__unsyncCheckedIndexOnInputChange() {
    const autoselect = this._autoSelectCondition();
    const checkedElement = this.formElements[/** @type {number} */ (this.checkedIndex)];
    if (!this.multipleChoice && !autoselect && checkedElement) {
      const textboxValue = this._getTextboxValueFromOption(checkedElement);
      if (!this._inputNode.value.startsWith(textboxValue)) {
        this.setCheckedIndex(-1);
      }
    }
  }
When textbox value doesn't match checkedIndex anymore, update accordingly...
@protected