void onMouseButton(int button, int action, int mods) {
    switch(action) {
      case GLFW_PRESS:
        for(MouseButtonEvent e : _mouseDown) { e.run(button, mods); }
      break;
      
      case GLFW_RELEASE:
        for(MouseButtonEvent e : _mouseUp) { e.run(button, mods); }
      break;
    }
  }