TextField设置属性scrollPadding: EdgeInsets.zero
    TextField的decoration设置属性contentPadding: EdgeInsets.zero， 设置isDense: true

    1. TextField(
    2.   style: TextStyle(fontSize: setFont(28), color: Color(0xff4a4a4a), height: 1, textBaseline: TextBaseline.alphabetic),
    3.   scrollPadding: EdgeInsets.zero,  // 关键
    4.   decoration: InputDecoration(
    5.     contentPadding: EdgeInsets.zero,  // 关键
    6.     isDense: true,  // 关键
    7.     border: InputBorder.none, // 去除边框
    8.     hintText: '搜索问题',
    9.     hintStyle: TextStyle(fontSize: setFont(28), color: Color(0xffCCCCCC), height: 58/28, textBaseline: TextBaseline.alphabetic),
    10.     counterText: '',
    11.   ),
    12.   controller: _searchInput,
    13.   keyboardType: TextInputType.text,
    14.   autocorrect: false,
    15.   maxLines: 1,
    16.   minLines: 1,
    17.   maxLength: 20,
    18.   textInputAction: TextInputAction.search,
    19.   onEditingComplete: () {
    20.     _searchQuestion();
    21.   },
    22. ),