Flutter: Using NumberFormat in TextInputFormatter

This is because after you format the value you are adding a new char but the text selection remains at the same position, one char less, this cause an expected behavior

You can modify your TextInputFormatter like this:

Fixed to support all locales and to remember cursor position

class NumericTextFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    if (newValue.text.isEmpty) {
      return newValue.copyWith(text: '');
    } else if (newValue.text.compareTo(oldValue.text) != 0) {
      final int selectionIndexFromTheRight =
          newValue.text.length - newValue.selection.end;
      final f = NumberFormat("#,###");
      final number =
          int.parse(newValue.text.replaceAll(f.symbols.GROUP_SEP, ''));
      final newString = f.format(number);
      return TextEditingValue(
        text: newString,
        selection: TextSelection.collapsed(
            offset: newString.length - selectionIndexFromTheRight),
      );
    } else {
      return newValue;
    }
  }
}

Based on the answer and for people from Europe looking for a quick fix

class NumericTextFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    final currencySymbol = '€';
    if (newValue.text.isEmpty || newValue.text.trim() == currencySymbol) {
      return newValue.copyWith(text: '');
    } else if (newValue.text.compareTo(oldValue.text) != 0) {
      var selectionIndexFromTheRight =
          newValue.text.length - newValue.selection.end;
      final f =
          NumberFormat.currency(locale: 'de', decimalDigits: 0, symbol: '');
      var num = int.parse(newValue.text.replaceAll(RegExp('[^0-9]'), ''));
      final newString = '$currencySymbol ' + f.format(num).trim();
      return TextEditingValue(
        text: newString,
        selection: TextSelection.collapsed(
            offset: newString.length - selectionIndexFromTheRight),
      );
    } else {
      return newValue;
    }
  }
}