st: Delay handling of :first/:last-child changes

Updating the :first/:last-child pseudo classes can result in a lot
of unnecessary style changes when bulk-adding children to a container,
as every child ends up as the new last child.

Address this by deferring the style change to an idle, so we only do
the work once for the actual first and last child.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/529

src/st/st-widget.c

@@ -65,6 +65,8 @@ struct _StWidgetPrivate
   StThemeNodeTransition *transition_animation;
 
   guint is_style_dirty : 1;
+  guint first_child_dirty : 1;
+  guint last_child_dirty : 1;
   guint draw_bg_color : 1;
   guint draw_border_internal : 1;
   guint track_hover : 1;
@@ -72,6 +74,7 @@ struct _StWidgetPrivate
   guint can_focus : 1;
 
   gulong texture_file_changed_id;
+  guint update_child_styles_id;
 
   AtkObject *accessible;
   AtkRole accessible_role;
@@ -323,6 +326,8 @@ st_widget_dispose (GObject *gobject)
   g_clear_object (&priv->last_visible_child);
 
   G_OBJECT_CLASS (st_widget_parent_class)->dispose (gobject);
+
+  g_clear_handle_id (&priv->update_child_styles_id, g_source_remove);
 }
 
 static void
@@ -1608,47 +1613,85 @@ find_nearest_visible_forward (ClutterActor *actor)
   return next;
 }
 
+static gboolean
+st_widget_update_child_styles (StWidget *widget)
+{
+  StWidgetPrivate *priv = st_widget_get_instance_private (widget);
+
+  if (priv->first_child_dirty)
+    {
+      ClutterActor *first_child;
+
+      priv->first_child_dirty = FALSE;
+
+      first_child = clutter_actor_get_first_child (CLUTTER_ACTOR (widget));
+      st_widget_set_first_visible_child (widget,
+                                         find_nearest_visible_forward (first_child));
+    }
+
+  if (priv->last_child_dirty)
+    {
+      ClutterActor *last_child;
+
+      priv->last_child_dirty = FALSE;
+
+      last_child = clutter_actor_get_last_child (CLUTTER_ACTOR (widget));
+      st_widget_set_last_visible_child (widget,
+                                        find_nearest_visible_backwards (last_child));
+    }
+
+  priv->update_child_styles_id = 0;
+  return G_SOURCE_REMOVE;
+}
+
+static void
+st_widget_queue_child_styles_update (StWidget *widget)
+{
+  StWidgetPrivate *priv = st_widget_get_instance_private (widget);
+
+  if (priv->update_child_styles_id != 0)
+    return;
+
+  priv->update_child_styles_id = g_idle_add ((GSourceFunc) st_widget_update_child_styles, widget);
+}
+
 static void
 st_widget_visible_notify (StWidget   *widget,
                           GParamSpec *pspec,
                           gpointer    data)
 {
+  StWidgetPrivate *parent_priv;
   ClutterActor *actor = CLUTTER_ACTOR (widget);
   ClutterActor *parent = clutter_actor_get_parent (actor);
 
   if (parent == NULL || !ST_IS_WIDGET (parent))
     return;
 
+  parent_priv = st_widget_get_instance_private (ST_WIDGET (parent));
+
   if (clutter_actor_is_visible (actor))
     {
       ClutterActor *before, *after;
 
       before = clutter_actor_get_previous_sibling (actor);
       if (find_nearest_visible_backwards (before) == NULL)
-        st_widget_set_first_visible_child (ST_WIDGET (parent), actor);
+        parent_priv->first_child_dirty = TRUE;
 
       after = clutter_actor_get_next_sibling (actor);
       if (find_nearest_visible_forward (after) == NULL)
-        st_widget_set_last_visible_child (ST_WIDGET (parent), actor);
+        parent_priv->last_child_dirty = TRUE;
     }
   else
     {
       if (st_widget_has_style_pseudo_class (widget, "first-child"))
-        {
+        parent_priv->first_child_dirty = TRUE;
-          ClutterActor *new_first;
-
-          new_first = find_nearest_visible_forward (CLUTTER_ACTOR (widget));
-          st_widget_set_first_visible_child (ST_WIDGET (parent), new_first);
-        }
 
       if (st_widget_has_style_pseudo_class (widget, "last-child"))
-        {
+        parent_priv->last_child_dirty = TRUE;
-          ClutterActor *new_last;
+    }
 
-          new_last = find_nearest_visible_backwards (CLUTTER_ACTOR (widget));
+  if (parent_priv->first_child_dirty || parent_priv->last_child_dirty)
-          st_widget_set_last_visible_child (ST_WIDGET (parent), new_last);
+    st_widget_queue_child_styles_update (ST_WIDGET (parent));
-        }
-    }
 }
 
 static void
@@ -1656,10 +1699,10 @@ st_widget_first_child_notify (StWidget   *widget,
                               GParamSpec *pspec,
                               gpointer    data)
 {
-  ClutterActor *first_child;
+  StWidgetPrivate *priv = st_widget_get_instance_private (widget);
 
-  first_child = clutter_actor_get_first_child (CLUTTER_ACTOR (widget));
+  priv->first_child_dirty = TRUE;
-  st_widget_set_first_visible_child (widget, find_nearest_visible_forward (first_child));
+  st_widget_queue_child_styles_update (widget);
 }
 
 static void
@@ -1667,10 +1710,10 @@ st_widget_last_child_notify (StWidget   *widget,
                              GParamSpec *pspec,
                              gpointer    data)
 {
-  ClutterActor *last_child;
+  StWidgetPrivate *priv = st_widget_get_instance_private (widget);
 
-  last_child = clutter_actor_get_last_child (CLUTTER_ACTOR (widget));
+  priv->last_child_dirty = TRUE;
-  st_widget_set_last_visible_child (widget, find_nearest_visible_backwards (last_child));
+  st_widget_queue_child_styles_update (widget);
 }
 
 static void