[gtk/expander-ref-leak] Release an extra reference on GtkExpander:child

[Emmanuele Bassi <ebassi src gnome org>]

commit 5e311d4d85d235a53191558a2337b534bf1a8a3a
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Tue Aug 24 01:11:15 2021 +0100

    Release an extra reference on GtkExpander:child
    
    The child of a GtkExpander is owned directly by the expander whenever
    the "expanded" flag is unset.
    
    We are adding an additional reference to the child of an expander when
    expander is not expanded.
    
    Additionally, if a GtkExpander is disposed while not expanded, we need
    to explicitly release the reference on the child widget that we own.
    
    This reference leak was masked in GTK3 by GtkContainer removing each
    child from the parent container by recursively calling
    gtk_widget_destroy().

 gtk/gtkexpander.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)
---
diff --git a/gtk/gtkexpander.c b/gtk/gtkexpander.c
index 00b9a52e38..27d94b0112 100644
--- a/gtk/gtkexpander.c
+++ b/gtk/gtkexpander.c
@@ -582,6 +582,10 @@ gtk_expander_dispose (GObject *object)
       expander->expand_timer = 0;
     }
 
+  /* If the expander is not expanded, we own the child */
+  if (!expander->expanded)
+    g_clear_object (&expander->child);
+
   if (expander->box)
     {
       gtk_widget_unparent (expander->box);
@@ -900,6 +904,9 @@ gtk_expander_set_expanded (GtkExpander *expander,
 
   if (child)
     {
+      /* Transfer the ownership of the child to the box when
+       * expanded is set, and then back to us when it is unset
+       */
       if (expander->expanded)
         {
           gtk_box_append (GTK_BOX (expander->box), child);
@@ -1196,25 +1203,31 @@ gtk_expander_set_child (GtkExpander *expander,
   g_return_if_fail (GTK_IS_EXPANDER (expander));
   g_return_if_fail (child == NULL || GTK_IS_WIDGET (child));
 
+  if (expander->child == child)
+    return;
+
   if (expander->child)
     {
-      gtk_box_remove (GTK_BOX (expander->box), expander->child);
       if (!expander->expanded)
         g_object_unref (expander->child);
+      else
+        gtk_box_remove (GTK_BOX (expander->box), expander->child);
     }
 
   expander->child = child;
 
   if (expander->child)
     {
+      /* We only add the child to the box if the expander is
+       * expanded; otherwise we just claim ownership of the
+       * child by sinking its floating reference, or acquiring
+       * an additional reference to it. The reference will be
+       * dropped once the expander is expanded
+       */
       if (expander->expanded)
         gtk_box_append (GTK_BOX (expander->box), expander->child);
       else
-        {
-          if (g_object_is_floating (expander->child))
-            g_object_ref_sink (expander->child);
-          g_object_ref (expander->child);
-        }
+        g_object_ref_sink (expander->child);
 
       gtk_accessible_update_relation (GTK_ACCESSIBLE (expander),
                                       GTK_ACCESSIBLE_RELATION_CONTROLS, expander->child, NULL,