The description of GObject contains a code snippet, describing how the floating
state of an object can be saved and restored. The code snippet is almost
identical to code in GtkMenu, but one line is missing.

  /* save floating state */
  gboolean was_floating = g_object_is_floating (object);
  g_object_ref_sink (object);
  /* protected code portion */
  ...;
  /* restore floating state */
  if (was_floating)
    g_object_force_floating (object);
  g_object_unref (object); /* release previously acquired reference */

The last few lines should be

  /* restore floating state */
  if (was_floating)
    g_object_force_floating (object);
  else
    g_object_unref (object); /* release previously acquired reference */

If the object was floating, g_object_ref_sink() did not add a reference, and
no reference shall be released. That's how it's done in gtk_menu_reparent().