[propose] onSubmitted can be called when TextField loses focus and Spinbox is enabled.

[zhang-peter]

This behavior is very like valueModified of SpinBox in Qt Quick Control.

  void _handleFocusChanged() {
    setState(() {
      if (hasFocus) {
        _selectAll();
      } else {
        if (widget.enabled) {
          final value = fixupValue(_controller.text);
          widget.onSubmitted?.call(value);
        }
      }
    });
  }