Basically, if I request a Gdk::Display object using the open() method, the
RefPtr returned causes an exception when the reference leaves scope. This
short program reproduces the issue on Gentoo with the latest ebuild version
(2.2.5) installed:

#include <iostream>
#include <gdkmm/display.h>
#include <gtkmm/main.h>

int main(int argc, char **argv) {
    Gtk::Main kit(argc, argv);
    std::cout << "Print this." << std::endl;
    {   
        Glib::RefPtr<Gdk::Display> display = Gdk::Display::open(":0.0");
        std::cout << "RefPtr is still good." << std::endl;
    }
    std::cout << "This will never print." << std::endl;

    return 0;
}

Compiling this against the gtkmm and gdkmm libraries will have this output:

$ g++ -o test test.cc `pkg-config --libs --cflags gdkmm-2.0 gtkmm-2.0`
$ ./test
Print this.
RefPtr is still good.
Segmentation fault

It's pretty clear where the problem is. To make it even more explicit
adding a call to display.clear() right above the "RefPtr is still good."
line will keep that from being printed.