[gtk+] Re-fix opacity group handling

[Alexander Larsson <alexl src gnome org>]

commit c4539df2f26dd42a96f5fe965ae5f8967b2e6d62
Author: Alexander Larsson <alexl redhat com>
Date:   Fri Mar 1 14:56:30 2013 +0100

    Re-fix opacity group handling
    
    The last change fixed the windowed widget case but broke
    opacity group handling for windowed child widgets. This fixes
    up the code by making sure we norender_children in when there
    is an opacity group.
    
    This also cleans up the comments about how this works to something
    that is hopefully more understandable.

 gtk/gtkwidget.c |   32 ++++++++++++++++++--------------
 1 files changed, 18 insertions(+), 14 deletions(-)
---
diff --git a/gtk/gtkwidget.c b/gtk/gtkwidget.c
index eefadde..47b3e26 100644
--- a/gtk/gtkwidget.c
+++ b/gtk/gtkwidget.c
@@ -14230,21 +14230,25 @@ gtk_widget_propagate_alpha (GtkWidget *widget)
 
   parent = priv->parent;
 
-  norender =
-    /* If this widget has an opacity group and no window don't render it */
-    (priv->opacity_group && !gtk_widget_get_has_window (widget)) ||
-    /* If the parent has norender_children, propagate that here */
-    (parent != NULL && parent->priv->norender_children);
-
-  /* Windowed widget children should norender if: */
+  /* Norender affects only windowed widget and means don't render widget->window in the
+     normal fashion.
+     We only set this if the parent has norender_children, because:
+     a) For an opacity group (that does not have a norender_children parent) we still
+     need to render the window or we will never get an expose event.
+     b) For alpha we set the opacity of window->widget directly, so no other
+     work is needed.
+  */
+  norender = (parent != NULL && parent->priv->norender_children);
+
+  /* windows under this widget should not render if:
+     a) This widget has an opacity group
+     b) This widget has alpha and is no-windowed (otherwise we'd set alpha on widget->window)
+     c) This widget has norender but is no-windowed (a windowed widget would "swallow" the norender)
+  */
   norender_children =
-    /* The widget is no_window (otherwise its enought to mark it norender/alpha), and */
-    !gtk_widget_get_has_window (widget) &&
-     ( /* norender is set, or */
-      norender ||
-      /* widget has an alpha */
-      priv->alpha != 255
-       );
+    priv->opacity_group ||
+    (!gtk_widget_get_has_window (widget) &&
+     ( norender || priv->alpha != 255));
 
   if (gtk_widget_get_has_window (widget))
     {