Flutter textfield background color on focus

It looks like it’s caused by the splash effect on the textfield. I couldn’t find a way to disable it for that specific widget but you can make it transparent by wrapping your TextField in a Theme widget and setting the splashColor to transparent:

Theme(
  data: Theme.of(context).copyWith(splashColor: Colors.transparent),
  child: TextField(
    autofocus: false,
    style: TextStyle(fontSize: 22.0, color: Color(0xFFbdc6cf)),
    decoration: InputDecoration(
      filled: true,
      fillColor: Colors.white,
      hintText: 'Username',
      contentPadding:
          const EdgeInsets.only(left: 14.0, bottom: 8.0, top: 8.0),
      focusedBorder: OutlineInputBorder(
        borderSide: BorderSide(color: Colors.white),
        borderRadius: BorderRadius.circular(25.7),
      ),
      enabledBorder: UnderlineInputBorder(
        borderSide: BorderSide(color: Colors.white),
        borderRadius: BorderRadius.circular(25.7),
      ),
    ),
  ),
);