[gnome-shell] st: Delay handling of :first/:last-child changes

[Florian Müllner <fmuellner src gnome org>]

commit 0afd600ea4bcbb04cef87c17e12609807afd5d1b
Author: Florian Müllner <fmuellner gnome org>
Date:   Thu May 2 21:12:01 2019 +0200

    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 | 83 +++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 63 insertions(+), 20 deletions(-)
---
diff --git a/src/st/st-widget.c b/src/st/st-widget.c
index d416374d5..a3feb1503 100644
--- a/src/st/st-widget.c
+++ b/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"))
-        {
-          ClutterActor *new_first;
-
-          new_first = find_nearest_visible_forward (CLUTTER_ACTOR (widget));
-          st_widget_set_first_visible_child (ST_WIDGET (parent), new_first);
-        }
+        parent_priv->first_child_dirty = TRUE;
 
       if (st_widget_has_style_pseudo_class (widget, "last-child"))
-        {
-          ClutterActor *new_last;
-
-          new_last = find_nearest_visible_backwards (CLUTTER_ACTOR (widget));
-          st_widget_set_last_visible_child (ST_WIDGET (parent), new_last);
-        }
+        parent_priv->last_child_dirty = TRUE;
     }
+
+  if (parent_priv->first_child_dirty || parent_priv->last_child_dirty)
+    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));
-  st_widget_set_first_visible_child (widget, find_nearest_visible_forward (first_child));
+  priv->first_child_dirty = TRUE;
+  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));
-  st_widget_set_last_visible_child (widget, find_nearest_visible_backwards (last_child));
+  priv->last_child_dirty = TRUE;
+  st_widget_queue_child_styles_update (widget);
 }
 
 static void