Makefile

sockfoc: sockfoc.cc window_ext.h
	g++ -ggdb sockfoc.cc -o sockfoc `pkg-config gtkmm-3.0 --cflags --libs` -lX11

sockfoc.cc

#include <gtkmm.h>
#include <gtkmm/socket.h>

#include "window_ext.h"

#define XEMBED_FOCUS_IN 4
#define XEMBED_FOCUS_CURRENT 0

class TheWindow : public WindowExt
{
  Gtk::Box* box;
  Gtk::Socket* socket;
  Gtk::Button* button_close;

  public:
    TheWindow() : WindowExt(Gtk::WINDOW_POPUP)
    {
      signal_realize().connect( sigc::mem_fun(*this,&TheWindow::add_term) ); 

      button_close = Gtk::manage( new Gtk::Button("Close Window.") );
      box = Gtk::manage( new Gtk::Box(Gtk::ORIENTATION_VERTICAL) );
      box->pack_start( *button_close );
      add(*box);

      button_close->signal_clicked().connect(
        sigc::mem_fun(*this,&Window::hide)
      );

      show_all();
      grab_keyboard(this->get_window());
    }

    void add_term()
    {
      socket = Gtk::manage( new Gtk::Socket );
      box->pack_start( *socket );
      int xid = socket->get_id();
      char buffer[50];
      //sprintf(buffer, "xterm -into %d &", xid);
      sprintf(buffer, "urxvt -embed %d &", xid);
      system(buffer);
      box->show_all();

      socket->signal_plug_added().connect(
          sigc::mem_fun(*this, &TheWindow::send_focus_in)
      );

      socket->signal_plug_removed().connect(
          sigc::mem_fun(*this, &TheWindow::on_plug_removed)
      );

    }

    bool on_plug_removed()
    { 
      hide();
      return false;
    }

    void send_focus_in()
    {
      GdkWindow* plug = socket->get_plug_window()->gobj();
      GdkDisplay *display = gdk_window_get_display(plug);

      XClientMessageEvent xclient;
      memset(&xclient, 0, sizeof (xclient));

      xclient.window = GDK_WINDOW_XID(plug);
      xclient.type = ClientMessage;
      xclient.message_type =
          gdk_x11_get_xatom_by_name_for_display(display, "_XEMBED");
      xclient.format = 32;
      xclient.data.l[0] = gtk_get_current_event_time();
      xclient.data.l[1] = XEMBED_FOCUS_IN;
      xclient.data.l[2] = XEMBED_FOCUS_CURRENT;
      xclient.data.l[3] = 0;
      xclient.data.l[4] = 0;

      XSendEvent(
          GDK_WINDOW_XDISPLAY(plug),
          GDK_WINDOW_XID(plug), False, NoEventMask, (XEvent *)&xclient
      );
    }
};

int main(int argc, char** argv)
{
  Glib::RefPtr<Gtk::Application> app;
  app = Gtk::Application::create(argc, argv, "socket.focustest");

  TheWindow win;
  return app->run(win);
}

window_ext.h

#ifndef WINDOW_EXT_H
#define WINDOW_EXT_H

#include <gtkmm.h>
#include <iostream>

class Overlay;

class WindowExt : public Gtk::Window
{
  private:
    Cairo::RefPtr<Cairo::Region> shape;

  public:
    void grab_keyboard(Glib::RefPtr<Gdk::Window> dest)
    {
      Glib::RefPtr<Gdk::Display> display = get_display();
      Glib::RefPtr<Gdk::DeviceManager> deviceM = display->get_device_manager();

      std::vector< Glib::RefPtr<Gdk::Device> >devices =
        deviceM->list_devices( Gdk::DEVICE_TYPE_MASTER );

      std::vector< Glib::RefPtr<Gdk::Device> >::iterator it;

      for(it = devices.begin(); it != devices.end(); ++it)
      {
        for(int grab = Gdk::GRAB_NOT_VIEWABLE; grab == Gdk::GRAB_NOT_VIEWABLE;)
        {
          grab = (*it)->grab(
            dest, Gdk::OWNERSHIP_NONE, true,
            Gdk::ALL_EVENTS_MASK, GDK_CURRENT_TIME
          );
        }
      }
    }

    WindowExt(Gtk::WindowType type=Gtk::WINDOW_TOPLEVEL) : Window(type) {}

    virtual ~WindowExt(){}
};

#endif