Modifier key events are sent continuously

If you press a modifier key like Shift, the event will be sent again and again in a continuous loop forever. In EphyWindow's handle_key_cb(), if when gtk_event_controller_key_forward() returns FALSE (propagate), then the handle_key_cb() gets called in an infinite loop. The good news is control does return to the main loop, so there is no hang. But the web content is receiving all those key press events.

Exalm and I aren't sure what's wrong and both plan to investigate it more.