g_return_val_if_fail (CLUTTER_IS_STAGE (stage), NULL);
+
+  root = g_object_get_qdata (G_OBJECT (stage), st_ui_root_quark ());
+
+  if (root != NULL)
+    return root;
+  else
+    return CLUTTER_CONTAINER (stage);
+}

src/st/st-widget.h

@@ -162,6 +162,10 @@ char  *st_describe_actor       (ClutterActor *actor);
 void   st_set_slow_down_factor (gfloat factor);
 gfloat st_get_slow_down_factor (void);
 
+void              st_set_ui_root (ClutterStage     *stage,
+                                  ClutterContainer *container);
+ClutterContainer *st_get_ui_root (ClutterStage     *stage);
+
 G_END_DECLS
 
 #endif /* __ST_WIDGET_H__ */