We have discovered some irregularities in the gobject reference counting
when using objects of short live time, e.g. dialogs. The sample program
shows the behavior by incresing the number of toplevel windows. This
vanishs when we apply the silly workaround by defining WITH_UNREF.

The real reason for this misbehavior is that in the destructor of Object
(gtk/gtkmm/object.cc) the reference counter will not be decremented if
gobj() equals NULL. But other methods as disconnect_cpp_wrapper() which is
called by Object::destroy_ for instance set gobj() to NULL, so that at the
end the destructor forgets to decrease the reference counter resulting in
the shown resource leak.

Below the example program and test output:

$ cat test.cc
#include <gtkmm/main.h>
#include <gtkmm/window.h>
#include <gtkmm/dialog.h>
#include <gtkmm/button.h>
#include <iostream>

using namespace std;
using namespace Gtk;
using SigC::slot;

class MyDialog : public Dialog {
public:
  MyDialog() { add_button("Ok", 0); }
  MyDialog::~MyDialog() {
#ifdef WITH_UNREF
    g_object_unref(G_OBJECT(gobj()));
#endif
  }
};

class MyWindow : public Window {
public:
  MyWindow() {
    set_size_request(200, 200);
    Box* mainBox = manage(new HBox);
    Button* button = manage(new Button("Show Dialog"));
    button->signal_clicked().connect(slot(*this, &MyWindow::ShowDialog));
    mainBox->pack_start(*button);
    add(*mainBox);
  }
  void ShowDialog() {
    { 
      MyDialog d; 
      d.run();
    }
    list<Window*> toplevelwindows = list_toplevels();
    cout << "toplevelwindows.size = " << toplevelwindows.size() << endl;
  }
};

int main(int argc, char* argv[])
{
  Gtk::Main kit(argc, argv);
  MyWindow win;
  win.show_all();
  kit.run(win);
  return 0;
}

$ ./test
toplevelwindows.size = 2
toplevelwindows.size = 3
toplevelwindows.size = 4