void render_thread() {
    glfwMakeContextCurrent(win);
    while( !glfwWindowShouldClose(win) ) {
        sleep(10); // emulate heavy rendering, to be sure that main loop stops event processing during this sleep
        glfwSetWindowTitle(win, "some title"); // posts message? to window queue to update title
    }
    glfwMakeContextCurrent(nullptr);
}

void main_thread() {
    set_thread_priority(highest); // to be sure this is first priority in OS sheduler, compared to render thread
    win = glfwCreateWindow(...);
    std::thread rt(&render_thread);
    while (!glfwWindowShouldClose(_glWindow))
        glfwWaitEvents(); // could put thread to sleep
    rt.join();
}