function isFallbackCompositionEnd(domEventName, nativeEvent) {
    switch (domEventName) {
      case 'keyup':
        // Command keys insert or clear IME input.
        return END_KEYCODES.indexOf(nativeEvent.keyCode) !== -1;

      case 'keydown':
        // Expect IME keyCode on each keydown. If we get any other
        // code we must have exited earlier.
        return nativeEvent.keyCode !== START_KEYCODE;

      case 'keypress':
      case 'mousedown':
      case 'focusout':
        // Events are not possible without cancelling IME.
        return true;

      default:
        return false;
    }
  }
Does our fallback mode think that this event is the end of composition?