function isKeypressCommand(nativeEvent) {
  return (nativeEvent.ctrlKey || nativeEvent.altKey || nativeEvent.metaKey) && // ctrlKey && altKey is equivalent to AltGr, and is not a command.
  !(nativeEvent.ctrlKey && nativeEvent.altKey);
}
As of v27, Firefox may fire keypress events even when no character
will be inserted. A few possibilities:

- `which` is `0`. Arrow keys, Esc key, etc.

- `which` is the pressed key code, but no char is available.
  Ex: 'AltGr + d` in Polish. There is no modified character for
  this key combination and no character is inserted into the
  document, but FF fires the keypress for char code `100` anyway.
  No `input` event will occur.

- `which` is the pressed key code, but a command combination is
  being used. Ex: `Cmd+C`. No character is inserted, and no
  `input` event will occur.