_syncToTextboxMultiple(modelValue, oldModelValue = []) {
    if (this.requireOptionMatch) {
      const diff = modelValue.filter(x => !oldModelValue.includes(x));
      const newValue = this.formElements
        .filter(option => diff.includes(option.choiceValue))
        .map(option => this._getTextboxValueFromOption(option))
        .join(' ');
      this._setTextboxValue(newValue);
    }
  }
@overridable
Allows to control what happens when checkedIndexes change
@param {string[]} modelValue
@param {string[]} oldModelValue
@protected