I know this Question is a older. But for people like me who are still searching for an answer. This is the solution. You are searching for the gapPadding in the focusedBorder BorderStyle.

const InputDecoration dialogPointInputStyle = InputDecoration(
  isDense: true,
  border: OutlineInputBorder(
    borderSide: BorderSide(color: CaboTheme.tertiaryColor, width: 2),
  ),
  enabledBorder: OutlineInputBorder(
    borderSide: BorderSide(color: CaboTheme.tertiaryColor, width: 2),
  ),
  focusedBorder: OutlineInputBorder(
    borderSide: BorderSide(color: CaboTheme.tertiaryColor, width: 2),
    gapPadding: 0,
  ),
  contentPadding: EdgeInsets.all(8.0),
  filled: true,
  fillColor: CaboTheme.secondaryColor,
);

TextField(
 keyboardType: TextInputType.number,
 onChanged: (String points) {},
 minLines: 1,
 style: const TextStyle(
   fontSize: 18,
   fontFamily: 'Aclonica',
   color: CaboTheme.primaryColor,
 ),
 decoration: dialogPointInputStyle.copyWith(
   labelStyle: CaboTheme.secondaryTextStyle.copyWith(
   fontSize: 14,
   color: CaboTheme.primaryColor,
   backgroundColor: CaboTheme.tertiaryColor,
 ),
 labelText: 'Max. Game Points'),
),