If GDK threading is initialized and you call gdk_display_put_event() from another thread, it won't wake up the main loop in the main thread to deliver the event.

Pseudo-patch:

void
gdk_display_put_event (GdkDisplay     *display,
                       const GdkEvent *event)
{
  g_return_if_fail (GDK_IS_DISPLAY (display));
  g_return_if_fail (event != NULL);

  _gdk_event_queue_append (display, gdk_event_copy (event));
+  /* If the main loop is blocking in a different thread, wake it up */
+  g_main_context_wakeup (NULL); 
}