[gimp] app: reduce time complexity of layer-group translation

[Ell <ell src gnome org>]

commit ebb3ec4925f86c0a1242f6ae116f4252ff779424
Author: Ell <ell_se yahoo com>
Date:   Tue Jul 23 10:46:55 2019 +0300

    app: reduce time complexity of layer-group translation
    
    When translating a layer group, avoid separately updating the
    original area of the child layers before translating them (as per
    the fix to issue #3484), as this results in quadratic time
    complexity w.r.t. to the maximal subgroup nesting level.  Instead,
    simply defer the updating of the group's offset until *after*
    translating the child layers, so that their original area isn't
    clipped by the parent, while their new area is still properly
    updated even if the parent's size changes (see comment in code).

 app/core/gimpgrouplayer.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)
---
diff --git a/app/core/gimpgrouplayer.c b/app/core/gimpgrouplayer.c
index 7468f540ba..b65f7ff40f 100644
--- a/app/core/gimpgrouplayer.c
+++ b/app/core/gimpgrouplayer.c
@@ -790,15 +790,25 @@ gimp_group_layer_translate (GimpLayer *layer,
   /*  redirect stack updates to the drawable, rather than to the projection  */
   private->direct_update++;
 
-  /*  if this is a nested group layer, we need to update the child-layers'
-   *  original area *before* updating the group's offset.  once we update the
-   *  group's offset, our parent's size will also be updated, and the
-   *  subsequent area-updates of the child layers during translation will be
-   *  clipped to the updated parent bounds, potentially failing to update their
-   *  original area.  see issue #3484.
+  /*  translate the child layers *before* updating the group's offset, so that,
+   *  if this is a nested group, the parent's bounds still reflect the original
+   *  layer positions.  This prevents the original area of the child layers,
+   *  which is updated as part of their translation, from being clipped to the
+   *  post-translation parent bounds (see issue #3484).  The new area of the
+   *  child layers, which is likewise updated as part of translation, *may* get
+   *  clipped to the old parent bounds, but the corresponding region will be
+   *  updated anyway when the parent is resized, once we update the group's
+   *  offset.
    */
-  if (gimp_viewable_get_depth (GIMP_VIEWABLE (group)) > 0)
-    gimp_drawable_update_all (GIMP_DRAWABLE (group));
+  for (list = gimp_item_stack_get_item_iter (GIMP_ITEM_STACK (private->children));
+       list;
+       list = g_list_next (list))
+    {
+      GimpItem *child = list->data;
+
+      /*  don't push an undo here because undo will call us again  */
+      gimp_item_translate (child, offset_x, offset_y, FALSE);
+    }
 
   gimp_item_get_offset (GIMP_ITEM (group), &x, &y);
 
@@ -818,16 +828,6 @@ gimp_group_layer_translate (GimpLayer *layer,
   /*  update the group layer offset  */
   gimp_item_set_offset (GIMP_ITEM (group), x, y);
 
-  for (list = gimp_item_stack_get_item_iter (GIMP_ITEM_STACK (private->children));
-       list;
-       list = g_list_next (list))
-    {
-      GimpItem *child = list->data;
-
-      /*  don't push an undo here because undo will call us again  */
-      gimp_item_translate (child, offset_x, offset_y, FALSE);
-    }
-
   /*  redirect stack updates back to the projection  */
   private->direct_update--;